The El Salvador earthquakes of January and February 2001...

UNCORRECTED PROOF

The El Salvador earthquakes of January and February 2001:

context, characteristics and implications for seismic risk

J.J. Bommera,*, M.B. Benitob, M. Ciudad-Realc, A. Lemoined, M.A. Lopez-Menjıvare,R. Madariagad, J. Mankelowf, P. Mendez de Hasbung, W. Murphyh,

M. Nieto-Lovoe, C.E. Rodrıguez-Pinedai, H. Rosaj

aDepartment of Civil Engineering, Imperial College, London SW7 2BU, UKbUniversidad Politecnica de Madrid, Spain

cKinemetrics, 222 Vista Avenue, Pasadena, CA 91107, USAdEcole Normale Superieure, Paris, France

eEscuela de Ingenierıa Civil, Universidad de El Salvador, San Salvador, El SalvadorfBritish Geological Survey, Keyworth, UK

gDpto. Mecanica Estructural, Universidad Centroamericana “Jose Simeon Canas”, San Salvador, El SalvadorhSchool of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK

iFacultad de Ingenierıa, Universidad Nacional de Colombia, Santafe de Bogota, Colombia.jFundacion PRISMA, San Salvador, El Salvador

Abstract

The small Central American republic of El Salvador has experienced, on average, one destructive earthquake per decade during the last

hundred years. The latest events occurred on 13 January and 13 February 2001, with magnitudes Mw 7.7 and 6.6, respectively. The two

events, which were of different tectonic origin, follow the patterns of the seismicity of the region although neither event has a known

precedent in the earthquake catalogue in terms of size and location. The earthquakes caused damage to thousands of traditionally built houses

and triggered hundreds of landslides, which were the main causes of fatalities. The earthquakes have clearly demonstrated trends of

increasing seismic risk in El Salvador due to rapid population expansion in areas of high shaking and landslide hazard, exacerbated by

deforestation and uncontrolled urbanisation. The institutional mechanisms required for the control of land use and building practice are very

weak and present a major obstacle to risk mitigation. q 2002 Elsevier Science Ltd. All rights reserved.

Keywords: El Salvador; Earthquakes; Strong motion; Landslides; Seismic risk; Vulnerability

1. Introduction

The earthquake of 13 January 2001 that struck El

Salvador was the first major seismic disaster of the third

millennium and the fifth destructive earthquake to affect the

small Central American republic in 50 years. The earth-

quake was followed exactly 1 month later by a second event,

of different tectonic origin, on 13 February, which

compounded the destruction. These two earthquakes

claimed almost 1200 lives. In addition, 20% of houses

were damaged, with 12% either completely destroyed or

declared uninhabitable. Economic losses were estimated by

the UN Economic Commission for Latin America (ECLA/-

CEPAL) at US$ 1.6 billion, which is equivalent to 12% of

the GDP of the previous year [1]. Estimates by the IMF and

the World Bank give a higher figure of US$ 1.9 billion [2].

This paper presents the context in which these earth-

quakes occurred, including their precedent in the seismic

history of El Salvador, and describes the characteristics of

the events and their impact on the built and natural

environments, and on the population. The primary objec-

tives of the paper are firstly to provide an overview of the

characteristics and effects of the earthquakes, and secondly

to assess the relative importance of the different factors,

physical and social, which have been demonstrated as

contributing to the high level of seismic risk in El Salvador.

2. Geophysical, geological and geographical contexts

With an area of just over 20,000 km2 El Salvador is the

smallest of the Central America republics, located on the

0267-7261/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.

PII: S0 26 7 -7 26 1 (0 2) 00 0 24 -6

Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

www.elsevier.com/locate/soildyn

* Corresponding author. Tel.: þ44-171-594-5984; fax: þ44-171-225-

2716.

E-mail address: [email protected] (J.J. Bommer).

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5

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Pacific coast of the isthmus and bordered by Guatemala to

the west, and Honduras to the north and east (Fig. 1).

2.1. Tectonics, seismicity and seismic hazard

El Salvador is affected by earthquakes from two main

sources of seismicity. The largest earthquakes are generated

in the Benioff–Wadati zones of the subducted Cocos plate,

which is converging with the Caribbean plate in the Middle

America Trench (Fig. 2) at an estimated rate of 7 cm/year

[3]. The largest earthquake in this zone during the 20th

century, in the vicinity of El Salvador, occurred on 7

September 1915, with a reported magnitude of Ms 7.8 and a

focal depth between 45 and 60 km [4]. This earthquake

caused widespread destruction in western El Salvador,

affecting particularly the town of Juayua [5]. Large

subduction earthquakes on 28 March 1921 (Ms 7.4) and

21 May 1932 (Ms 7.1) caused minor and moderate damage

in eastern and central El Salvador, respectively; the

relatively small impact of these earthquakes was probably

the result of their focal depths of 170 and 150 km,

respectively [4]. An earthquake on 19 June 1982, offshore

from western El Salvador, did cause widespread damage in

the southwest of the country, mainly in adobe (sun-dried

clay brick) and bahareque (wattle-and-daub) houses, and

Fig. 1. El Salvador.

Fig. 2. Focal mechanisms determined for the earthquakes of 13 January and

13 February 2001. Stars are epicentral locations from NEIC. Red line is the

Middle America Trench and yellow triangles are volcanoes. Locations of

previous earthquakes are indicated by black stars.


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UNCORRECTED PROOF

triggered many landslides [6,7]. This earthquake shared

many similarities with the earthquake of 13 January in terms

of mechanism and focal depth, although somewhat smaller

with a magnitude of Mw 7.3. The damage patterns were

similar to those of the January 2001 earthquake, but much

less severe with a total of just eight fatalities. The worst

destruction occurred in the town of Comasagua, which was

also very severely affected by the January 2001 earthquake.

The second source of seismicity affecting El Salvador is a

zone of upper-crustal earthquakes that coincide with the

Quaternary volcanoes that extend across the country from

west to east, forming part of a chain extending throughout

the isthmus from Guatemala to Panama. Due to their

shallow foci and their coincidence with main population

centres, these earthquakes (Fig. 2) have been responsible for

far more destruction in El Salvador, as in neighbouring

Nicaragua, than larger earthquakes in the subduction zone

[8]. During the 20th century, such shallow focus earth-

quakes caused destruction on at least seven occasions,

sometimes occurring in clusters of two or three similar

events separated by periods of minutes or hours. On 8 June

1917 an earthquake occurred west of the capital, San

Salvador, assigned a magnitude Ms 6.7 by Ambraseys and

Adams [4] and Ms 6.5 by White and Harlow [8], causing

destruction in Armenia, Ateos, Quetzaltepeque and other

towns. The earthquake was followed by an eruption of the

San Salvador volcano, which resulted in lava flows to the

north. White and Harlow [8] report a second event of Ms 6.4,

on the eastern side of San Salvador, less than an hour later,

but this is contested by Ambraseys and Adams [4]. On 28

April 1919 San Salvador was again damaged, this time by a

shallow earthquake of Ms 5.9. On 20 December 1936, an

earthquake of Ms 6.1 caused very heavy damage to the town

of San Vicente, 40 km east of San Salvador, with more than

100 deaths [9]. The 1936 earthquake is of particular interest

since the location was similar to that of the earthquake of 13

February 2001.

On 25 December 1937 an earthquake of Ms 5.8 near the

Salvadorian border with Guatemala caused damage and a

few deaths in the towns of Ahuachapan and Atiquizaya. A

series of three destructive earthquakes occurred in eastern El

Salvador on 6–7 May 1951, with magnitudes Ms 5.9, 6.0

and 5.5, destroying the towns of Jucuapa and Chinameca,

leaving about 400 dead [10].

The capital city of San Salvador, amongst the Latin

American city most frequently damaged by earthquakes,

was badly hit on 3 May 1965 (Ms 5.9) and on 10 October

1986 (Ms 5.4, Mw 5.7). The 1965 earthquake left about 120

dead [11] whereas the 1986, despite being of smaller

magnitude, resulted in 1500 deaths and more than 100,000

homeless [12–14]. Many engineered structures that col-

lapsed in 1986 had been damaged by the 1965 earthquake

and possibly further weakened by the 1982 subduction

event.

The shallow focus, moderate magnitude earthquakes that

occur along the volcanic chain are generally tectonic rather

than volcanic in origin, and are probably the result of a

right-lateral shear zone caused by an oblique component of

the Cocos–Caribbean collision [15]. However, swarms,

which may have volcanic origin, are also relatively

frequent. In March and April 1999 an important seismic

swarm occurred in an area close to the San Vicente

(Chichontepec) volcano, with almost 1000 small earth-

quakes registered, none exceeding M 4.5, registered, and as

many as 160 occurring per day. A similar swarm had

affected approximately the same area in July 1975. The

1999 swarm, despite the size of the individual events,

caused minor to moderate damage to a number of adobe

houses and also the church in Apastepeque. The same area

was also affected by the earthquakes of January and

February 2001; it is very likely that the level of damage

was exacerbated by the damage inflicted during the 1999

swarm.

Major earthquakes also occur on the Motagua and

Chixoy-Polochic faults that traverse Guatemala and mark

the boundary between the Caribbean and North American

plates, but they are sufficiently distant to not produce

damaging motions in El Salvador. The Ms 7.5 Guatemala

earthquake of 4 February 1976 caused shaking that did not

exceed MM intensity of V within El Salvador [16].

White [15] also describes a fourth source of seismicity as

a zone of tensional tectonics near the common borders of El

Salvador, Guatemala and Honduras, bounded by the

Motagua fault to the north, the volcanic chain to the south

and the Honduran Depression to the east. White [15] asserts

that an earthquake of Ms 7.5 occurred in this zone in June

1765. The largest earthquake during the 20th century in this

zone was that of 29 December 1915 (Ms 6.4), for which

Ambraseys and Adams [4] relate press reports alleging two

deaths in San Salvador due the collapse of walls, although

the effects in El Salvador were clearly not overly important.

There have been a number of probabilistic seismic

hazard assessments carried out for El Salvador [17–19] and

for Central America [20,21]. The hazard maps produced for

a 475-year return period, despite being based on generally

similar seismological and strong-motion data, differ signifi-

cantly in terms of the geographical distribution of the hazard

and by more than a factor of three in terms of the maximum

ground accelerations [22]. Following the San Salvador

earthquake of May 1965, Rosenblueth and Prince [23]

proposed two separate seismic zonations for El Salvador,

one for suduction earthquakes and one for upper-crustal

seismicity. Although the application of this proposal has

been explored [24,25], it has not been incorporated into

seismic design codes.

2.2. Geology, geomorphology and landslide hazard

El Salvador is made up of four morphological-geological

units, each of which forms an east–west strip across the

country parallel to the coast [26]. The northernmost unit,

along the border with Honduras, is a mountain range


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UNCORRECTED PROOF

consisting mainly of plutonic rocks from the Tertiary. To the

south of these mountains is the Great Interior Valley that

forms the central area of the country; the southern part of the

valley includes the Salvadorian segment of the chain of

Quaternary volcanoes, six of which are active. To the south

of the valley are three coastal mountain ranges: Tacuba on

the western border with Guatemala; the Cordillera del

Balsamo to the south and west of the capital; and the

Jucuaran range bordering the Gulf of Fonseca to the east.

Between the coastal ranges are two coastal plains, the larger

one, in the centre and east of the country, including the

estuary of the Rıo Lempa, El Salvador’s main river.

The geology of El Salvador is entirely volcanic. The

youngest and most commonly encountered volcanic soil is

the tierra blanca ash, which originates from eruptions in the

Coatepeque and Ilopango calderas. The tierra blanca occurs

as silty sand or sandy silt, and is generally relatively well-

consolidated and stable only where it is thick [27]. The

strength of the tierra blanca, which permits it to stand in

near-vertical slopes of up to 15 m or more, derives from a

combination of weak cementation, probably due to silica

gels, and negative pore water pressure, resulting from partial

saturation, which have been measured in the range 400–

500 kPa [28].

Earthquake-induced landslides are common in Central

America although different mechanisms of slope failure

dominate in the northern countries of Guatemala and El

Salvador compared to southern countries of Costa Rica and

Panama [29]. In the south, the most abundant and most

damaging slope failures are translational soils in residual

soils, whereas in the north soil and rock slides on volcanic

slopes, and more commonly, soil falls and slides in steep

slopes of pumitic ash, dominate. Although these volcanic

ash deposits are able to form almost vertical slopes in

incised ravines (barrancas ) and in road cuts, they are

susceptible to sudden and catastrophic failure under

sustained or intense rainfall and under earthquake shaking.

The numbers of landslides triggered by earthquakes in these

ash deposits tend to be disproportionately high compared to

the numbers of landslides triggered by earthquakes of

similar magnitude in other parts of the world. In the global

database of Keefer [30], the 1976 Guatemalan earthquake

stands out as having caused an order of magnitude more

landslides than any other earthquake of the same size; the

1986 San Salvador stands out in the same way amongst the

cases in the database of Rodrıguez et al. [31], which extends

the Keefer [30] database from 1980 to 1997.

The record of landslides induced by earthquakes in El

Salvador dates back to 1576, when landslides in the Sierra

Los Texacuangos were reported to be triggered by an

earthquake [32]. Since then more than 20 earthquakes have

been found to cause widespread landsliding within the

Salvadorian territory [33]. Areas affected by earthquake-

induced landslides in El Salvador are much higher than

those affected by earthquakes of comparable magnitude that

occur in other geological, geomorphological and climatic

environments [29,31]. Historical evidences show that land-

slides triggered by earthquakes in El Salvador occur as soil

and rock slides on volcanic slopes but more abundantly as

soil falls and slides in slopes of pumitic volcanic ash [29,

33]. Subduction earthquakes generally trigger landslides

over areas that are large compared to crustal earthquakes,

which tend to concentrate landsliding around the epicentral

area. The 13 January and 13 February earthquakes have

confirmed these trends.

Rymer and White [34] reviewed topography, lithology,

rainfall, seismic hazard and historical cases of earthquake-

induced landslides, and concluded that landslide hazard in

El Salvador is high, the susceptible areas being the coastal

mountain ranges, the volcanic chain and the interior valley

areas. This evaluation has been confirmed by observations

during the 2001 earthquakes.

Fig. 3 shows a landslide hazard map prepared by the

Planning Office for the Metropolitan Area of San Salvador

(OPAMSS) as part of PLAMADUR in 1997. The map

identifies most of the metropolitan area of the capital as

being of medium landslide hazard with several areas

highlighted, in dark red, as being of high hazard: amongst

these is the area affected by the catastrophic landslide at Las

Colinas (Section 5.1).

2.3. Demographic and socio-economic conditions

The current population of El Salvador is about 6.3

million, very unevenly distributed throughout the national

territory. There has been a steady trend for the population to

concentrate in the south-western third of the country, which

was home to 53% of the population in 1971, a figure that had

risen to 64% by 1992 [35]. Probably three-quarters of the

population now live in the region west of Lake Ilopango and

south of Santa Ana, which is also the area of greatest seismic

hazard [36].

The main agricultural export of El Salvador is coffee,

having replaced anil (indigo) as the main cash crop at the

turn of the 20th century, following the introduction of

synthetic dyes in Europe. A large section of the rural

population depends directly or indirectly on the cultivation

of coffee for its livelihood, often in precarious conditions.

Even before the earthquakes of 2001, the coffee industry

was in a difficult situation as a result of low prices on the

international market, partly as a result of a bumper crop in

1999–2000, and a delay in the previous year’s harvest due

to particularly wet weather that affected Mexico, Guate-

mala, El Salvador and Honduras [37].

During recent years the relative importance of the coffee

industry in El Salvador has declined, with its contribution to

the GDP dropping from close to 10% in the early 1980s to

around 3% in recent years [38]. The main source of income

to the Salvadorian economy is now the dollars sent back to

relatives by Salvadorians living, often illegally, in the USA.

The migration of Salvadorians to the United States was

accelerated by the civil war that engulfed the country from



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1980 to 1992, but poverty and increasing crime rates have

maintained the exodus since the peace accords were signed

between the Salvadorian government and the FMLN

(Farabundo Martı National Liberation Front). Income

from remittances (known in Spanish as remesas ) reached

US$ 1751 million in 2000, almost six times the total value

of coffee exports (US$ 298 million) and 2.7 times the net

foreign exchange generated by the assembly (maquila )

industry (US$ 654 million).

El Salvador is classified as a lower middle-income

Fig. 3. OPAMSS map of landslide hazard in the Metropolitan Area of San Salvador.



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economy with an average GDP per capita of a little over

$2000, although the distribution of wealth is enormously

uneven. The economic fragility of the small republics of

Central America when subjected to natural disasters is well

established. Coburn and Spence [39] list the economic

losses inflicted by major earthquakes from 1972 to 1990; the

three highest losses, when expressed as a percentage of the

GNP for the year of the earthquake, of 40%, 18 and 31%

were caused by the 1972 Managua, 1976 Guatemala and

1986 San Salvador earthquakes, respectively.

3. Source characteristics of the 2001 earthquakes

The earthquake sequence that began on the 13 January

2001 lasted for a total of over 6 weeks. Within this sequence

were distinct and tectonically separate main shocks, on 13

January and 13 February, whose characteristics are

described in the next sections. A third event that occurred

on 17 February, with an epicentre on the western side of San

Salvador, was sufficiently remote from the two main shocks

not to be considered as an aftershock of either. However,

this was a small event, assigned a magnitude of ML 5.1 by

the Centre for Geotechnical Investigations (CIG) of the

Ministry of Public Works and mb 4.1 by NEIC, and caused

only very minor damage and caused less intense ground

shaking than many of the aftershocks of the 13 January

event; its impact was primarily psychological, owing to the

understandably agitated state of the population after 5 weeks

of tremors.

3.1. The 13 January 2001 earthquake

The first earthquake struck just after 11:30 am (local

time) on Saturday 13 January 2001. Table 1 gives the source

parameters determined by different agencies, which in terms

of size and depth of the earthquake are remarkably

consistent.

We determined fault mechanism, depth, source time

function and seismic moment of earthquakes of 13 January

and 13 February using very broadband digital data. In order

to avoid multi-pathing, upper mantle and core arrivals, we

only inverted body-waveforms from stations in the range

308 , D , 908. We modelled the earthquakes as single

point double-couple sources. The velocity structure near the

source and beneath the stations was approximated by a half

space with standard upper mantle wave velocities. We

modelled the direct waves (P and S) and the reflected phases

from free surface (pP, sP, sS, pS). In order to simulate

seismic attenuation; we assumed tp ¼ 11 s for P waves and

4 s for SH waves. We used a maximum likelihood principle

to obtain the source parameters that provide the best fit

between observed and synthetic waveforms [40,41]. During

the inversion, we solved simultaneously for focal mechan-

ism and source time function using the CMT solutions as a

priori models. We selected a set of teleseismic stations that

gave us the best azimuthal coverage as possible in order to

have a good constraint of the fault plane parameters. We

used displacement seismograms, deconvolving them from

their instrumental response and then reconvolving each

signal to a common instrumental response. Band-passed

filters were applied to the displacement records with a band-

pass Butterworth filter of order 3.

Fig. 4 shows body-waveform inversion results for the

earthquake of 13 January. For this event, P waves were

relatively well fitted by our simple point source model. For

P- and S-waves the first arrivals were not very well

modelled because the S-waves—which are much stron-

ger—dominate the inversion. The strong SH arrivals force

the source time function of the earthquake to start with a

very strong impulse. The peak appears to be much weaker

for P-waves. The depth we found was 50 km. The more

vertical fault plane was well constrained by the azimuthal

coverage. The source time function can be divided into two

sub-events: the first one had higher amplitude and a duration

of 22 s; the second sub-event occurred during 24 seconds.

The seismic moment was 5.54 £ 1020 N m; it is difficult to

observe any directivity effect. However, if we compare

signals from LBNH and KIP stations with their synthetics

(which were modelled with a point source), we observe a

possible directivity effect. If we assume that the fault plane

is the sub-vertical one, as is commonly observed for

intraplate events, there could be an upward rupture

propagation (Fig. 4); more data is required in order to

constrain this directivity. The Mw 7.7 earthquake of 13

January 2001 was an intermediate depth earthquake that

occurred inside the down-going Cocos plate; its tension axis

was sub-parallel to the dip direction of the descending slab.

Large magnitude, normal faulting earthquakes are not

unknown in subduction zones, indeed the El Salvador

earthquake of 19 June 1982 was of very similar rupture

mechanism. The highly destructive Peru earthquake (Ms

7.7) of 31 May 1970 was also associated with a normal

rupture, as was the large M 8.1 Chillan (Chile) earthquake in

1939. In the case of the Peruvian earthquake, the large-scale

extensional fracture in the underthrusting Nazca plate was

interpreted as being due to tensional stresses caused by the

denser descending plate [42]. In the case of the Cocos plate

in Central America, the cause of normal faulting may be

both extensional stresses due to slab pull and flexural

stresses induced as the slab begins to descend at a greater

dip angle inside the mantle [43].

Table 1

Source parameters for 13 January 2001 earthquake

Time

(UTC)

Epicentre Depth

(km)

Magnitudes Agency

N8 W8

17:33:32 13.049 88.660 60 Mw 7.7, Ms 7.8, mb 6.4 NEIC

17:33:46 12.97 89.13 56 Mw 7.7, Ms 7.8, mb 6.4 HRV

17:33:30 12.868 88.767 60 Mw 7.7 CASC



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In view of the agreement that the focal depth was of

the order of 50–60 km, the earthquake would not have

been expected to generate tsunami, even though there

have been reports of a minor seismic sea wave [44].

Fig. 5 shows a tide gauge record from the port of

Acajutla in which it can be seen that no tsunami

occurred; the fluctuation in sea level at the time of the

earthquake was comparable with ambient noise levels,

and possibly due to the arrival of P-waves at the

surface.

The earthquake was felt from Mexico City in the north to

Colombia in the south. Our field observations from extensive

travel throughout the interior valley and the coastal areas of El

Salvador suggest that MM intensities throughout the southern

half of the country were between VI and VII with local pockets

of higher intensity between VII and VIII.

Fig. 4. Analysis of broadband body waves for the 13 January 2001 earthquake. Top: observed P-wave seismograms (solid lines) are in general fitted well by

synthetics (dashed lines) computed for a point source model with focal mechanism and source time function shown. Bottom: as above but for SH waves.



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UNCORRECTED PROOF

3.2. The 13 February 2001 earthquake

Aftershocks from the 13 January earthquake decayed

approximately according to Omori’s law in the period up to

13 February and were gradually dying out when a second

earthquake occurred. The parameters for Omori’s equation

for the aftershocks prior to the second major event were

obtained as follows:

log½NðtÞ� ¼ 2:7 2 0:7 logðtÞ R2 ¼ 0:9 ð1Þ

The source parameters for the second earthquake are listed

in Table 2. Wave-form modelling was also carried out for

this second event. Fig. 6 shows displacement seismograms

filtered between the same corner frequencies as the 13

January event. Signals were noisier but we managed to

constrain the mechanism using the envelope of the signal.

The depth was 14 km and the seismic moment was

6.05 £ 1018 N m. The total source time function duration

was 12 s. P and SH waves were very well fit even if at some

stations P waves were very noisy (PAS, KDAK). In spite of

these problems, the two fault planes were well determined.

The event of 13 February 2001 was totally different from

that of 13 January: it was a strike–slip event that took place

inside the upper continental plate, in the zone of weakness

of the volcanic axis. The fault plane must be the one sub-

parallel to the volcanic axis, i.e. sub-parallel to the

subduction trench, which is confirmed by the distribution

of aftershocks located by CIG.

The 13 February earthquake, despite its size and

relatively shallow focus, did not produce surface rupture,

although there are mapped faults to the east of Chichonte-

pec volcano whose rupture would be compatible with the

fault plane solution [45]. An important issue in the

interpretation of these earthquakes is the focal depth of

the 13 February earthquake, which appears to be of the

order of 15 km from our well-determined solution. Focal

depth is the most difficult seismic source parameter to

determine reliably and seismograph coverage in Central

America, although improved by recent regional collabor-

ations [46], is still limited, hence reported focal depths

Fig. 5. Tide gauge record from the port of La Libertad showing height above mean sea level (metres) from 12 to 15 January 2001.

Table 2

Source parameters for 13 February 2001 earthquake

Time

(UTC)

Epicentre Depth

(km)

Magnitudes Agency

N8 W8

14:22:06 13.671 88.938 10 Mw 6.5, Ms 6.5, mb 5.5 NEIC

14:22:16 13.98 88.97 15 Mw 6.6, Ms 6.5, mb 5.5 HRV

14:22:07 13.927 88.743 9.5 MC 5.9, ML 5.7 CASC



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UNCORRECTED PROOF

carry a considerable degree of uncertainty. A clear

example of this is the earthquake sequence of Jucuapa–

Chinameca on 6–7 May 1951; contemporary catalogues

list the earthquakes with focal depths between 80 and

100 km, and re-determinations using teleseismic data by

Ambraseys and Adams [4] confirms the intermediate focus

of the events. However, wave-form modelling, the

presence of well-developed surface waves on a seismo-

gram from Guatemala City, and the distribution on damage

and intensity, all point compellingly towards very shallow

focal depths, probably less than 10 km [10]. On the basis of

the very limited evidence available, there does appear to be

some correlation between magnitude and focal depth for

crustal earthquakes in the Central America region, with

events of this size occurring below the upper crust.

Ambrasyes and Adams [4] report that the 20 December

1936 earthquake in the region of San Vicente, one of the

towns most heavily affected by the 13 February event, was

of sub-crustal origin. The empirical relationship of Wells

and Coppersmith [47] for strike–slip faults yields a mean

value of 10.5 km for the rupture width of an earthquake of

this size; if the rupture did not advance more than 5–8 km

from the surface, this may at least partly explain why the

13 February earthquake was less destructive than may have

Fig. 6. Analysis of body waves for the 13 February 2001 earthquake. As for Fig. 4 except that only P-waves are considered.



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UNCORRECTED PROOF

been expected from an event of this size occurring so close

to population centres.

A preliminary isoseismal for the 13 February 2001

earthquake published by CIG reported a maximum MM

intensity of VII–VIII in the area from Lake Ilopango to San

Vicente and VI in San Salvador. Our field observations

suggest that these are overestimates and that the maximum

intensity generally did not exceed VII.

An obvious question to be addressed is whether the 13

February earthquake was in some way triggered by the

subduction event a month earlier. Stress transfer due to

relaxation of one crustal area leading to heightened tectonic

stresses in an adjacent area has been clearly observed, for

example, in the sequence of earthquakes from 1939 to 1999

along the North Anatolian fault in Turkey [48]. However,

the situation in Central America is much less clear because

the two earthquakes are of entirely different tectonic origin,

even though they are both ultimately the result of the same

general tectonic process. Lomnitz and Rodrıguez Elizar-

raras [44] report that normal faulting subduction earth-

quakes in Mexico tend to be followed by either large thrust

events or shallow intraplate events four or five years later. A

similar pattern may possibly exist in El Salvador, whereby

large magnitude subduction earthquakes in some way

trigger crustal events within the Caribbean plate within

similar, or in some cases much smaller, intervals. The large

subduction earthquake of 1915 in western El Salvador was

followed by crustal earthquakes in San Salvador and to the

west in 1917 and 1919; the subduction earthquake of 1932

offshore of central El Salvador was followed by the crustal

earthquake in San Vicente in 1936; the 1982 subduction

earthquake was followed in 1986 by the San Salvador

upper-crustal earthquake. Therefore, the events of January

and February 2001 may be simply a highly accelerated case

of a process that is characteristic of the region. Earlier

studies have alluded to relationships between Quaternary

faulting in the Caribbean plate and the nature of the

subducted Cocos plate [49] but the highly complex system

of stress transfer and the exact nature of the plate

interactions are not sufficiently well known to infer any

definitive model at this stage.

4. Strong ground-motion

Both the 13 January and 13 February 2001 earthquakes

were well recorded by three accelerograph networks in

operation in El Salvador: a network of SMA-1 analogue

instruments operated by the CIG, a network of digital and

analogue instruments operated at geothermal and hydro-

electric plants by GESAL, and the TALULIN network of

digital SSA-2 instruments operated by the Universidad

Centroamericana (UCA) ‘Jose Simeon Canas’ [50]. Records

were also obtained from the network of INETER in

Nicaragua. The records from the CIG network were

digitised and processed by the USGS.

4.1. Characteristics of accelerograms

Tables 3 and 4 list the main characteristics of the

accelerograph recordings of the 13 January and 13 February

earthquakes; the station locations are shown in Fig. 7. A

major difficulty in performing detailed analysis and

interpretation of the recorded accelerograms is the lack of

information about soil profiles at recording sites other than

the CIG stations in San Salvador for which investigations

were carried out as part of a microzonation study following

the 1986 earthquake [51]. Nearly all of the stations that

recorded the two earthquakes are located on pyroclastic

deposits such as tierra blanca and the older tobas color cafe.

Exceptions to this are the Presa 15 de Septiembre hydro-

electric dam site (alluvium), La Libertad (alluvium) and

Panchimaclo (volcanic rocks). It is very likely that the

ground motions at several of the recording sites are also

affected by topographical features: the Panchimalco station

of the UCA network is located within a N–S trending

valley, whereas the San Pedro Nonualco station sits atop an

E–W trending narrow ridge. Recordings of distant subduc-

tion events off the coast of Nicaragua have consistently

produced relatively strong recordings at the latter site,

whereas at Panchimalco recordings have generally been

weak, frequently below the instrument trigger level [50].

Regrettably several potential records were lost due to

malfunction of instruments. The CIG station at Santiago de

Marıa in eastern El Salvador did not trigger during the 13

January event; the instruments in the north-western towns of

Santa Ana and Metapan also failed to produce records,

although it is not clear whether this was due to malfunction

or due to accelerations not reaching the triggering level,

which may have been the case at Metapan at least. It is clear

from comparison of Tables 3 and 4 that the functioning of

the CIG instruments was not consistent. More important

cases of malfunction concerned the San Vicente instrument

of the UCA network, which did not record either of the

earthquakes, and the San Pedro Nonualco station that would

have produced the most important recording of the 13

February earthquake. The station operated by GESAL at the

Berlın geothermal energy plant also failed to yield an

accelerogram of the 13 February earthquake.

4.2. Comparisons of strong-motion parameters with

predictions

For earthquakes of magnitude greater than about 6, for

which the source dimensions are of the order of more than a

few kilometres, the use of epicentral distance can seriously

overestimate the separation of the site and the source of

energy release. For the 13 January earthquake, distances

have been measured from the assumed fault rupture, since

this is the distance measure proposed by Youngs et al. [52]

for subduction zone earthquakes. The actual location of the

fault rupture has been fixed by the angle of dip of the fault,

which coincides with the angle of dip of Cocos plate as



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UNCORRECTED PROOF

proposed by Burbach et al. [43]. Taking account of the focal

depth of the main shock, the seismic moment and the spatial

distribution of aftershocks, the fault plane is modelled as a

fault plane with a strike of 3008 dipping 558 to the NE,

which corresponds to a plane sub-parallel to the subduction

trench. The dimensions of the rupture plane were con-

strained by the distribution of aftershock hypocentres from

13 January until the end of August, concentrated at focal

depths between 20 and 40 km. The dimensions of the

inferred fault rupture plane are 65 km in length and 55 km in

width. The uppermost part of the rupture is assumed to

extend to a depth of 20 km and extends from (12.958N,

89.258W) in the west to (12.678N, 88.748W) in the east. The

distances from this assumed rupture are given in Table 3.

For the crustal earthquake of 13 February, a more

appropriate measure of the source-to-site distance is that

proposed originally by Joyner and Boore [53], namely the

shortest distance from the surface projection of the fault

rupture. The distances from the fault rupture for the 13

February earthquake were determined by modelling the

fault as a line striking N948E, extending from (13.668N,

89.08W) to (13.638N, 88.618W). This places the fault

rupture as extending eastwards from the western shore of

Lake Ilopango for about 42 km; this is longer than would be

expected from the relationships of Wells and Coppersmith

[47], which may indicate a narrow rupture and hence the

effective depth of the source that may explain the relatively

low amplitudes recorded. This rupture plane was con-

strained from aftershock distributions from 13 February

until the end of August with depths up to 15 km. Seismic

activity west of Ilopango has been reported after the 13

February earthquake but it is probably related to the 17

February event near San Salvador. The calculated distances

from this assumed source are presented in Table 4; since it is

possible that the length of the fault rupture has been

overestimated, there is the possibility that some of the

distances are underestimated. The uncertainty, however, lies

mainly in the eastward extension of the fault rupture, which

Table 3

Strong-motion records of 13 January 2001 earthquake

Network Station Location drupa (km) PGA (g) PGV (cm/s)

N8 W8 N–S E–W V N–S E–W V

GESAL Berlın Geoth. 13.50 88.53 54 0.459 0.370 0.235 21.3 24.0 12.3

UCA Armenia 13.744 89.501 93 0.601 0.454 0.223 28.8 29.4 19.6

UCA La Libertad 13.468 89.327 60 1.113 0.575 0.617 53.2 35.5 16.0

UCA Panchimalco 13.614 89.179 75 0.177 0.154 0.089 9.2 9.4 7.3

UCA San Bartolo 13.705 89.106 85 0.157 0.199 0.166 25.2 31.2 15.2

UCA S Pedro Nonualco 13.602 88.927 50 0.580 0.488 0.439 37.5 26.4 18.2

UCA San Salvador ESJb 13.707 89.201 85 0.301 0.278 0.154 25.4 17.4 11.9

UCA Santa Tecla 13.671 89.279 83 0.496 0.243 0.487 57.0 34.2 18.5

UCA Tonacatepeque 13.778 89.114 93 0.234 0.205 0.263 23.1 23.2 9.8

UCA Zacatecoluca 13.517 88.869 47 0.260 0.314 0.253 12.3 21.9 10.4

CIG Ahuachapan 13.925 89.805 123 0.146 0.214 0.124 14.9 16.6 10.8

CIG Acajutla 13.567 89.833 95 0.098 0.108 0.050 14.6 18.6 4.2

CIG Cutuco 13.333 87.817 125 0.078 0.079 0.063 13.8 8.6 4.0

CIG Presa 15 de Sept.c 13.616 88.550 66 0.152 0.187 0.122 23.5 16.0 10.2

CIG San Salvador DBd 13.733 89.150 84 0.225 0.250 0.160 23.2 19.2 11.3

CIG San Salvador REe 13.692 89.250 83 0.304 0.323 0.329 22.9 27.6 15.3

CIG San Miguel 13.475 88.183 107 0.136 0.120 0.089 12.8 12.1 6.0

CIG Sensuntepeque 13.867 88.663 81 0.082 0.061 0.058 8.5 9.1 6.2

INETER Boaco 12.473 85.658 336 0.004 0.003 0.002 0.5 0.5 0.4

INETER Chinandega 12.632 87.133 175 0.090 0.070 0.042 6.3 4.6 2.1

INETER DEC 12.124 86.267 276 0.045 0.044 0.028 3.1 3.3 1.7

INETER Estelı 13.092 86.355 263 0.014 0.011 0.009 2.3 2.5 0.9

INETER Granada 11.937 85.976 312 0.009 0.009 0.006 1.7 1.3 0.9

INETER Jinotega 13.086 85.995 302 0.006 0.005 0.004 0.7 0.9 0.5

INETER Juigalpa 12.107 85.372 371 0.003 0.003 0.002 0.6 0.6 0.5

INETER Leon 12.117 86.266 276 0.040 0.037 0.026 2.3 2.6 1.4

INETER Managua (ESSO)f 12.144 86.320 270 0.057 0.045 0.022 3.8 3.9 1.5

INETER Managua (INET)g 12.149 86.248 277 0.034 0.041 0.014 2.6 2.7 1.1

a Distance from fault rupture as defined by Youngs et al. [52].b Externado de San Jose.c Ground level instrument adjacent to dam.d Ciudadela Don Bosco.e Ministerio de Relaciones Exteriores; there are two accelerographs at the this site, the reported values are from the instrument at ground level, the other is at

the base of a well.f ESSO Refinery.g INETER.



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would only affect the distances to the stations at Berlın and

the 15 de Septiembre dam.

Algermissen et al. [18] derived an attenuation relation-

ship from 82 recordings of strong-motion obtained in the

vicinity of San Salvador, without distinguishing between

subduction and crustal earthquakes. Taylor Castillo et al.

[54] derived an equation from 89 records from Costa Rica,

El Salvador and Nicaragua, again combining crustal and

subduction earthquakes. Dahle et al. [55] subsequently

produced attenuation equations for response spectral

ordinates, using a database of 280 records, including 157

from Costa Rica and more than 60 from Mexico, and

making no distinction between different sources of seismi-

city. Some other studies have separated subduction zone and

crustal earthquakes: Alfaro et al. [17] derived two separate

equations for PGA, but used only 20 records for each.

Schmidt et al. [56] have derived equations for spectral

ordinates from a database of 200 accelerograms recorded in

Costa Rica, presenting coefficients for the entire dataset and

for subduction and crustal sub-sets. Climent et al. [57]

derived spectral acceleration equations for Central America

using 280 records from Costa Rica, Mexico, Nicaragua and

El Salvador; these relationships also did not separate crustal

and subduction events.

There are shortcomings in all of the above attenuation

relationships in terms of applicability to El Salvador, either

because they do not discriminate between subduction and

crustal earthquakes, or because they are based on insuffi-

cient datasets. The equations of Schmidt et al. [56] are the

only exceptions, but there are important tectonic and

geologic differences between Costa Rica and El Salvador,

on the one hand, and on the other they make use of

epicentral and hypocentral distance, which are unsuitable

for large events as was noted previously. For these reasons,

comparisons have been made with predictions from

relationships derived for other regions. For the subduction

earthquake of 13 January, the most appropriate attenuation

relationships are those of Youngs et al. [52] derived from

regressions on almost 500 accelerograms from Alaska,

Chile, Cascadia, Japan, Mexico, Peru and the Solomon

Islands. These equations have been proposed for intra-slab

and interface subduction earthquakes, for events larger than

Mw 5 and distances from the fault rupture between 10 and

500 km, making them ideally suited to this situation. The

recorded PGA values are compared with those predicted by

the intra-slab equation of Youngs et al. [52] in Fig. 8;

ground conditions corresponding to more than 20 m of soil

overlying rock have been assumed. The equation appears to

Table 4

Strong-motion records of 13 February 2001 earthquake

Network Station Location drupa (km) PGA (g) PGV (cm/s)

N8 W8 N–S E–W V N–S E–W V

GESAL Berlın (town) 13.50 88.53 17 0.032 0.070 0.031 4.1 6.0 2.9

UCA Armenia 13.744 89.501 55 0.029 0.037 0.026 4.0 2.3 1.3

UCA La Libertad 13.468 89.327 41 0.091 0.093 0.037 4.7 4.5 3.1

UCA Panchimalco 13.614 89.179 20 0.185 0.106 0.045 9.4 4.6 2.0

UCA San Bartolo 13.705 89.106 13 0.106 0.141 0.123 25.6 22.3 6.9

UCA San Salvador ESJb 13.707 89.201 22 0.124 0.099 0.052 18.3 6.6 2.7

UCA Santa Tecla 13.671 89.279 30 0.047 0.040 0.023 6.4 4.8 2.0

UCA Tonacatepeque 13.778 89.114 18 0.345 0.251 0.240 30.0 24.7 10.5

UCA Zacatecoluca 13.517 88.869 18 0.408 0.305 0.262 20.1 20.4 9.6

CIG Presa 15 de Sept.c 13.616 88.550 7 0.020 0.026 0.017 6.4 5.0 2.4

CIG S. Salvador CIGd 13.698 89.173 19 0.138 0.071 0.059 19.9 8.4 3.8

CIG San Salvador DBe 13.733 89.150 18 0.100 0.094 0.055 14.8 12.2 4.6

CIG S. Salvador DUAf 13.737 89.209 24 0.077 0.059 0.046 8.2 8.7 3.5

CIG S. Salvador OBSg 13.681 89.198 22 0.107 0.104 0.068 6.7 13.9 3.3

CIG San Salvador REh 13.692 89.250 27 0.058 0.063 0.034 3.9 8.1 2.2

CIG S. Salvador SEMi 13.705 89.225 25 0.065 0.071 0.044 5.7 10.8 2.6

CIG S. Salvador UCAj 13.677 89.236 26 - 0.058 0.040 - 8.5 2.1

CIG Santa Tecla 13.675 89.300 32 0.039 0.042 0.019 6.4 7.4 2.2

a Distance from fault rupture as defined by Joyner and Boore [53].b Externado de San Jose.c Ground level site adjacent to dam.d Centro de Investigaciones Geotecnicas.e Ciudadela Don Bosco.f Viveros, Direccion de Urbanismo y Arquitectura; there are two accelerographs at the this site, the reported values are from the instrument at ground level,

the other is at the base of a well.g Observatorio Sismologico.h Ministerio de Relaciones Exteriores, ground-level instrument.i San Jose de la Montana Seminary, ground-level instrument.j Universidad Centroamericana.



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UNCORRECTED PROOF

fit the data well at distances of less than 300 km, with values

from greater distances being overestimated. It is worth

noting that in the distance range from 50 to 130 km, the

values obtained from the CIG network are consistently

lower than those from the UCA network.

One particularly interesting feature of the motions

recorded during the 13 January earthquake is the fact that

the response spectra are rich in high frequencies whereas for

such a large magnitude event greater energy at intermediate

and long periods would have been expected. Similar

features have been observed in accelerograms from large

subduction zone earthquakes in Japan and also in Peru [58,

59]. Since high-frequency ground motions were recorded in

both the 1966 and 1970 Peruvian earthquakes, which were,

respectively, associated with thrust and normal ruptures

[42], it would appear that this feature may not be exclusively

a function of source mechanism. Nonetheless, Prvance and

Anderson [60] identify normal faulting earthquakes in the

Mexican subduction zone as producing consistently more

high-frequency radiation than thrusting events. There is

evidence that the recording from La Libertad (Fig. 9), where

PGA exceeds 1 g, displays strong site effects at a period of

about 0.2 s (Fig. 10). This is visible on many recordings

from this station from previous smaller or more distant

earthquakes and would be consistent with a relatively thin

layer (,10 m) of alluvium overlying bedrock (lavas).

For the crustal earthquake of 13 February, one possibility

would be to use the relationships derived by Spudich et al.

[61] for zones of extensional tectonics, but the two

recordings from the 1986 San Salvador earthquake used in

that study were found to be outliers whose amplitudes were

significantly underestimated by the median predictions.

Other candidate equations derived from crustal recordingsFig. 8. Recorded PGA values from the 13 January earthquake compared

with predictions from the attenuation relationships of Youngs et al. [52].

Fig. 7. Location of strong-motion recording stations in El Salvador.



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UNCORRECTED PROOF

elsewhere in the world would include the western USA

relationships of Boore et al. [62], the global relationships of

Abrahamson and Silva [63], the European relationships of

Ambraseys et al. [64] and the Italian relationships of Sabetta

and Pugliese [65]; the latter may be particularly suitable

since large areas of Italy are also volcanic. Fig. 11a

compares the recorded PGA values with the median values

from these relationships, in which the equations seem to

consistently overestimate the observed values beyond about

20 km. Fig. 11b confirms that most of the recorded PGA

values are within the ^s values predicted by Ambraseys

et al. [64]. Worthy of particular note in this figure are the

low PGA values obtained at Berlın in this event, which

raises questions about the reliability of this particular

recording: GESAL operates digital accelerographs in the

Fig. 9. Accelerogram recorded at La Libertad during 13 January 2001 earthquake.

Fig. 10. Absolute acceleration response spectrum (5% damping) of record

in Fig. 9.

Fig. 11. (a) Recorded PGA values from the 13 February earthquake

compared with median values predicted by the equations of Abrahamson

and Silva [63], Boore et al. [62], Ambraseys et al. [64] and Sabetta and

Pugliese [65]. (b) Recorded values of PGA compared with the 16, 50 and

84% predictions from Ambraseys et al. [64].



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UNCORRECTED PROOF

town of Berlin and at the nearby geothermal energy plant,

with records having been obtained only from the latter

instrument on 13 January and reportedly from the former on

13 February. The even lower amplitudes recorded at the 15

de Septiembre dam may point to the fault rupture being

shorter than 42 km and not extending so far east as has been

assumed.

4.3. Implications of recorded motions for seismic design

code

The first seismic design code in El Salvador was

introduced in 1966 following the San Salvador earthquake

of the previous year; this code was adapted for El Salvador

from the code from Acapulco, Mexico [23]. A revised code

was drafted by the Salvadorian Association of Engineers

and Architects (ASIA) in 1989, issued as an emergency

regulation following the 1986 San Salvador; the design

spectra in this code took account of the nature of the ground

motions recorded in the earthquake. The current seismic

code, published in 1994, forms part of a comprehensive set

of regulations for building and civil works produced by the

Ministry of Public Works. The current code has several

merits, including the fact that it is the first to have involved a

probabilistic assessment of seismic hazard in El Salvador

[19]. Furthermore, the regulations cover a wide range of

practices, including geotechnical works, and also provides

guidance on construction using adobe despite initial

opposition from contractors who were concerned that

promotion of vernacular building techniques would be

detrimental to their business.

The elastic spectra in the current code appear to be

sufficient for most of the ground motions recorded in these

earthquakes. The somewhat exceptional record of 13

January at La Libertad exceeds the code spectrum

(Fig. 12), but it would not seem reasonable to increase the

code spectrum to a maximum level of 5g just to

accommodate the narrow-band amplification due to specific

site effects at this location. The strongest recording from the

13 February earthquake, obtained at Zacatecoluca, is

covered by the spectral ordinates specified in the code, as

shown in Fig. 13a. Fig. 13b shows the spectrum at the same

station from the 13 January earthquake, which is also

adequately covered by the code spectrum. In passing it can

be noticed that as at other stations [66], the shape of the

spectra from the two earthquakes are generally similar,

confirming the importance of site effects in determining the

nature of the ground motion.

5. Effects of the earthquakes

The impact of the January and February 2001 earth-

quakes was strong in many parts of the south of the country,

particularly the coastal cordilleras and locations around the

volcanic centres. The area around the San Vicente volcano,

where buildings had been weakened by the 1999 swarms,

and where both the 13 January and 13 February earthquakes

caused strong shaking, was particularly affected. The

patterns of damage, however, were very uneven and the

capital city, San Salvador, was largely unaffected. None-

theless, the overall impact was devastating to the fabric of

the country, with an estimated 40% of the health service and

30% of schools severely damaged.

The death tolls due to the two earthquakes have been

reported as 844 and 315, respectively, with the majority of

the casualties, particularly in the 13 January event, being

due to landslides. It is worth highlighting here that the loss

Fig. 12. Envelope of horizontal acceleration spectra from the 13 January

recording at La Libertad compared with the elastic spectra from the 1994

design code for soil class S3 and importance categories I and III.

Fig. 13. (a) Envelope of horizontal acceleration spectra from the 13

February recording at Zacatecoluca compared with the elastic spectra from

the 1994 design code for soil class S3 and importance categories I and III.

(b) Envelope of horizontal acceleration spectra from the 13 January

recording at Zacatecoluca compared with the elastic spectra from the 1994

design code for soil class S3 and importance categories I and III.



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UNCORRECTED PROOF

of life in these earthquakes underestimates their impact;

more people were killed by the Mw 5.7 San Salvador

earthquake of October 1986. The death toll is also small

compared with the 75,000 lives lost in the fratricidal war

from 1980 to 1992, and indeed when compared with the

numbers of victims of violent crime, which has grown to

epidemic proportions in recent years. The true impact of the

earthquakes is more accurately represented by the fact,

mentioned previously, that a significant proportion of the

population was either made homeless or suffered substantial

damage to their housing. Even before the earthquakes it had

been estimated that El Salvador had a housing deficit of

more than half a million dwellings.

5.1. Landslides and liquefaction

The 13 January earthquake triggered more than 500

landslides across in El Salvador and a further 70 occurred as

a result of the 13 February earthquake. Landslides were an

extensive secondary effect of the earthquake of 13 January

2001. The landslides could be divided into three broad types

according to the classification scheme proposed by Dikau

et al. [67]. These were rock and debris falls, slides and

debris flows. Many of the landslides occurred on the slopes

of volcanoes that are used for coffee cultivation; coinciding

with the time of the coffee harvest, many coffee pickers

were killed by these slides.

Rockfalls and debris falls were common throughout the

area and ranged from single block falls (some of which were

up to 3 m in diameter, Fig. 14) to the collapse of slopes cut

in pyroclastic ashfall deposits, which exist as a result of

weak cementation and high negative pore pressure [28,68].

Such failures were largely independent of lithology, but

occurred only on steep slopes. Individual block falls were

more common in the rocks of the Balsamo Formation

because of the prevalence of persistent discontinuities in the

form of bedding and cooling joints. Highly altered layers of

volcanic rock also acted as aquitards.

The occurrence of rock and debris falls in the tierra

blanca affected an extensive area around the Comasagua

Road on the Balsamo Ridge. Here the steep slopes which

were formed as part of the road construction and also by

quarrying failed in spectacular manner giving rise to an

extensive area of slope instability which extended from the

landslide at Las Barrioleras to the Jayaque–Comasagua

Junction (Fig. 15) and in Comasagua itself (Fig. 16). This

formed a classic shattered ridge. In many cases, it was

impossible to tell where one landslide ended and another

began.

Large landslides were observed along roads to Comasa-

gua, Talnique, Jayaque, Tepecoyo and Sacacoyo. The

principal cases were reported along the road between

Nueva San Salvador and Comasagua on slopes of volcanic

ashes mainly tierra blanca. The Pan-American Highway

was blocked between Los Chorros and Colon by landslides

to the west of San Salvador. At the Las Leonas location, to

Fig. 14. Single block fall, triggered by an aftershock of the 13 January earthquake, on the coastal motorway near La Libertad.



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the east of San Salvador, this road was blocked by a large

slide of approximately 500,000 to 700,000 m3 of rock and

soil debris. Roads to San Agustın, Santiago de Marıa and

Berlın were also blocked. Several landslides were also

observed along the road between Cojutepeque and Santiago

Texacuangos, and around Lake Ilopango.

The most important group of landslides, which were

triggered by the earthquake, were the debris flows. These

landslides were responsible for more than half the deaths

during the 13 of January earthquake. The two most

important of these slides were at Las Colinas (Fig. 17)

and Las Barioleras. These landslides, which occurred in the

Tierra Blanca showed significant travel distances of 735 m

and c. 1140 m, respectively. The former of these two slides

destroyed part of the residential area at Las Colinas, while

the latter killed many people who were working on the

coffee plantations and travelled onto the Pan-American

Highway. The long travel distances of these landslides

indicate low coefficients of internal friction of between 6

and 98, inferred from the ratio of slope height to run-out

length of the slide [69]. This indicates a significant drop in

frictional strength from the undisturbed state, which may be

as high as 388. Debris flows were common throughout the

Cordillera del Balsamo resulting from the steep terrain

mantled with weak volcanic debris and the presence of

aquitards in the underlying Balsamo Formation.

The Las Colinas landslide in Santa Tecla was the most

notorious slide triggered by the earthquakes due to its

devastating impact on population. This slope failure buried

as many as 500 people. This slide was approximately 790 m

long, 150 m wide and left a scarp 50 m high. The total

volume of the slide was approximately 200,000 m3. The

slide affected a part of the northern flank of the Balsamo

Ridge composed of the Balsamo Formation. This formation

is formed mainly of andesitic cinders and some interbedded

tephra. Extensive cracking was observed on the ridge crest

in areas that did not slide, which was cause for additional

concern. Some authors attributed this slide to liquefaction of

saturated tierra blanca deposits [70–72], however, a

rotational slope failure of the upper part of the slope has

also been attributed as initial failure mechanism [73].

Failure has been found to be related to high water content of

the lower part of the slope, which has been attributed to

natural drainage blockage by a retaining wall observed on

the bottom of the slope [72], although a perched aquifer on

the slope due to the impermeable nature of the Balsamo

Formation has also been proposed as the cause of this high

water content. Soil saturation was observed only locally,

due mainly to the earthquake occurring after 5 months of

dry season. The destructiveness of the landslide may have

been due to its high mobility, which may have been the

result of an unfavourable combination of high water content

and material brittleness.

Harp and Wilson [74] have identified Arias intensity

(sum of the two horizontal components) as a useful indicator

of the capacity of the ground shaking to trigger landslides.

Fig. 15. Debris fall from the Tierra Blanca near Comasagua.



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UNCORRECTED PROOF

Fig. 17. The Las Colinas landslide, Santa Tecla.

Fig. 16. Landslide damage in Comasagua.



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From studies of the 1987 Superstition Hills and Whittier

Narrows earthquakes in California, Harp and Wilson [74]

proposed thresholds of about 0.25–0.3 m/s for landslide

triggering. The values of Arias intensity for the records of

the 13 January earthquake are as high as 14 m/s and in all

cases (excluding the Nicaraguan records), exceed this

threshold [75].

In terms of geographical distribution, landslides were

triggered across most of the southern half of El Salvador,

with a particular high concentration in the Cordillera del

Balsamo to the southwest of San Salvador, between Nueva

San Salvador and Armenia (Fig. 18), affecting a much larger

area than in previous earthquakes [29]. In a general way, the

geographical distribution of landslides roughly corresponds

to the distribution of young ash, tuff, and tephra deposits on

steep slopes, incised valley walls and river channels.

Landslides were also reported to occur in Guatemala [76].

Slides blocked roads between Quesada and Monte Verde

and between Moyuta and El Obraje in the Jutiapa District.

Landslides were also reported along the Guatemala-Mexico

and Quetzaltenango–Retalhuleu roads, and along the road

to Ixtahuacan, Solola.

The 13 February earthquake triggered additional land-

slides to those reported by the 13 January event. Along the

Pan-American Highway new landslides were observed at

Las Leonas and adjacent locations. A large landslide was

reported in the water head part of the Rio Jiboa; it was

estimated that volume of sediments yielded in this area

reaches between 10 and 15 £ 106 m3 of debris, mainly

tierra blanca [77]. This landslide blocked the river course

for 600–700 m causing an artificial lake to be formed.

Another large landslide blocked the course of Rio El

Desague; in this case a volume between 1 and 2 £ 106 m3

was mobilised, consisting of andesitic breccia blocks of

around 0.5–2 m in diameter embedded into a tierra blanca

matrix [77].

On the slopes of the San Vicente volcano landslides were

reported along the El Muerto and El Blanco creeks. The El

Muerto landslide was estimated to have mobilised around

700,000–800,000 m3 of andesitic rock blocks, whereas the

El Blanco landslide mobilised silty and sandy gravels and

blocks coming from pyroclastic flows. This slide becomes a

latent hazard against the Tepetitan town, which was flooded

in 1930 by a mudflow resulting in four deaths. New

landslides were also reported around the Lake Ilopango

[77].

Fig. 18. Distribution of areas affected by landslides (solid line), by concentrated landslides (small dashes) and by liquefaction (large dashes).



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UNCORRECTED PROOF

Analysis of SPOT image data after the earthquake of 13

of January 2001 with 10 m ground resolution, reveals many

flowslides in the Balsamo Cordillera. A similar scene

collected after the earthquake of 13 February shows that

many of these landslides have expanded in size either as a

result of aftershocks from the first earthquake, or from the

effects of the second, much closer, event.

The susceptibility of slopes to earthquake-induced

instability has been shown to be strongly dependent on the

rainfall in the months and weeks prior to the seismic event

[33]. Although ACPC [37] reports that the 2000/2001 coffee

harvest was delayed due to wet weather, average annual

precipitations reported by the Meteorlogy Department of the

Salvadorian Ministry of Agriculture (MAG) indicate that

rainfalls for the year 2000 were in fact slightly low in many

parts of the country, at least compared to the previous 2

years (Table 5), although it should be noted that 1998 was an

exceptional year because of Hurricane Mitch.

The hazard of rainfall-induced landslides in the rainy

season (normally starting in April or May) following the

earthquakes became a major concern. On 19 September

1982, after a rainfall of 223 mm in less than 2 days, a

landslide began to move on the slopes of San Salvador

Volcano (El Picacho) and then descended rapidly into the

densely populated neighbourhood of Montebello. The slide

had an estimated volume of 200,000 m3 and killed an

estimated 500 people, leaving another 2400 homeless

(CEPRODE, 1994). This slide happened exactly three

months after an Mw 7.3 subduction earthquake, which is

reported to have caused extensive cracking on slopes.

Extensive cracking along ridges, especially along the road

to Comasagua in the Cordillera del Balsamo, caused by the

13 January earthquake led to concerns that a similar

sequence of events might follow in the 2001 rainy season.

However, the hazard did not materialise during the first

months of the rainy season since rainfall levels were

exceptionally low, to the point of creating drought and

consequently severe problems with water supply and

agriculture. Nonetheless, heavy rainfalls have occurred

since the earthquakes and a large mud and debris flow was

triggered on the lower slopes of the San Vicente

(Chichontepec) volcano on 15 September 2001.

Liquefaction was observed at various locations along the

coast in central and eastern El Salvador, accompanied by

lateral spreading and consequent damage to some houses.

Similar observations were made on the shores of Lake

Ilopango, where lateral spreading was significant and some

houses were rendered uninhabitable due to foundation

damage. The most serious effects of lateral spreading

occurred on the banks of the Lempa River at San Nicolas

Lempa that resulted in collapse of a railway bridge (Figs. 19

and 20).

5.2. Damage to housing

The initial estimates by the Committee for National

Emergency (COEN) of the Salvadorian government of the

number of homes destroyed by the earthquake was about

150,000, with another 185,000 damaged. There has been

some debate regarding the damage statistics, with reports

that the COEN figures are overestimated [2], but all sources

agree that more than one million people were made

homeless by the earthquakes. The overwhelming majority

of the damaged houses were of adobe and bahareque, with

the former being the most susceptible type of housing.

Timber frames and reinforced masonry houses performed

significantly better and it was not uncommon to visit

locations where most adobe houses were in a state of at least

partial collapse whereas reinforced masonry houses were

practically unscathed.

In the rural area of El Salvador the dwelling construction

types mostly used are adobe, bahareque, reinforced brick

masonry (mixto ), wood frames cover by thin metal sheets

(lamina ), and wood frames cover by palm fronds (ranchos ).

Other building practices, which are less widely used,

include concrete and soil-cement block masonry using

soil-cement blocks, and steel frames cover by precast walls

(Fig. 21).

Roofs of adobe houses may be of metal sheets and/or

clay tiles supported by wood trusses or thatched roof

supported on wood timber purlins. Load transfer between

the roof and walls, or between walls, is often not effective.

This building system has high mass and stiffness but low

strength.

Bahareque consists of timber vertical elements and

horizontal timber, cane or bamboo elements, infilled with

mud and finished with plaster. The seismic resistance of

bahareque depends primarily on the condition of the timber

and cane elements, having low vulnerability when carefully

maintained. Bahareque is a more expensive building system

than adobe. Roofs are similar to those for adobe and show the

same problems. Mixto is composed of fired clay bricks with

mortar and slender elements of concrete with thin steel

reinforcement, or the same thickness as the wall, which are not

properly reinforced concrete and are known as nervios (nerves

or tendons). This system, in which the load bearing system is

Table 5

Annual average rainfalls (mm) at selected meteorological stations

Year Ilopango Santiago de Marıa La Union San Miguel Ahuachapan Acajutla Puente Cuscutlan

1998 1958 2338 2123 1648 1623 2280 2037

1999 1504 1902 1859 1470 1554 1953 1303

2000 1454 1890 1783 1543 1052 1761 1637



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Fig. 19. Lateral spreading on river bank at San Nicolas Lempa.

Fig. 20. Collapse of railway bridge due to lateral spreading on banks of the Lempa River.



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UNCORRECTED PROOF

provided by the masonry walls, has relatively good seismic

resistance but is considerably more expensive than both adobe

and bahareque. Lamina is the name given to buildings of

timber or metal frames covered by thin metal sheets, usually

founded on a 50 cm high block wall. Lamina has good seismic

resistance due to its low weight and mass. Wood frames

covered by palm fronds have excellent seismic response

characteristics but this building system is rapidly disappearing

due to scarcity of materials. The construction systems most

severely affected by both the January and February earth-

quakes were adobe and bahareque.

The damage patterns clearly revealed the social vulner-

ability of poor forced to live in susceptible locations and

vulnerable houses. Small towns such as San Agustın (Fig.

20), where 80% of the houses were made from adobe, were

particularly hard hit. The same pattern was visible in small

hamlets and villages, where adobe was even more dominant

and where the quality of construction was generally poor.

Even in San Salvador, where damage to engineered

structures was very limited, extensive damage was observed

in shanty dwellings such as in the Jose Cecilio del Valle area

adjacent to the exclusive Escalon neighbourhood, due to

both shaking and to movements on slopes.

5.3. Damage to engineered structures

The majority of engineered structures in El Salvador are

located in the metropolitan area of San Salvador, and mainly

in the cities of San Salvador and Nueva San Salvador (Santa

Tecla). When seismic design has been considered at all, the

approach has generally been focused on the ultimate limit

state. Most damage in engineered structures due to the 13

January earthquake was non-structural, with damage to

partition walls, ceilings, A/C ducts and windows. Many

major hospitals were put out of service because they lost

their functionality and non-structural damage needed to be

repaired before they could be used again. Several buildings

that suffered light damage in the 1986 earthquake and were

not adequately repaired suffered some damage due to the

2001 earthquakes. One structure that had been badly

damaged in 1986, a building housing a hair salon next to

the Externado de San Jose High School (itself destroyed in

the 1986 earthquake and subsequently rebuilt), withstood

the shock without collapse, although the owners have since

taken the decision to demolish.

There are only a few buildings in San Salvador that stand

more than 70 m height, most of which behaved very well

during the earthquakes. One of these, the Torre Cuscutlan

(formlerly known as Torre de Democracia), located in the

south west of San Salvador, is an irregular tower with

external glass walls; none of these were broken. One high-

rise structure did suffer some internal damage, the

Ministerio del Interior building in the centre of San

Salvador, which was also damaged by the 1986 earthquake.

Fig. 21. Collapsed adobe house in San Agustın, of which only the door remains standing after the 13 January earthquake; behind are houses of mixto and

lamina, which have survived the earthquake.



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UNCORRECTED PROOF

There are a few other examples of structures having suffered

severe damage, such the Regis Condominium in the San

Jacinto area of south San Salvador. Most structures that

suffered some damage are of reinforced concrete; steel

structures behaved well.

Outside the capital there are few engineered structures

compared to San Salvador. The hospitals in Usulutan and

San Miguel were left non-operational due to non-structure

damage. Public buildings erected in the 1960’s were

severely damaged in Santiago de Marıa. Another case of

damage outside of the capital city was El Salvador’s

International Airport located near Comalapa on the coastal

plain. The airport suffered important non-structural damage

including cracking of infill walls, breakage of windows and

collapse of ceilings, as well as some minor cracking in

columns and beams in the older sections of the airport

buildings.

The 13 February earthquake damaged some engineered

structures mainly in the central area of El Salvador. The city

that shows most of this kind of destruction is San Vicente,

where at least two schools, one of them built in the early

1970s, have suffered severed damage; one branch of the

Universidad de El Salvador suffered non-structural damage

that limited its function. In addition, this earthquake

damaged the Zacatecoluca hospital, which is a twin

structure of the Usulutan Hospital. However, in general

damage levels were low for the size and location of the

earthquake. In the towns of Guadalupe (Fig. 22), Verapaz

and Santa Cruz Analquito, which are located very close to

the assumed fault rupture, there was total collapse of many

houses built from adobe and bahareque, but mixto

(reinforced masonry) constructions generally survived

intact. Even structures that had been weakened by both

the 1999 seismic swarm and the 13 January earthquake,

such as the church in the town of San Esteban Catarina, did

not suffer as much damage as might have been expected. In

the town of Apastepeque, close to the source of the 13

February earthquake and badly affected by the 1999 swarm,

residents reported that the most severe effect of the

earthquake was to dislodge roof tiles. That a crustal

earthquake of Mw 6.6 did not cause greater levels of

damage in reinforced masonry nor in some cases in

weakened adobe buildings, suggests that the earthquake

was less superficial than is typical of volcanic chain

earthquakes such as those in Jucuapa–Chinameca in 1951

and in San Salvador in 1986.

In terms of the cultural heritage of El Salvador, there are

relatively few examples of colonial architecture surviving in

many parts of the country, indeed in San Salvador all

buildings from the colonial period have been destroyed by

fire or earthquake. The earthquakes caused damage to more

than 400 churches in El Salvador.

An important question that immediately presents itself is

why the damage to engineered structures, particularly

Fig. 22. Guadelupe following the 13 February earthquake, contrasting total collapse of adobe houses with the almost undamaged state of the reinforced

masonry building to the right.



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UNCORRECTED PROOF

during the 13 January earthquake, was so limited? Even at

the Health Centre in La Libertad, where the maximum 5%

damped spectral acceleration exceeded 4.5g, damage was

limited to the fall of part of the ceiling (non-structural) and

minor cracks in an external wall. Comparison of accel-

erograms obtained in San Salvador during the 1982

subduction-zone and 1986 upper-crustal earthquakes pro-

vides insight into possible reasons, since the latter event

caused significantly higher levels of damage in engineered

structures, despite the fact that the response spectral shapes

were not very dissimilar, hence the frequency content of the

motions is unlikely to provide the explanation. The 1982

and 1986 accelerograms were found, however, to contain

almost identical levels of energy, as measured by the Arias

intensity but with very different durations, so that the rate of

energy input was an order of magnitude greater in the 1986

earthquake [50]. The total energy input, which was actually

higher in the January 2001 earthquake than for the 1982 and

1986 records, is a good indicator of the damage potential in

brittle and degrading materials such as adobe and volcanic

soils. It would appear that for damage to be inflicted on

engineered structures it is necessary that the motion has both

a high energy content and a high rate of energy input, as

indicated by the root-mean-square acceleration.

5.4. Performance of lifelines

The performance of lifelines in the two earthquakes has

been reported in detail by Lund [79] and EERI [80].

Telecommunications were not seriously affected and service

was fully restored in the capital within one day of the first

event. Electricity generation was not seriously affected but

the distribution system was affected by a large number of

transmission lines broken by landslides. There are no gas

distribution lines in El Salvador since all household use

imported propane distributed in canisters. The diesel and

petrol refinery in the port of Acajutla was not damaged and

production was not interrupted by the earthquakes.

The distribution of potable and waste water in El

Salvador is managed by the state-owned company ANDA.

The earthquakes caused disruption to the water distribution

system but breakage of pipes was limited; for example, only

three repairs in the northern area supply line in San Salvador

were reported by ANDA. The most serious disruption to the

water distribution system was the damage caused by the 13

February earthquake to the treatment plant at Cacahuatal

that supplies the San Vicente area. Although the disruption

to the water distribution system by the earthquakes was

limited, it is worth noting that even under normal conditions

there are problems with water distribution in El Salvador,

with chronic shortages and few households, even in urban

areas, have uninterrupted water supply 24 h a day.

The most seriously affected lifelines were transport lines.

There are three railway lines in El Salvador, connecting the

ports of Acajutla and Cutuco (La Union) and the cement

production plants in Metapan in the northwest of the

country, used predominantly for transportation of cargo

rather than passengers. The eastern line connecting Cutuco

has not been operational for many years. The only damage

to the railway system was the collapse of the steel arch truss

bridge at San Nicolas Lempa due to lateral spreading.

The two main highways in El Salvador run across the

country from east to west. The Panamerican Highway

(CA-1) runs along the Great Interior Valley; it was

originally constructed to serve the coffee industry. The

second major artery is the coast road (CA-2) whose original

purpose was to serve the cotton plantations that previously

occupied the coastal plains. Transport on both roads was

severely disrupted by landslides. The coast road between the

ports of La Libertad and Acajutla in the west was partially

blocked by a number of rock falls and relatively small

landslides; the five tunnels on this section of motorway were

undamaged apart from minor cracks in their lining. The

Panamerican Highway was completely blocked by major

landslides both east and west of San Salvador for several

weeks. To the west, major slides at Los Chorros blocked the

road and even after several weeks traffic was only able to

circulate in one direction, with vehicles entering the capital

in the morning and leaving in the afternoon. East of San

Salvador the highway was completely blocked in both

directions by the huge slide, re-activated by the 13 February

earthquake, at Las Leonas, obliging traffic to use the old and

practically abandoned road running approximately parallel

to the north.

The motorway joining San Salvador and the international

airport at Comalapa on the coastal plain was damaged by

cracks at several locations and during several weeks traffic

was reduced to a single lane in each direction over part of

the road. The airport was closed for one day following the

13 January earthquake to allow clearing up of debris and

inspections of buildings and runways.

6. Implications for seismic risk: physical, social and

institutional vulnerability

The 2001 earthquakes have revealed the extreme levels

of vulnerability to natural hazards that exist in El Salvador.

Moreover, the failure to mitigate earthquake risk in El

Salvador is a reflection of institutional vulnerabilities that

have not been addressed; chief among these are the

capacities for emergency response, monitoring of natural

hazards, land-use planning, and seismic design and its

enforcement.

6.1. Emergency response

Some observers have claimed that the government

response to the disaster in El Salvador has been poorly

organised and in particular that the lessons from Hurricane

Mitch were clearly not learnt [81]. Although this study is not

primarily concerned with emergency aid following the



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UNCORRECTED PROOF

earthquakes, there were some obvious shortcomings, at least

in the initial phases of the response. For example, most aid

arriving at Comalapa International Airport, located on the

coastal plain, was transported almost 30 km to the Feria

Internacional in San Salvador for centralised logging and

thence distributed to affected areas, several of which were

within 1 hour’s drive of the airport. Our visits to badly

affected rural areas generally indicated that government

assistance, in the first few weeks of the crisis, was not

getting through to many of the earthquake victims,

particularly in more remote rural areas.

The main response to the emergency seems to have been

provided by the affected people themselves, although

important contributions by NGOs and others, including

contingents of the Venezuelan Armed Forces and, changing

their historical role, the Salvadorian Armed Forces as well,

should not be overlooked. Despite the huge numbers of

people made homeless by the earthquakes, there were very

few examples of victims living in temporary shelters in the

streets of the cities, as there were after the 1986 earthquake.

Most rural communities, except where affected by land-

slides, appear to have remained to rebuild their homes and

continue with their lives. Middle class people made

homeless, such as those from Las Colinas and adjacent

neighbourhoods, were either absorbed by relatives or added

to the exodus to the USA. May 2001 saw the highest ever

influx of remesas into El Salvador, with a monthly total of

US$ 197.1 millions.

6.2. Seismic design of buildings

Although damage to engineered structures was limited,

at least in terms of structural collapse, there is still a

significant danger of many large engineered structures

having been weakened by the earthquakes and therefore

urgently requiring intervention. This is, in the majority of

cases, which may be unlikely to happen given that seismic

design requirements are not imposed even for new

buildings. Lara [82] reports that prior to the 1986 earthquake

in San Salvador, the seismic design code was rarely applied,

and there is little evidence to suggest that the codes of 1989

and 1994 have been more widely implemented. Indeed,

although it has many technical merits, there is no effective

mechanism for the imposition of the current code for

earthquake-resistant design in El Salvador [83].

The current seismic design code in El Salvador has many

technical merits but the lack of a credible system for its

enforcement severely limits its effectiveness in mitigating

seismic risk. There are almost many aspects of seismic risk in

buildings that fall outside the remit of the code, one being

repair and strengthening. As noted previously, the code does

include an appendix of guidelines for the improved earth-

quake-resistant construction of adobe although this, logically,

does not form part of the actual regulations. These guidelines,

and other publications [84], affirm that adobe buildings can be

constructed with a degree of earthquake resistance, with

minimal requirements in terms of additional costs and

building skills. There is clearly a need, however, for a

transfer of this knowledge to the most isolated and vulnerable

rural communities where these forms of housing are most

abundant and also where they are built with the highest levels

of susceptibility. Amongst the many obstacles to this effective

mitigation are the relatively high rate of illiteracy in rural

areas and the lack of confidence in adobe construction

following its poor performance in the 2001 earthquakes.

6.3. Land use planning

The high level of landslide hazard in El Salvador makes

land-use planning an issue of great importance. The high

population density of El Salvador and the housing deficit

also makes it a sensitive and controversial issue. There is

currently almost no effective control over land develop-

ment. It is interesting to note that the landslide hazard map

shown in Fig. 2 clearly indicates that the area affected by the

catastrophic landslide at Las Colinas was identified as being

of high hazard. The hazard map was prepared some years

after Las Colinas was developed in 1985, but nonetheless no

remedial action was taken to stabilise the slopes or to protect

the area from landslides. The irrelevance of geohazards in

planning decisions is very clearly demonstrated by a recent

housing development to the north of San Salvador, called

Santısima Trinidad (Fig. 23). The development consists of

several rows of four-storey apartment blocks built on

terraces on a natural slope with an inclination of about

358, above which three huge water tanks have been

constructed. The constructors apparently did not face any

serious obstacles in obtaining permission to build.

Despite the apparent lack of control and accountability in

land use planning, the earthquakes may lead to important

changes in this area. Following the earthquakes, 200

survivors from Las Colinas, supported by the Salvadorian

Foundation for the Application of Law (FESPAD) brought a

case against the State to the Supreme Court of Justice for

their failure to prevent or mitigate the risk of landslides on

the slopes of Cerro La Gloria, which were well known and

identified in the PLAMADUR hazard map (Fig. 3). The case

was unsuccessful, being dismissed by the Supreme Court;

similarly, the attempt by the Santa Tecla municipality to

prosecute the developers also failed [81].

More generally, after the earthquake there was a renewed

interest in addressing hazard-related land use issues, not

only in urban areas but in the country as a whole. However,

it is not yet clear how this process will evolve. Certainly,

both public and private sectors in El Salvador will need to

make rapid advances in their risk management and

evaluation practices if large-scale losses are to be avoided

in future due to earthquakes or other natural hazards.

6.4. Seismic monitoring

The monitoring of earthquakes, volcanoes and landslides



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Fig. 23. Santısima Trinidad residential area north of San Salvador built on terraces along a steep slope (a) above which are three large water tanks (b).



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UNCORRECTED PROOF

has traditionally been the responsibility of the Centre for

Geotechnical Investigations (CIG), which is part of the

Ministry of Public Works. Monitoring capacities for natural

hazards were severely weakened in the 1980s due to the war

and efforts to re-build and re-generate these activities since

have been limited. Some responses to this situation have

taken the form of independent initiatives by private

institutions, a clear example of which is the digital

accelerograph network established in 1996 by the UCA.

The government of El Salvador has now responded to this

situation by forming, in October 2001, SNET (National

Service for Territorial Studies), which will unify and

strengthen current natural hazards monitoring capacities.

The structure of SNET includes four different national

services: Geological Service (including earthquake, volca-

noes and landslides); Meteorological Service; Hydrological

Service; and Risk Management Service. The United States

and Japan are expected to provide equipment and technical

assistance for SNET. The Spanish government has approved

a project to expand and upgrade the existing strong-motion

network previously managed by CIG and a convention has

been agreed for collaboration amongst the three strong-

motion networks in El Salvador.

7. Discussion and conclusions

The El Salvador earthquake of 13 January 2001 was the

first major earthquake disaster of the new millennium and

serves perhaps as a warning that in many countries of the

Third World seismic risk is growing. The combination of

population expansion and increasing urbanisation, in the

case of El Salvador with cities expanding in the zones of

highest seismic hazard, together with the increasing

susceptibility of the terrain to landslides, has led to

increased levels of risk both to lives and to the livelihood

of the country.

The impact of the earthquake of 13 January was

compounded by the second event on 13 February, which

came as the aftershocks of the former event were beginning

to diminish in frequency and intensity. The 13 February

event was followed by many aftershocks, both around the

crustal source of this earthquake but also offshore in the

subduction zone. This would tend to indicate interaction

between the two earthquakes, a topic that will be the focus

of future research. Interactions between earthquakes are

well recognised, with stress release in one location causing,

by transfer, stress increase in adjacent zones and hence

inducing or accelerating rupture on adjacent faults or fault

segments. The clearest example of such interaction is the

progression of earthquakes from 1939 to 1999 along the

North Anatolian fault in Turkey [48,85,86]. Interactions

between different earthquakes has also been identified

within subduction zones, as for example in the 1997–1998

sequence in central Chile [87]. That there is interaction

between subduction and crustal earthquakes in Central

America seems probable: it has been noted that the

subduction zone from central El Salvador to the northern

Nicaragua has a far lower rate of moment release than the

zones offshore from Guatemala and Nicaragua either side

[88]. There is also evidence that destructive shallow-focus

earthquakes along the volcanic chain opposite the El

Salvadorian section of the Middle America Trench are

more frequent than in Guatemala and Nicaragua. The exact

nature of the interaction and the mechanism of stress

transfer between the two seismogenic sources is, however,

far from clear at this stage.

The large numbers of accelerograms recorded during the

two earthquakes provide a very useful basis for the

characterisation of strong ground-motion in Central Amer-

ica, although the lack of any near-source recordings of the

13 February earthquake—due to malfunction of the San

Vicente and San Pedro Nonualco stations of the TALULIN

network—is an unfortunate gap in the data set. This is

particularly the case because the indications from the

recorded motions of the second earthquake, and the

observed levels of damage, are that the ground motions

generated were less intense than would be expected from a

shallow earthquake of magnitude Mw 6.6, indicating either

very high attenuation with distance or a focus within the

lower part of the crust. Macroseismic observations and the

limited strong-motion recordings from other earthquakes

point towards high rates of attenuation in the volcanic chain

zone, as has been found elsewhere including the volcanic

region of the North Island of New Zealand [89].

Notwithstanding this observation, the 13 February earth-

quakes appears not to have been as shallow as other slightly

smaller but more destructive events along the volcanic chain

in El Salvador and neighbouring countries. There are

several features of the ground motion that warrant further

research:

† The differences between ground motions from crustal

and subduction events in Central America, and the

development of separate predictive relationships for the

two sources of seismicity.

† The influence of site effects due to both surface geology

and topographical features; the apparent predominance

of these influences suggests that microzonation is a

potentially very useful tool in El Salvador.

† The specification of earthquake loads for seismic design,

taking account of both the different geographical

distributions of the hazard from crustal and subduction

earthquakes and the different natures of the resulting

ground motions.

† The relationship between the nature of the recorded

motion and its capacity to produce damage: it is

abundantly clear that PGA is of very little significance

in this respect, and to some extent this is also true for

spectral accelerations (whence the current trend towards

displacement-based approaches to assessment and

design).



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UNCORRECTED PROOF

The lack of extensive structural damage in reinforced

concrete buildings due to these earthquakes should not be

interpreted as a vindication of the success of the current

seismic design code in El Salvador nor its predecessors,

since regardless of their technical merits these codes have

generally been applied only sporadically. The lack of major

structural damages and collapse of large buildings appears

to be more closely related to the nature of the ground

motions generated than the quality of engineering design or

construction. Particularly in San Salvador there is now a real

danger of complacency regarding the capacity of existing

buildings, despite the fact that it is widely known that many

buildings have been left damaged by the 10 October 1986

earthquake and these may have been further weakened by

the 2001 earthquakes. Destructive moderate magnitude

earthquakes occur in San Salvador on average every 20–25

years [14] and the next event could cause terrible damage

and loss of life in the overcrowded and expanding capital.

The most devastating impact of the 2001 earthquakes has

been the triggering of hundreds of landslides in volcanic soils,

which have buried houses and blocked roads, causing most of

the deaths in these earthquakes and bringing massive

disruption: the Pan-American Highway remained closed for

more than 10 months due to the landslide at Las Leonas. The

number of landslides triggered by these earthquakes, the size

of the slides and their geographical distribution, all indicate

increasing susceptibility of the terrain when compared to

patterns in previous earthquakes, with no indication that this

was due to precedent rainfall. The hazard of earthquake- and

rainfall-induced landslides in the volcanic soils that dominate

much of El Salvador, and particularly the most densely

populated areas, urgently requires attention. The identification

of zones of high landslide hazard is an important component of

any programme of mitigation, but relocation to lower hazard

zones will often not be an option in this densely populated

country with a long history of conflicts over land ownership.

Stabilisation measures cannot necessarily be imported from

regions of the world with entirely different soil characteristics,

since one of the distinguishing features of volcanic soils such

as tierra blanca is the complete loss of cementation at small

strains, followed by the collapse of its matrix structure and a

drastic loss of strength [68]. One of the most important fields

of research in El Salvador is the engineering characterisation

of the tierra blanca, in order to model its behaviour in slopes

subjected to rainfall and due to earthquake shaking, and

similarly to model its modified behaviour after the application

of different stabilisation techniques.

Seismic risk in El Salvador clearly cannot be viewed in

complete isolation from other risks, including those due to

other natural hazards such as floods and volcanic eruption but

also anthropogenic risks such as pollution, deforestation,

crime, poverty, disease and social conflict. The failure to

tackle the challenges of seismic risk, or even to hold back its

increasing levels, is not due to any lack of awareness amongst

Salvadorians of the very high earthquake hazard that affects

their country. Rather the lack of effective measures against

earthquake risk reflects the fact that there are many urgently

pressing needs on limited resources, exacerbated by the

weakness of central and local government. A pessimistic

view of the situation may conclude that earthquake risk

mitigation will only be possible following the solution of

other major social problems in El Salvador. An alternative

view holds that recognition of the interaction of seismic

vulnerability with other features of vulnerability, including

institutional vulnerability, means that concerted programmes

of seismic risk mitigation could provide a vehicle and a

stimulus to the solution of many other issues, including the

current concentration of more than half of the population in

one-third of the national territory. El Salvador will need

external assistance, both in terms of material resources and

technology transfer, to make this vision a reality.

8. Uncited reference

[78].

Acknowledgments

Field investigations of the 13 January earthquake were

funded by the Natural Environment Research Council

(NERC), the Royal Academy of Engineering and the

Universidad Nacional de Colombia. Records from GESAL

instrument in Berlın were provided by Javier Rivas,

Salvador Handal Candray and Jose Antonio Rivas; Griselda

Marroquın kindly provided the records from Nicaragua.

Jose Mauricio Cepeda of the Universidad Centroamericana

provided useful assistance in deciphering incorrect time

codes on records from the TALULIN network and in

providing additional information about the performance of

the network during the earthquakes. We are grateful to

Dominic Dowling of the University of Technology, Sydney,

for his very thorough review of the manuscript. We also

wish to express our sincere thanks to all the people in El

Salvador who, despite the situation into which they had been

thrown, generously assisted us in our field studies of the

earthquakes: their fortitude, resilience and vitality are El

Salvador’s most valuable resources in tackling the problems

posed by natural hazards.

References

[1] Zapata R. ECLAC: summary of the damage caused by the earthquakes

of 13 January and 13 February in El Salvador. ISDR Informs 2001;3:

12–17.

[2] Acevedo C, Romano L. Economıa, Desastre y Desarrollo Sostenible.

San Salvador, El Salvador: FLACSO; 2001.

[3] Dewey JW, Suarez G. Seismotectonics of Middle America. In:

Slemmons, et al., editors. Neotectonics of North America. Geological

Society of America Decade Map, Vol. 1. GSA; 1991. p. 309–21.

[4] Ambraseys NN, Adams RD. The Seismicity of Central America: A



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3108

3109

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3111

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3113

3114

3115

3116

3117

3118

3119

3120

3121

3122

3123

3124

3125

3126

3127

3128

3129

3130

3131

3132

3133

3134

3135

3136

UNCORRECTED PROOF

Descriptive Catalogue 1898–1995. London: Imperial College Press;

2001.

[5] Larde J. El terremoto del 6 de Septiembre de 1915 y los demas

terremotos de El Salvador. Obras Completas, San Salvador, El

Salvador: Ministerio de Cultura; 1960.

[6] Alvarez S de J. Informe tecnico sobre apsectos sismologicos del

terremoto en El Salvador del 19 de Junio de 1982. San Salvador, El

Salvador: Centro de Investigaciones Geotecnicas, Ministerio de Obras

Publicas; 1982.

[7] Lara MA. The El Salvador earthquake of June 19, 1982. EERI

Newslett 1983;17(1):87–96. Oakland, CA: Earthquake Engineering

Research Institute.

[8] White RA, Harlow DH. Destructive upper-crustal earthquakes of

Central America since 1900. Bull Seismol Soc Am 1993;83:1115–42.

[9] Levin SB. The Salvador earthquakes of December, 1936. Bull

Seismol Soc Am 1940;30:1–45.

[10] Ambraseys NN, Bommer JJ, Buforn E, Udıas A. The earthquake

sequence of May 1951 at Jucuapa, El Salvador. J Seismol 2001;5(1):

23–39.

[11] Lomnitz C, Schulz R. The San Salvador earthquake of May 3, 1965.

Bull Seismol Soc Am 1966;56:561–75.

[12] Bommer J, Ledbetter S. The San Salvador earthquake of 10th October

1986. Disasters 1987;11(2):83–95.

[13] EERI, The San Salvador earthquake of October 10, 1986. Earthquake

Spectra 1987;3(3):415–634.

[14] Harlow DH, White RA, Rymer MJ, Alvadrado S. The San Salvador

earthquake of 10 October 1986 and its historical context. Bull Seismol

Soc Am 1993;83(4):1143–54.

[15] White RA. Tectonic implications of upper-crustal seismicity in

Central America. In: Slemmons, et al., editors. Neotectonics of North

America,. Geological Society of America Decade Map, Vol. 1. GSA;

1991. p. 323–38.

[16] Espinosa AF, editor. The Guatemala earthquake of February 4, 1976:

a preliminary report. US Geological Survey Professional Paper 1002;

1976.

[17] Alfaro CS, Kiremidjian AS, White RA. Seismic zoning and ground

motion parameters for El Salvador. Report No. 93, The John A. Blume

Earthquake Engineering Center, Stanford University; 1990.

[18] Algermissen ST, Hansen SL, Thenhaus PC. Seismic hazard evaluation

for El Salvador. Report for the US Agency for International

Development; 1988.

[19] Singh SK, Gutierrez C, Arboleda J, Ordaz M, Peligro sısmico en El

Salvador. Universidad Nacional Autonoma de Mexico, Mexico DF;

1993.

[20] Lindholm C, Rojas W, Bungum H, Dahle A, Camacho E, Cowan H,

Laporte M. New regional seismic zonation for Central America.

Proceedings Fifth International Conference on Seismic Zonation,

Nice 1995;1:437–44.

[21] Villagran M. Problems related to seismic hazard in Central America:

magnitude estimation, attenuation of seismic waves, seismic hazard

assessment for Guatemala city and site response. Master of Science

Thesis, Institute of Solid Earth Physics, University of Bergen,

Norway; 1995.

[22] Bommer JJ, Hernandez DA, Navarrete JA, Salazar WM. Seismic

hazard assessments for El Salvador. Geofısica Internacional 1996;

35(3):227–44.

[23] Rosenblueth E, Prince J. El temblor de San Salvador, 3 de mayo 1965:

ingenierıa sısmica. Ingenierıa, vol. 36. Mexico DF: UNAM; 1966. p.

31–58.

[24] Bommer J. Cargas sısmicas para el diseno estructural en El Salvador.

Rev ASIA 1999;132:15–25. San Salvador, El Salvador: Asociacion

Salvadorena de Ingenieros y Arquitectos.

[25] Bommer JJ, White N. Una propuesta para un metodo alternativo de

zonificacion sısmica en los paıses de Iberamerica. Proceedings of

Segundo Congreso Iberoamericano de Ingenierıa Sısmica, Madrid;

2001.

[26] Weyl R. Geology of Central America. Berlin: Gebruder Borntraeger;

1980.

[27] Schmidt-Thome M. The geology in the San Salvador (El Salvador,

Central America), a basis for city development and planning. Geol Jb

B 1975;13:207–28. Hannover, Germany.

[28] Bommer J, Rolo R, Mendez P. Propiedades mecanicas de la tierra

blanca y la inestabilidad de taludes. Rev ASIA 1998;129:15–21. San

Salvador, El Salvador: Asociacion Salvadorena de Ingenieros y

Arquitectos.

[29] Bommer JJ, Rodrıguez CE. Earthquake-induced landslides in Central

America. Engng Geol 2002;63(3/4):189–220.

[30] Keefer DK. Landslides caused by earthquakes. Geol Soc Am Bull

1984;95:406–21.

[31] Rodrıguez CE, Bommer JJ, Chandler RJ. Earthquake-induced land-

slides: 1980–1997. Soil Dynam Earthquake Engng 1999;18:325–46.

[32] Montessus de Balore F. La Geologie Sismologique: Les Tremble-

ments de Terre. Paris: Libraire Armand Cilin; 1924.

[33] Rodrıguez CE. Hazard assessment of earthquake-induced landslides

on natural slopes. PhD Thesis, University of London; 2001.

[34] Rymer MJ, White RA. Hazards in El Salvador from earthquake-

induced landslides. In: Brabb, Harrod, editors. Landslides: extent and

economic significance. Rotterdam: Balkema; 1989. p. 105–9.

[35] Rosa H, Barry D. Poblacion, territorio y medio ambiente en El

Salvador. Boletın PRISMA 1995; No. 11, San Salvador, El Salvador:

Programa Salvadorena de investigacion sobre Desarrollo y Medio

Ambiente.

[36] Bommer JJ, McQueen C, Salazar W, Scott S, Woo G. A case study of

the spatial distribution of seismic hazard (El Salvador). Natural

Hazards 1998;18:145–66.

[37] ACPC. Coffee Market Report Number 21, May, Association of Coffee

Producing Countries; 2001.

[38] ACES, Monografıa del Cafe: dos siglos de historia en la cafecultura de

El Salvador. San Salvador, El Salvador: Asociacion Cafetalera de El

Salvador; 1999.

[39] Coburn, A., Spence, R., Earthquake Protection. Chichester, England:

Wiley, 1992.

[40] Nabelek J. Determination of earthquake source parameters from

inversion of body waves. PhD Thesis, MIT, Cambridge, MA; 1984.

[41] Nabelek J. Geometry and mechanism of faulting of the 1980 El

Asnam, Algeria, earthquake from inversion of teleseismic body waves

and comparison with field observations. J Geophys Res 1985;90(12):

12713–28.

[42] Abe K. Mechanisms and tectonic implications of the 1966 and 1970

Peru earthquakes. Phys Earth Planetary Interiors 1972;5:367–79.

[43] Burbach GV, Frolich C, Pennington WD, Matumoto T. Seismicity and

tectonics of the subducted Cocos plate. J Geophys Res 1984;89:

7719–35.

[44] Lomnitz C, Rodrıguez Elizarraras S. El Salvador 2001: earthquake

disaster and disaster preparedness in a tropical volcanic environment.

Seismol Res Lett 2001;72(3):346–51.

[45] IGN, Mapa Geologico de la Republica de El Salvador/America

Central 1:100,000. Bundesantalt fur Bodenforschung, San Salvador,

El Salvador: Instituto Geografico Nacional; 1978.

[46] Alvarenga E, Barquero R, Boschini I, Escobar J, Fernandez M, Mayol

P, Havskov J, Galvez N, Hernandez Z, Ottemoller L, Pacheco P,

Redondo C, Rojas W, Vega F, Talavera E, Taylor W, Tapia A,

Tenorio C, Toral J. Central America Seismic Center (CASC). Seismol

Res Lett 1998;69(5):394–9.

[47] Wells DL, Coppersmith KJ. New empirical relationships among

magnitude, rupture length, rupture width, rupture area, and surface

displacement. Bull Seismol Soc Am 1993;83(4):974–1002.

[48] Stein RS, Barka AA, Deitrich JH. Progressive failure on the North

Anatolian fault since 1939 by earthquake stress triggering. Geophys J

Int 1997;128:594–604.

[49] Carr MJ. Underthrusting and Quaternary faulting in northern Central

America. Geol Soc Am Bull 1976;87:825–9.

[50] Bommer JJ, Udıas A, Cepeda JM, Hasbun JC, Salazar WM, Suarez A,



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3244

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3247

3248

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Ambraseys NN, Buforn E, Cortina J, Madariaga R, Mendez P,

Mezcua J, Papastamatiou D. A new digital accelerograph network for

El Salvador. Seismol Res Lett 1997;68(3):426–37.

[51] Faccioli E, Battistella C, Alemani P, Tibaldi A. Seismic microzoning

investigations in the metropolitan area of San Salvador, El Salvador,

following the destructive earthquake of October 10, 1986. Proceed-

ings International Seminar on Earthquake Engineering, Innsbruck;

1988. p. 28–65.

[52] Youngs RR, Chiou SJ, Silva WJ, Humphrey JR. Strong ground motion

attenuation relationships for subduction zone earthquakes. Seismol

Res Lett 1997;68(1):58–73.

[53] Joyner WB, Boore DM. Peak horizontal acceleration and velocity

from strong-motion records including records from the 1979 Imperial

Valley, California, earthquake. Bull Seismol Soc Am 1981;71:

2011–38.

[54] Taylor Castillo W, Santos Lopez P, Dahle A, Bungum H. Digitization

of strong-motion data and estimation of PGA attenuation. NORSAR

Technical Report 2–4; 1992.

[55] Dahle A, Climent A, Taylor W, Bungum H, Santos P, Ciudad Real M,

Lindholm C, Strauch W, Segura F. New spectral strong motion

attenuation models for Central America. Proceedings of the Fifth

International Conference on Seismic Zonation, Nice 1995;II:1005–12.

[56] Schmidt V, Dahle A, Bungum H. Costa Rican spectral strong motion

attenuation. Norway: NOSAR; 1997.

[57] Climent A, Taylor M, Ciudad Real M, Strauch W, Villagran M, Dahle

A, Bungum H. Spectral strong motion attenuation in Central America.

Technical Report No. 2–16, Norway: NORSAR; 1994.

[58] Cloud WK, Perez V. Unusual accelerograms recorded at Lima, Peru.

Bull Seismol Soc Am 1971;61:633–40.

[59] Knudson CF, Perez V. Accelerograph records from Lima, Peru.

Proceedings of the Sixth World Conference on Earthquake Engin-

eering, New Delhi 1977;1:338–44.

[60] Purvance MD, Anderson JG. The behavior of spectral decay in strong

ground motion accelerations recorded in Guerrero, Mexico. Proceed-

ings of the Seventh US National Conference on Earthquake

Engineering, Boston; 2002.

[61] Spudich P, Joyner WB, Lindh AG, Boore DM, Margaris BM, Fletcher

JB. SEA99: a revised ground motion prediction relation for use in

extensional tectonic regimes. Bull Seismol Soc Am 1999;89(5):

1156–70.

[62] Boore DM, Joyner WB, Fumal TE. Equations for estimating

horizontal response spectra and peak accelerations from western

North American earthquakes: a summary of recent work. Seismol Res

Lett 1997;68(1):128–53.

[63] Abrahamson NA, Silva WJ. Empirical response spectral attenuation

relations for shallow crustal earthquakes. Seismol Res Lett 1997;

68(1):94–127.

[64] Ambraseys NN, Simpson KA, Bommer JJ. Prediction of horizontal

response spectra in Europe. Earthquake Engng Struct Dynam 1996;

25:371–400.

[65] Sabetta F, Pugliese A. Estimation of response spectra and simulation

of nonstationary earthquake ground motions. Bull Seismol Soc Am

1996;86(2):337–52.

[66] Lopez Casado C, Benito B, Bommer JJ, Ciudad Real M, Pelaez JA.

Analisis de los acelerogramas registrados en los terremotos de El

Salvador de 2001. Proceedings of the Second Iberoamerican

Conference on Earthquake Engineering, Madrid; 2001. p. 771–80.

[67] Dikau R, Brunsden D, Schrott L, Ibsen M-L. Landslide recognition.

Chichester: Wiley; 1996.

[68] Bommer J, Rolo R, Mitroulia A, Berdousis P. Geotechnical properties

and seismic slope stability of volcanic soils. Proceedings of the 12th

European Conference on Earthquake Engineering, London; 2002,

Paper No. 695.

[69] Hsu KJ. Catastrophic debris streams (sturzstroms) generated by

rockfalls. Geol Soc Am Bull 1975;86:129–40.

[70] Devoli G, Egger C, Ferres D, Rubio J. Estudio geologico preliminary

de la ladera norte de la Sierra del Balsamo, Santa Tecla, Departamento

de La Libertad, El Salvador. Geologos del Mundo; 2001.

[71] Jibson RW, Crone AJ. Observations and recommendations regarding

landslide hazards related to the January 13, 2001 M 7.6 El Salvador

earthquake. US Geological Survey Open-File Report 01-141; 2001.

[72] Mendoza MJ, Dominguez L, Melara EE. Deslizamientos y flujo de

tierras en la ladera Las Colinas, Nueva San Salvador, El Salvador,

C.A., disparado por el sismos del 13 de Enero de 2001. Proceedings of

the Second Iberoamerican Conference on Earthquake Engineering,

Madrid; 2001. p. 771–80.

[73] Bernal A. Establidad de taludes en terremotos: el deslizamiento de Las

Colinas, El Salvador, en el terremoto del 13 de Enero de 2001.

Proceedings of the Second Iberoamerican Conference on Earthquake

Engineering, Madrid; 2001. p. 781–91.

[74] Harp EL, Wilson RC. Shaking intensity thresholds for rock falls and

slides: evidence from Whittier Narrows and Superstition Hills

earthquake strong-motion records. Bull Seismol Soc Am 1995;85:

1739–57.

[75] Murphy W, Petley DN, Bommer JJ, Mankelow JM. Uncertainty in

ground motion estimates for the evaluation of slope stability during

earthquakes. Q J Engng Geol Hydrogeol 2002;35:71–8.

[76] CEPREDENAC. Informe sobre terremotos en El Salvador.http://

www.cepredenac.org; 2001.

[77] Baum RL, Crone AJ, Escobar D, Harp EL, Major JJ, Martınez M,

Pullinger C, Smith ME. Assessment of landslide hazards resulting

from the February 13, 2001. El Salvador earthquake. US Geological

Survey Open-File Report.01-119; 2001.

[78] CEPRODE, Caracterizacion de los desastres en El Salvador: tipologıa

y vulnerabilidad socioeconomica. San Salvador, El Salvador: Centro

de Proteccion para Desastres; 1994.

[79] Lund LV, editor. Lifeline performance in the El Salvador earthquakes

of January 13th and February 13th, 2001: preliminary reconnaissance

survey. Technical Council on Lifeline Earthquake Engineering,

ASCE; 2001.

[80] EERI, Preliminary observations on the El Salvador earthquakes of

January 13 and February 13, 2001. EERI Newslett 2001;35(7):12.

[81] Wisner B. Risk and the neoliberal state: why post-Mitch lessons didn’t

reduce El Salvador’s earthquake losses. Disasters 2001;25(3):251–68.

[82] Lara MA. The San Salvador earthquake of October 10, 1986—history

of construction practices in San Salvador. Earthquake Spectra 1987;

3(3):491–6.

[83] Bommer J. Terremotos, urbanizacion y riesgo sısmico en San

Salvador. Boletın PRISMA 1996; No. 18. San Salvador, El Salvador:

Programa Salvadorena de investigacion sobre Desarrollo y Medio

Ambiente.

[84] Asociaion Equipo Maız, La Casa de Adobe Sismorresistente. San

Salvador, El Salvador: Associacion Equipo Maız; 2001.

[85] Barka AA. Slip distribution along the North Anatolian Fault

associated with large earthquakes of the period 1939 to 1967. Bull

Seismol Soc Am 1996;86:1238–54.

[86] Parsons T, Toda S, Stein RS, Barka A, Dietrich JH. Heightened odds

of large earthquakes near Istanbul: an interaction-based probability

calculations. Science 2000;288:661–5.

[87] Lemoine A, Madariaga R, Campos J. Evidence for earthquake

interaction in Central Chile: the July 1997–September 1998

sequence. Geophys Res Lett 2001;28(14):2743–6.

[88] Ambraseys NN, Adams RD. Large Central American earthquakes

1898–1994. Geophys J Int 1996;127:665–92.

[89] Cousins WJ, Zhao JX, Perrin ND. A model for the attenuation of

peak ground acceleration in New Zealand earthquakes based on

seismograph and accelerograph data. Bull NZ Soc Earthquake Engng

1999;32(4):193–220.



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The El Salvador earthquakes of January and February 2001...

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