INTRODUCTION 3

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P o s t - W W I I r e s e a r c h

Little geological work was possible during the years immediately

after the end of WWII, but following Indonesian Independence in

1947 the Geological Survey of Indonesia (GSI) was established in

the old Bureau of Mines building in Bandung. From 1969 to 1974

the Mapping Division of (GSI) commenced a systematic pro-

gramme of mapping in the Padang area of West Sumatra, in col-

laboration with the United States Geological Survey (USGS), as

part of the First Five Year Development Plan (PELITA I).

Several 1:250 000 Geological Map Sheets were published as a

result of this programme (Silitonga & Kastowo 1975; Rosidi

e t al . 1976; Kastowo & Leo 1973). As part of this collaboration

a senior geologist of the USGS, Warren Hamilton, was commis-

sioned to prepare a series of maps and a memoir reviewing the

geology of the Indonesian region in plate-tectonic terms

(Hamilton 1977, 1979). Hamilton's (1979)Tectonic Map, which

includes Sumatra, shows clearly present views of the tectonic

setting of Sumatra.

SEATAR P r o g r a m m e

In 1973 a meeting was convened by the United Nations Committee

for the Coordination of Joint Prospecting for Mineral Resources

in Asian Off-shore waters (CCOP) in Bangkok which established

the Studies in East Asian Tectonics and Resources (SEATAR)

Programme. At that time a review of the current understanding

of the tectonics of eastern Asia was prepared by Deryck Laming

on behalf of CCOP-IOC (1974). As a result of the meeting it

was proposed to concentrate research along a series of transects

across the island arc systems of East and SE Asia. Subsequently

A. J. Barber (University of London) and Derk Jongsma (BMR)

were engaged by CCOP as Technical Consultants to prepare a

report on the current state of knowledge along the lines of

these transects (CCOP-IOC 1980). One of the selected transects

ran from the Malay Peninsula across northern Sumatra and the

forearc island of Nias to the Sunda Trench. Although the final

report for this transect was never published, a great deal of

important research was carried out by American researchers

4 C H A P T E R 1

INDIAN OCEAN NIAS MALAY

Present BARISAN MOUNTAINS PENINSULA Accretionary

Sunda Complex Sumatran Fault MALACCA STRAIT Toba Caldera Backarc Basin NE

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km

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Fig. 1.3. Diagrammat ic section across the Sumat ran Subduct ion System from the floor of the Indian Ocean to the Malay Peninsula , drawn to scale.

under the auspices of the SEATAR Programme, particularly in Nias and the surrounding seas (Curray et al. 1982; Karig et al. 1980; Moore & Karig 1980). Also in conjunction with the SEATAR Pro- gramme, Cobbing et al. (1992) made a detailed study, including isotopic dating, of the granites on the Tin Islands of Bangka and Billiton, supported by the UK Overseas Development Adminis-

tration as a contribution to the work of :COP. Since the effective termination of the SEATAR Programme,

US research in Sumatra has been concentrated on neotectonics, an important part of which has been the monitoring of movement along the Sumatran Fault System, using GPS location systems (Prawirodirdjo et al. 1997).

Indonesian Petroleum Association

In 1971 the Indonesian Petroleum Association (IPA) was estab- lished by petroleum companies operating in Indonesia, in associ- ation with the Indonesian national oil company, Pertamina. Since its inception the IPA has held Annual Conventions which continue to the present day. At these conventions papers on the geology of Indonesia are presented and published as the Pro- ceedings of the Indonesian Petroleum Association. The IPA Proceedings provide an invaluable source of information on the geology of Indonesia. Most of the papers deal with Tertiary depos- its and details of the stratigraphy and structure of the oil and gas fields of Indonesia, including those of Sumatra, but more general papers on geology and tectonics have also been published. The publication of the IPA Proceedings has resolved van Bemme- len's (1949) complaint of the pre-WWII situation, in which large amounts of geological data, accumulated by the oil companies, remained unpublished for commercial reasons, and were not avail- able for the compilation of regional geological syntheses.

British and Indonesian Geological Surveys

Major UK involvement in the geology of Sumatra began in 1975 when the Institute of Geological Sciences (IGS, now the British Geological Survey, BGS), in collaboration with the Geological Survey of Indonesia (GSI), commenced a five-year mapping and reconnaissance geochemical survey of northern Sumatra to the north of the equator (Northern Sumatra Project, NSP). In 1978 GSI was reorganized into a number of semi-auton- omous directorates and the Directorate of Mineral Resources (DMR) became the designated Indonesian counterpart organisation in the NSP. The work of IGS in the Northern Sumatra Project, and subsequent projects by BGS in Sumatra, were funded from the Technical Assistance and Technical Cooperation budgets of the U.K. Overseas Development Administration (ODA).

The structural, stratigraphic, geochemical and tectonic results of the Northern Sumatra Project have been presented in a series of papers (Page et al. 1978, 1979; Cameron et al. 1980; Rock et al. 1982; Aldiss & Ghazali 1984) and unpublished reports. In a continuation of the NSP, geological maps and reports result- ing from the project were edited by BGS personnel, and published by the Indonesian Geological Research and Development Centre (GRDC), one of the constituent directorates of GSI, as a series of 18 Geological Map Sheets at 1:250 000 scale, with accompanying Explanatory Notes. Follow-up studies of fossil localities, with the view of establishing the stratigraphical ages of the sedimentary units in Sumatra, were carried out by Metcalfe (1983, 1986, 1989a, b; Metcalfe et al. 1979) and by Fontaine and his collaborators, under the auspices of : C O P (Fontaine & Gafoer 1989). The results of the regional geochemical stream sediment sampling survey were published in a joint IGS/DMR Geochemical Atlas (Stephenson et al. 1982) and sub- sequently DMR published sets of single element proportional symbol distribution maps at 1:250000, for many of the quadrangles to the north of the equator. Geochemical anomalies found during the NSP were followed up by BGS and DMR in the collaborative North Sumatra Geological and Mineral Exploration Project (NSGMEP, 1985-1988). The results of a separate programme of research into the mineralization in north Sumatra, also funded by UK ODA, have been published by Bowles et al. (1984, 1985) and Beddoe-Stephens et al. (1987).

University of London Southeast Asian Research Group,

BGS and LEMIGAS

In 1978 members of the University of London Southeast Asian Research Group which had previously been active in Eastern Indonesia, commenced a programme of research projects in Sumatra, in collaboration with BGS, DMR and GRDC. In 1984 a joint University of London/BGS North Sumatra Basins Study Project, was set up with funding from the UK Overseas Development Administration, in collaboration with the Indonesian Research and Development Centre for Oil and Gas Technology (LEMIGAS) (Kirby et al. 1993). This project built on the major involvement by LEMIGAS in this productive basin, where one of the largest exploration blocks is operated directly by Pertamina. The overall programme was largely concerned with the stratigraphy, sedimentology and geophysics of the Tertiary basins in northern Sumatra, with the University contribution Concentrating on field studies of the relationship of the Tertiary rocks to the underlying basement, with a view to understanding the tectonic evolution, of these basins (Turner 1983; Tiltman 1987, 1990; Kallagher 1990). More recently the University of London contribution, funded by the UK Natural Environment Research Council (NERC), ODA and a number of oil companies,

INTRODUCTION 5

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Tertiary Sediments and volcanics

Pre-Tertiary Basement

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Complex

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became increasingly concentrated in the forearc islands, where

a series of geological mapping and gravity surveys were

completed (Situmorang et al. 1987; Milsom et al. 1990; Harbury

& Kallagher 1991; Samuel & Harbury 1996; Samuel et al.

1997). At the same time LEMIGAS collaborated with the

French CNRS (Centre National pour Recherche Scientifique) in

a number of studies in the forearc region using the Indonesian

Marine Research Vessel Baruna Jaya III (Diament et al. 1992;

Izart et al. 1994). Outside the bounds of the NSP, University of

London Staff and research students with funding from NERC,

ODA and a Consortium of petroleum companies collaborated

with LEMIGAS on studies on the Ombilin interarc basin

in central Sumatra (Lailey 1989; Bartram & Nugrahaningsih

1990; Howells 1997b), the Woyla Group in North Sumatra

(Wajzer et al. 1991; Barber 2000; McCarthy et al. 2001) and a study of the Sumatran Fault System throughout the island

(McCarthy & Elders 1997).

Southern Sumatra Project

Geological mapping, gravity surveys and geochemical program-

mes in Sumatra south of the equator were conducted by GRDC

and DMR during PELITA II (1974-79) and in successive five

year development programmes, continuing into the 1980s. In

1988 the Southern Sumatra Geological and Mineral Exploration

Project (SSGMEP) was established, and BGS joined DMR and

GRDC in the completion of these surveys and in research

6 CHAPTER 1

r./i/o: d o [ /Banda Aceh ~ . . . . . . . . ,, ,. ......" / " . Lhokseumawe

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; t7

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t3 1 O0 200 300 ................. 400 500kin %'7 /

Fig. 1.5. Coverage, sheet numbers and names of the 1:250 000 Geological Maps published by the Indonesian Geological Survey, the Geological Research and

Development Centre, Indonesian Ministry of Mines and Energy.

programmes with funding from UK ODA Technical Cooporation budget. This programme was completed in 1995 with the publi- cation by GRDC of the last of the forty three Geological Map Sheets at 1:250000 scale, covering the whole of Sumatra (Fig. 1.5) and 18 1:250000 scale Bouguer gravity anomaly maps of southern Sumatra, including Bangka and Billiton islands, but excluding the coastal swamps and the Barisan Mountains. The collaborative geochemical survey was completed in 1994 with the publication by DMR of 14 quadrangle boxed sets of 1:250 000 single element proportional symbol geochemical maps (up to 15 elements) with accompanying reports on the geochemistry, geology and mineral occurrences. Subsequently the Sumatra geochemical data was made available on CD-ROM

(Version 2 in 1999). In 1995 following a one-year 'Sustainability Phase' of the SSGMEP a Geochemical Atlas of Southern Sumatra was issued in digital form on CD-ROM (Machali et al.

1995). Publication in book form followed in 1997, with text in both Bahasa Indonesia and English (Machali et al. 1997). An evaluation of tectonic models for the Pre-Tertiary history of Sumatra based on BGS/DMR/GRDC and University of London research programmes has been published by Barber & Crow (2003).With the completion of this major phase of UK involvement in the study of the geology of the Sumatra, the time is ripe to review the vast increase in our knowledge of the geology of Sumatra since van Bemmelen's (1949, 1970) synthesis.

Chapter 2

Seismology and neotectonics

JOHN MILSOM

Sumatra is an active (Andean) continental margin that would be linked by land to SE Asia if sea level fell by as little as 50 m. Present-day tectonic processes are controlled by three major fault systems, the most obvious of which is the subduction thrust which crops out in the Sunda Trench. The trench curves very little in the 800 km between Enggano and Nias, i.e. off central Sumatra (Fig. 2.1), but is markedly convex towards the Indian Ocean both further north and further south. Water depths of more than 6000 m are reached in the south but the maximum in the north may be less than 5000 m. The difference is usually, and convincingly, attributed to the presence on the Indian Ocean plate of the Nicobar Fan, consisting of sediments, derived ultimately from erosion of the Himalayas, which increase steadily in thickness towards the north (e.g. Hamilton 1979). Continuing subduction is attested by a Wadati-Benioff Zone (WBZ) that extends to depths of the order of 200 km (e.g. Newcomb & McCann 1987) and by volcanic activity in the Barisan mountains, the peaks of which generally lie within a few tens of kilometres of the coast. The change, of more than 45 ~ in the trend of the trench between 96~ and 97~ (the 'Nias Elbow') may have been initiated by subduction of the 2 km high Investigator Ridge (Investigator Fracture Zone), which trends approximately north- south at about 98~ Sieh & Natawidjaja (2000) defined a 'Central Domain' of mainland Sumatra between the Nias Elbow and the ridge intersection as anomalous in a number of ways (notably in the differing trends of the Sumatran Fault and the volcanic line) and as distinct from more regular Northern and Southern domains on either side (Fig. 2.1).

Inland, the dextral transcurrent Sumatran Fault runs the entire length of the island, from Banda Aceh to the Sunda Strait (Fig. 2.1). A variety of names have been used for both the overall fault system and parts of it, and new nomenclature developed by Sieh & Natawidjaja (2000) divided it into 19 indi- vidual segments. Even this detailed study failed to answer many fundamental questions, and estimates of total lateral displacement still vary from several hundred kilometres to as little as twenty kilometres. The 150km suggested by McCarthy & Elders (1997) seems to be about the mean of the published values. The fault trace coincides roughly with the watershed of the Barisans and with the volcanic line, although most of the volcanoes lie somewhat to the NE of the fault and only nine of the fifty youngest centres lie within 2 km of it (Sieh & Natawidjaja 2000). A more precise correlation is with the subduction thrust, since for most of its length the distance between the Sumatran Fault and the trench axis differs by no more than 30 km from the average value of 290 km. The largest deviations are a narrowing within the bight of the Nias Elbow and a broadening in the region further to the NW.

The third and most enigmatic of Sumatra's major fault systems is the Mentawai Fault, at the outer margin of the forearc basin (Fig. 2.1). In many publications the name is reserved for the segment extending from the Sunda Strait to Nias (Samuel & Harbury 1996) or the Batu Islands (Diament et al. 1992), but the same disturbance zone continues at least as far as the Andaman Sea (Malod & Kemal 1996) and possibly to the Andaman and Nicobar Islands. Movement has been variously interpreted as normal, strike slip or reverse (Sieh & Natawidjaja 2000). There are considerable changes in appearance on seismic sections even within the region from Nias southwards; the structure was

described by Sieh & Natawidjaja (2000) as a homocline and by Karig et al. (1980) as a 'fault-flexure'.

Magnetic anomalies in the Indian Ocean south of Sumatra trend east-west and were interpreted by Sclater & Fisher (1974) as indicating Palaeogene ages for most of the crust adjacent to the trench, with a possibility of Late Cretaceous crust in the extreme SE. Transforms such as the Investigator Fracture Zone, which may offset the anomalies by several hundred kilometres, run almost precisely north-south. With the trend of the trench varying from N40~ to N60~ and the direction of the Indian Ocean-Sumatra convergence vector being about N15~ (Fig. 2.1), Sumatra has long been recognized as a key area for studies of the partitioning of strain between thrust and transcurrent faults during oblique convergence (Fitch 1972; McCaffrey 1992, 1996; Malod & Kemal 1996). The suggestion, originally made by Fitch (1972), that the oblique motion is to a first approxima- tion accommodated by orthogonal subduction at the trench and dextral slip along the Sumatran Fault, is now widely accepted. To the extent that this is true, the forearc region must be decoupled from both the Indian Ocean and Eurasia. The commonly used term 'sliver plate' (e.g. Curray 1989) suggests more strength and rigidity than could reasonably be expected of such a long and narrow strip of lithosphere, and any analysis of subduction beneath Sumatra must take into account the probability of inde- pendent movements of forearc fragments (e.g. McCaffrey 1991).

Estimates of the movements of the Indian Ocean relative to Sumatra are shown in Figures 2.1 and 2.4. Changes in magni- tude and direction from NW to SE are dictated by the East African location of the pole of rotation (Larson et al. 1997). If par- titioning of orthogonal and transcurrent strain between, respect- ively, the trench and the Sumatran Fault were complete (and movement occurred only along these features), then sites in the forearc sliver would move parallel to the Sumatran Fault relative to SE Asia, but at right angles to the trench relative to the Indian Ocean. Trench-normal relative motion implies that the forearc sliver 'tracks' across linear features on the Indian Ocean Plate, such as the Investigator Fracture Zone, which have north- south trends (Fig. 2.1). If the long term movement between the forearc and the Indian Ocean has actually been approximately orthogonal, the intersection point of the Investigator Fracture Zone with the trench, now near the Batu Islands, would have been north of Nias less than 10 million years ago. The relief, of more than 2 km, on the Investigator Fracture Zone might not only impede such tracking but could be responsible for cyclical uplift and subsidence in the forearc basin and ridge.

Slip partitioning and subduction of Indian Ocean lithosphere produce high levels of seismicity in the Barisan Mountains, in the forearc basin and along the forearc ridge (Fig. 2.2). The poten- tial for extremely destructive earthquakes was most recently demonstrated by the Magnitude 9 event near Simeulue in Decem- ber 2004 and by the resulting tsunami, which gave rise to one of the worst natural disasters in recorded human history. However, and despite the geological evidence for a long history of subduc- tion (e.g. Page et al. 1979), shocks deeper than 200 km are rare (Fig. 2.3). Events below 300 km are confined to the extreme SE and may be associated with north-directed subduction beneath Java rather than NE-directed subduction beneath Sumatra. The abrupt change in orientation of the active margin between these two islands must produce considerable stress in the downgoing

8 CHAPTER 2

Northern Domain

Central Domain Southern Domain

1000 ~ LINE --~l 42-43 Batu

pora N.

Elbow'

F i g . 2.1. Sumatra: the neotectonic setting. The figure has been oriented on the main fault direction. The India-SE Asia convergence vector changes significantly in both

direction and magnitude over the length of the island, from 52 mm a-1 directed at N10~ (at 2~ 95~ to 60 mm a- l directed at N 17~ (at 6~ 102~ Convergence

data (and mainland structural domains) are from Sieh & Natawidjaja (2000). Elongated rectangles in the forearc region indicate the locations of the zeros on the seismicity

cross-sections in Figure 2.3. The seismic image along Line 42-43 is shown in Figure 2.7. The white stars mark the epicentres of the Enggano 2000 and Simeulue

2004 Great Earthquakes. Bathymetric contours at 200, 1000, 3000, 5000 and 6000 m are from GEBCO (1997). Shading indicates sea floor deeper than 6000 m. I.F.Z.,

Investigator Fracture Zone. Onshore topography derived from the Global Relief Data CD-ROM distributed by the National Geophysical Data Center, Boulder, Colorado.

slab but this is not obvious in the patterns of shallow seismicity

shown in Figure 2.2 and discussed below.

Shallow seismicity

As in most active continental margins, shallow (<60 km depth)

earthquakes in Sumatra are distributed over wide areas of the

upper plate and are not restricted to the WBZ (Fig. 2.2).

Maximum shallow earthquake activity occurs within the sliver

defined by the Sumatran Fault in the east and by the subduction

thrust in the west and at depth, and is most intense along the

line of the forearc ridge. There must be considerable forearc

extension (see McCaffrey 1991 ) if the estimates of large variations

in rates of transcurrent slip (more than 400 km of offset in Aceh

but negligible displacements in the Sunda Strait; Curray et al.

1978) are correct (see also Bellier & Sebrier 1995). Although

there have been relatively few shocks of Magnitude 6 or greater beneath the mainland, some have occurred, most notably in

the vicinity of the 'equatorial bifurcation' in the Sumatran Fault

identified by Prawirodirdjo et al. (2000).

The insets to Figure 2.2 attempt to show separately the distri-

butions of events within the uppermost 40 km of the crust and

at depths of between 40 and 60 km. Because of the uncertainties

inherent in determining the depths of shallow earthquakes (see

discussion in Engdahl et al. 1998), there will be events on one map that should properly have been plotted on the other, but the

overall differences between the plots are likely to be real. The

4 0 - 6 0 km events are concentrated in a narrow zone centred on

the forearc basin and most are probably directly associated with the subducted oceanic lithosphere, i.e. with the WBZ. There are,

however, some similarities with the patterns of shallower events,

noticeably in the tendency for epicentres to be concentrated in

short linear zones at right angles to the trench, presumably due

to some form of forearc segmentation. The most obvious examples

can be seen around Enggano and western Simeulue, i.e. close to

the sites of the Great Earthquakes (defined as earthquakes with

MW magnitudes greater than about 7.8) in June 2000 and Decem-

ber 2004 respectively. Interestingly, the Simeulue events cluster

along the crest of a basement ridge that defines the northwestern

boundary of a marine and sedimentary basin (Simeulue Basin)

where maximum water depths exceed 1000 m. The trend of the

linear alignments changes slightly north of the Nias Elbow to

partly match the change in orientation of the trench but, surpris-

ingly, N E - S W alignments of epicentres can be seen east of the

even more dramatic change between Sumatra and Java (Fig. 2.2).

A second feature of the shallow seismicity is the separation

of the shallowest earthquakes (Fig. 2.2; lower inset) into two

divergent zones, one along the forearc ridge (with a bend or

offset where the Investigator Fracture Zone enters the subduction

zone near the Batu islands), the other very approximately along

the west coast of Sumatra. The forearc basin itself is relatively

quiet seismically at these depths. The offset at the Investigator

Fracture Zone is interesting because Newcombe & McCann

(1987) noted that ruptures associated with Great Earthquakes do

not propagate across this region. In 1833 a Magnitude (Mw) 8.7

event faulted the plate margin for about 600 km from Enggano

to the Batu Islands, while the effects of the Mw 8.4 event in

1861 were confined to a 300 km segment between the Batu and

Banyak Islands.

The Wadati-Benioff Zone (WBZ)

In keeping with the continental margin setting, seismicity beneath Sumatra is more diffuse than beneath a typical intra-oceanic arc.

This is illustrated in Figure 2.3, which shows hypocentre dis-

tributions within three typical swathes, each 200 km wide. In the

extreme NE (Fig. 2.3a) the WBZ forms the lower boundary to a

seismogenic zone that extends up to the surface over a distance

of approximately 300 km from the trench. The greatest concen-

tration of events is at about I00 km from the trench and at

depths of about 50 km. In the swathe immediately south of the

equator, near the islands of Siberut and Sipora, there is a much

SEISMOLOGY & NEOTECTONICS 9

i Hypocentres above 40 km o "

oe L a t e * %* **" - . /Hypocentres 40-60 km N e,++"

. /

,i.

E q u a t o r

k '.++~+

+ + + 4 o - 6o km onJy

0,~" o

300 k i n - / 0

o+

+I~ COo "' ,

o Oo

o Oo o o

, , ' - ,

/ ~ / o ",

�9 , % : + 1 . . . . / +o /

40ku, o,,~* ] o '~0s / o

+ +++: + +~+ I ~ ' . I ~ ~ ' + o /

I 6+ ~ ,e : ~

+ �9 ........ ~ + , + +

"ore, o ~o o b o ( +,+ ++++ :+ +++ +++ +++:+ .... I ... /o'| + o " < " "V

++ ?++++++++++++++++++++;+++++++++ I + ....... . i o + , . +++ + ' % ~+ +++ ++ +'++ ++++ +I . . . . . . . . . o ...... o * ,i

�9 ,'+::~:~+;++++++++:~++I + o .~.~ ]++, ;.:,:)+ +i] o

............... + + :i~':+i. i I O 0 ~ 1 0 4 ~ o, oo

"..,

%o

Fig. 2.2. Shallow selsmicity of Sumatra. Data downloaded from supplementary

material to Engdahl et al. (1998) using only events occurring between 1980 and

1996. Rectangles show locations of swathes cross-sectioned in Figure 2.3. Thick

lines within the rectangles mark the cross-section zeros. Insets show 0-40 km

and 40-60 km hypocentres separately�9

clearer development of a linear WBZ but the scatter is still con-

siderable (Fig. 2.3b). Sieh & Natawidjaja (2000), among others,

have claimed that the depth of the WBZ beneath the volcanic

line is considerably greater in this Central Domain (Fig. 2.1)

than to either the NW or the SE, although the maximum depth

of the seismic zone is actually smaller. The effect is not,

however, obvious in Figure 2.3.

The most intensely active part of the WBZ is in the extreme

south, near Enggano, where there are two main event clusters, at

about 40 and 70 km (Fig. 2.3c). The seismogenic zone continues

down to at least 200 km. The two deepest shocks might be

associated with Java subduction but, if associated with Sumatra,

indicate a pronounced steepening of the WBZ between 200 and

300 km.

T o b a s e i s m i c i t y

A more comprehensive picture of Sumatra seismicity than is

provided by Figure 2.3 was presented by Hanus e t al . (1996),

who plotted hypocentres within 50 km wide, N E - S W swathes

that together covered the whole of the island. Arguably their

most interesting plot was A15, which included the northern part

of the forearc island of Nias and much of the Toba caldera

(Fig. 2.1). The WBZ in this region dips at an angle of a little

more than 30 ~ and the deepest shocks occur between 200 and

250km. There is a small but noticeable gap in seismicity

beneath the volcanic line at depths of about 150 to 180 km and

a corresponding region of shallow seismicity immediately

beneath the volcanoes.

In detail, the picture provided by Hanus e t al . (1996) is suspect

because of the reliance on International Seismological Centre

T R F 200 100 0 100 200 300

o 0+~+., o+..++ ~ I o ) o q t o o+o..~ ~ .

og~. o,~ ~ "~ . o : . o o

o % ~ ~ : o o o o 1 O0

North o o ?

T R o o F ~ I l~ 1200 o . . . . . . . : - i e o ' ~ ~, : o :

I I o 6

I . . . . . . ~ - : , ~ o o,~, o :

" . - $ 0 0 .: ~ a ) �9 �9 �9 o

Simeulue

T R F 200 100 0 100 200 300

o oGo o o o

o ~ " e ~ : : . . . . o , , o ~ o , ~ o

o

. . . . . . . . . . . . . . . . . . . . .

O

Central

I00

200

~g0 T 100 O R 100 F200 30g

o o bo 0 oo : o 0 o

~ o �9 oi~o.Oo < l l . _ , , . . o : ' + . . . . . . : : .o~ < ' * " ~ l R m ~ - T ~ . . , , ~

: :, .... i ~ + , ~ , . ~ . . ,~o ~++~_~l[llm~ o . . . . I oo

. . . . :~ : " : A~176176176 o o <>~ . . . . . .

. . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . o

~>o 2OO

South c) .:~ 300

M a g n i t u d e s 3 �9 4 o 5 o

Fig. 2.3. Cross-sections of seismicity, Sumatra subduction zone 1980 to 1996.

Each cross-section is based on a swathe drawn at right angles to arc. Each swathe

is 300 km across, except for the Simeulue swathe (inset, profile 2.3a), which is

only 100 km across. T, R and F in each case indicate the locations of,

respectively, the trench, the crest of the forearc ridge and the Sumatran Fault.

Locations of cross-section zeros are shown in Figure 2.1 and the swathe areas are

shown in Figure 2.2. For profiles 2.3b and 2.3c these zeros coincide with the crest

of the forearc ridge, but for 2.3a, where the ridge is poorly defined, the zero is in

the centre of the forearc basin. The star on the Simeulue swathe indicates the

location of hypocentre of the December 2004 event, after NEIC (2005). All other

data were downloaded from supplementary material to Engdahl et al. (1998).

Distances in kilometres, no vertical exaggeration.

(ISC, Thatcham, UK) hypocentre locations. These, being

derived from interpretations of teleseismic data based on global

velocity models, are inevitably of fairly low accuracy. The signifi-

cance of this limitation has been demonstrated by Fauzi e t al .

(1996), who used additional data from a newly established (but

now permanent) network of short-period digital seismometers to

study earthquakes in the vicinity of Toba. The primary aim of

the work reported, which covered the period from October 1990

to April 1993, was to investigate a hypothesized break in the

downgoing slab due to subduction of the Investigator Fracture

Zone. Seismic activity was found to be unusually high in the

appropriate area but no discontinuity was detected and a limit

of 20 km was placed on the magnitude of any possible displace-

ment. There was more success with a subsidiary objective of

defining the shape of the WBZ as it followed the bend in the

offshore trench between Nias and Simeulue. In contrast to both

the ISC and Engdahl e t al . (1998) data, hypocentres derived

from the local study and plotted for narrow cross-strike swathes

10 CHAPTER 2

~.Lake

%

Batu IS.

I Indian ~ : [Ocean/

SE Asia convergence

. . . . . . . . . . . . . . . . . . . . . . v e c t o r ;

52 mm/yr

0 300 km

Sipor k , pNgr taih ~'~f~ S~

N ~ l ' ~ g a l

Convergence velocity scale .... X 50mm/yr % ~ [ ! ~

Contours on the WBZ in the Toba region, after %. Fauzi et al., 1996 * . . . . . !~ I n d i a n .

June 2000 Enggano earthquake ~ Ocean/ I

Main ~ Aftershocks~ ~ ~" SEAsia / convergence [ shocks ~ k.' ~ vector I

December 2004 Simeulue & "~ 58 m m / v r / 7 ....

/ March 2005 Nias ,IL earthquakes Main shocks ~ ~

f

Equator

i �9

BengkulL

. . . . . o

Engganc o 0

........ ............................. 6"S

Fig. 2.4. Movements of sites in Sumatra as determined

by GPS observations during the period 1989-1993 (Prawirodirdjo et al. 1997). Vectors show rates of movement relative to a stable SE Asia. They imply stress

accumulation in parts of the forearc region, some of which would have been released by the June 2000 earthquake near Enggano, the December

2004 earthquake near Simeulue and the March 2005 earthquake near Nias. The locations and mechanisms of these earthquakes are indicated by the centres of the

lower hemisphere projection 'beachballs', from

Abercrombie et al. (2003) for Enggano and from NEIC (2005) for Simeulue and Nias. Locations of aftershocks of the Enggano earthquake for which fault-plane

solutions were calculated by Abercrombie et al. (2003)

are also shown. Major aftershocks to the Simeulue earthquake occurred almost entirely NW of the limits of the map. MS, Muara Siberut. S, Sinabang, PB, Pulau Babi. The pecked grey lines show the locations of

barriers to propagation of ruptures from Great Earthquakes inferred by Newcomb & McCann (1987).

were found to be tightly concentrated in very narrow zones that

changed in dip scarcely at all around the bend (Fig. 2.4). Estimated

depths also tended to be smaller than those based only on

teleseismic data, especially beneath the forearc basin.

A more recent seismological study of the Toba area used a tem-

porary network comprising 30 short-period and 10 broad-band

seismographs deployed for four months in the first half of 1995

(Masturyono et al. 2001). Tomographic methods were used

to define velocity variations beneath the caldera. The results

support the hypothesized existence of two distinct eruptive

centres, one in the south-central part of the lake and the other at

its northern end, which erupted at different times (Knight e t al.

1986). Low velocity zones underlying these two centres and

extending down into the mantle are separated by a region with a

more typical crustal velocity structure.

Relative horizontal movements

The information on present-day tectonic processes in Sumatra pro-

vided by seismology is now being supplemented by geodetic data

from Global Positioning System (GPS) satellites. Repeated

measurements at fixed pillars provide an essential complement

to earthquake studies, which record only episodic, although some-

times very large, displacements. During seismically quiet periods,

GPS measurements monitor aseismic creep and can indicate

regions in which stress is increasing and may be released catastro-

phically at some time in the future. Because of the short time

intervals over which observations are made (typically 3 to 5

years), GPS measurements must always be considered in the

context provided by estimates of long term relative plate motions.

Most of the GPS site markers in Sumatra were established by

BAKOSURTANAL, the Indonesian mapping and geodetic survey

authority, working in collaboration with various US institutes, and

most are located between 2~ and 2~ (Prawirodirdjo et al. 1997;

Genrich e t al. 2000). Additional measurements were made at sites

near Bengkulu and Medan and on Nias and Billiton in the course

of the GEODYSSEA study, which covered the whole of SE Asia.

The GEODYSSEA results defined a 'Sunda' Block that includes

Borneo, the Malay Peninsula and Indochina and moves east rela-

tive to Eurasia at 7 - 1 0 mm a -1 (Chamot-Rooke & Le Pichon

1999; Michel et al. 2001). Billiton Island and Medan are clearly

within this block, as is much of Sumatra east of the Sumatran

Fault, but motions near and to the west of the fault are much

more complex. The main B A K O S U R T A N A L campaign (sites

shown in Fig. 2.4) began in 1989. Detailed analyses of the data

obtained to 1996 in the Central Domain (Fig. 2.1) have been pro-

vided by McCaffrey et al. (2000) and by Genrich e t al. (2000). To

supplement these analyses, Prawirodirdjo et al. (2000) also con-

sidered the results of conventional triangulation surveys extending

over a period of 100 years in the same area. These generally con-

firmed the GPS estimates of 2 0 - 3 0 mm a-1 of dextral movement

SEISMOLOGY & NEOTECTONICS 11

on this portion of the Sumatran Fault, but revealed very consider-

able differences in detail in both movement magnitudes and

directions.

Figure 2.4 shows the site motions relative to SE Asia as

interpreted by Prawirodirdjo et al. (1997) and (also relative to

SE Asia) the averaged long term Indian Ocean movement

vectors (Demets et al. 1990). Strain partitioning was evidently

only partially achieved, at least over the short time interval

involved, nor were movements confined to the main fault

systems. Sites east of the Sumatran Fault but within 50 km of it

were not stationary with respect to SE Asia but recorded small

but significant displacements to the north and NW. Similar pat-

terns near other major strike-slip features have been interpreted

as recording stress accumulations in wide regions of deformed

rock that are ultimately released by faulting (e.g. Armijo et al.

1999).

Sites in the forearc experienced much larger trench-parallel

displacements, but McCaffrey et al. (2000) argued that only

about two-thirds of the necessary slip was accounted for and

that most of the remainder must have been accommodated ocean-

ward of the crest of the forearc ridge. However, the situation

varied considerably from place to place. On forearc islands in

the Central Domain (between the Batu and Banyak Islands) the

trench-normal components were small, suggesting strong parti- tioning of convergent and transcurrent movements, but it seems

that the forearc was largely coupled to the downgoing slab every-

where to the south of the Batu Islands. The boundary between

the two regimes occurs in the region where the Investigator

Fracture Zone enters the trench. Prawirodirdjo et al. (1997) tenta-

tively interpreted the northwestwards decrease in coupling as a consequence of the subduction of thick, water-rich sediments of

the Nicobar Fan, resulting in high pore pressures in the forearc

wedge and weakening of the upper plate by the introduction of

hydrothermal fluids. The change in coupling would thus be due

to the barrier to sediment flow from the NW presented by the

Investigator Fracture Zone, rather than directly to its presence as

an asperity on the lower plate. However, the magnitude of the

December 2004 Simeulue earthquake suggests a 'sticky', rather

than well-lubricated, fault zone.

The combination of gradual change in the orientation of the

Indian Ocean/SE Asia convergence vector and the change in

trench orientation at the Nias Elbow implies almost orthogonal

convergence across the trench in the vicinity of Simeulue

and the Banyak Islands. The Sumatran Fault, however, changes

direction much less noticeably, and the differences in curvature of structures on the mainland and along the forearc ridge

produce a widening and deepening of the forearc basin NW of

Simeulue. Rather surprisingly, the GPS motions of the two sites

in the Banyak Islands were almost perfectly parallel to the trend

of the Sumatran Fault, and so to the trench further south. The

lack of GPS sites on Simeulue means that short-term neotectonic

patterns in this critical area remain, for the moment, undefined.

The data from GPS measurements and triangulation surveys

can be compared with long-term slip estimates based on geolo-

gic and topographic offsets at the Sumatran Fault. Slip rates esti-

mated from stream offsets on SPOT imagery vary from

10 mm a -1 at the Sunda Strait to 23 mm a -1 near Lake Toba

(Bellier & Sebrier 1995). Much of this change occurs in the

Central Domain, where the rates estimated by Sieh & Natawid- jaja (2000) using geological offsets increase from 11 mm a i in

the SE to 27 mm a-1 in the NW. Slip rates estimated from GPS

observations vary much less, increasing by only 4 mm a -1, from 23 mm a-1 to 27 mm a-1, over the same distance (Genrich et al.

2000). Sieh & Natawidjaja (2000) suggested that the geologi-

cally indicated changes in slip rates along the fault must have

developed only during the last 100 ka, because of the absence of compressional accommodation structures, but left the geo-

logical-GPS discrepancy unexplained. They also suggested

that the total slip on the Sumatran Fault might be little more

than the 20 km of the maximum verifiable geological offset,

and that the remainder of the roughly 100 km offset required by stretching in the Sunda Strait might have been accommo-

T r e n c h - o r t h o g o n a l motion T r e n c h - p a r a l l e l motion

1 2

Fig. 2.5. GPS vectors and the Great Earthquake of June 2000. The upper diagram shows overall movement vectors relative to SE Asia and their trench-parallel and trench-orthogonal resolved components. The lower diagrams compare these components individually. Vector 1 is the regional convergence vector, after Demets et al. (1990). The remaining vectors are GPS vectors from the 1991-1993 campaign at sites at the bases of the arrows, after Prawirodirdjo et al. (1997). 'Beachballs' show the locations of the two subevents proposed by Abercrombie et al. (2002) for the June 2000 earthquake.

12 CHAPTER 2

dated by slip on the Mentawai Fault. Their proposed defor-

mation history (which they emphasized was only one of a mul-

titude of possibilities) involved arc-parallel stretching during the

Pleistocene but provided no role for the segment of the Menta- wai Fault north of the Nias Elbow.

GPS data, the Enggano and Simeulue earthquakes and the

Mentawai Fault

During the period covered by published GPS measurements,

the southern forearc islands (Siberut to Enggano) were moving NW relative to Sumatra at roughly the same rate as the underlying

Indian Ocean Plate (Fig. 2.4). Enggano, in particular, participated

in virtually all of the motion of the Indian Ocean during the period

of observation, which unfortunately in this particular case

extended only from 1991 to 1993 (Fig. 2.5). Much smaller relative

motions were recorded at two sites on the adjacent coast of the

mainland and therefore only a small part of the trench-parallel

motion required accommodation further inland, in the vicinity

of the Sumatran Fault. More than half the trench-parallel motion

and an even greater proportion of the trench-normal motion

must have been absorbed between Enggano and the coast, either

at one or more discrete faults or by distributed strain over the width of the forearc basin.

Seismic reflection sections from many parts of the basin

favour localized faulting in the forearc basin, since deformation

of Late Neogene sediments is generally confined to the narrow

zone close to the eastern coasts of the forearc islands which was

named the Mentawai Fault by Diament et al. (1992). However,

the now numerous published images of this feature obtained on

crossings reported by Karig e t al. (1980), Diament et al. (1992)

(Fig. 2.6a), Malod & Kemal (1996) and Schlfiter et al. (2002)

(Fig. 2.6b) and the excellent multichannel imagery obtained

by the Scripps Institution of Oceanography (SIO) south of

Nias (Fig. 2.7), indicate a very complex and variable structure. Considerable uncertainties remain as to its true nature. On some

seismic sections (e.g. Diament et al. 1992) it appears to be a

simple faulted anticline, while in other areas the zone of weakness

has been exploited by shale diapirs which conceal fundamental

structures (Milsom e t al. 1995). The extreme linearity has been

used as an argument for a fundamentally transcurrent role

(Sieh & Natawidjaja 2000) but subsidence of the forearc basin

and elevation of the forearc ridge imply either normal or thrust

components. Where it emerges on land, in southeastern Nias, the

fault was interpreted by Samuel & Harbury (1996) as an originally

extensional fault that has suffered Pliocene to Recent subduction-

related inversion. Significant transcurrent movement was regarded

as improbable. Interestingly, however, seismic section presented

by Schltiter et al. (2002) (Fig. 2.6b) shows the disturbance

as having moved away from the landward side of the forearc

ridge (which is itself fragmented in this region; see Fig. 3.1) to a

/ ' ~ ~ ~ ~ " r (a)

l" i . . . . . . . . �9 2.5

! Seismic sections from Diament et al. (1992) " ~

Fig. 2.6. (a) Interpreted single-channel seismic reflection sections across the Mentawai Fault in the southern part of the Sumatra forearc basin (after Diament et al. 1992). Line locations as shown. (b) Multi-channel seismic reflection section across the Mentawai Fault south of Enggano, after Schltiter et al. (2002). Location shown on (a). The greater penetration achieved on the more recent survey suggests a transcurrent origin for the feature which, in the nearby southernmost single-channel section, appears to be a simple faulted anticline.

SEISMOLOGY & NEOTECTONICS 13

Fig. 2.7. SIO Line 42-43, showing the Mentawai Fault immediately south of Nias. Section provided by Scripps Institution of Oceanography.

position within the forearc basin. This fact, and the image itself, are more compatible with transcurrent than vertical motion. Indeed, Schltiter et al. (2002) suggested that the transcurrent function of the Sumatran Fault might be in the process of shifting to the Mentawai Fault. This is an attractive hypothesis but difficult to reconcile with the suggestion by Sieh & Natawidjaja (2000) that the total offset on the Sumatran Fault is rather small, despite the abundant evidence (including occasional large earthquakes; Untung et al. 1985) for recent and continuing offsets along it.

A further complication is introduced by a possible relationship between the Mentawai Fault and the Batee Fault. The latter is a dextral splay from the Sumatran Fault that trends offshore near the Banyak Islands and was interpreted by Karig et al.

(1980) as displacing or terminating the Mentawai Fault near Nias (Fig. 2.1). The Mentawai Fault is often shown as either ending near Nias (e.g. Diament et al. 1992) or merging with the Batee Fault, but a very strong gravity gradient indicates a major structural discontinuity between the two westernmost islands in the Banyak group (see Fig. 3.5). This is roughly the position where a Mentawai Fault continuation would be expected if the Batee Fault were not present. Moreover, the existence of Mentawai-type structures still further north has been confirmed by Izart et al. (1994) and by Malod & Kemal (1996) using single-channel reflection data.

Additional insights into the role of the Mentawai Fault in the Enggano area were provided in June 2000 by an Mw 7.9 earth- quake followed by a train of strong aftershocks (Fig. 2.5). P and S wave studies of the primary event suggested that this comprised two subevents, involving strike-slip within the Indian Ocean Plate followed by thrust motion on the subduction fault (Abercrombie et al. 2003). The events were too deep, and in the wrong plate, to be due to failure on the Mentawai Fault, but they do provide important data on its relationship to the transition between the accretionary wedge and the continental margin. Matson & Moore (1992) suggested that this transition occurs near the east coast of Nias in the Central Domain and that the subduction fault originally reached the surface in this area. Its subsequent migration oceanwards was interpreted as a conse- quence of the development of the accretionary wedge that now forms the forearc ridge. This is consistent with the Malod & Kemal (1996) interpretation of the Mentawai Fault along its entire length as marking the transition between the wedge and a rigid backstop of pre-existing basement. On this hypothesis,

the linearity of the fault is a consequence of the linearity of the original subduction trace, which would, in turn, have been con- trolled by the linearity of the former passive margin.

The significance of the Enggano composite earthquake to the backstop concept is that the GPS results shown in Figure 2.5 indi- cate that in this area, and possibly only for short periods, the accre- tionary wedge moves with the subducting plate and must therefore compress against the backstop, resulting in folding and reverse faulting. Potential energy stored in this folded and faulted zone can be released in large earthquakes in which the wedge moves oceanwards and deformation near the backstop is reversed. Pre- sumably such reversals are only partial, so that deformation gradu- ally increases. At no point in this stick-slip cycle would large earthquakes necessarily occur within the wedge, because accreted material is usually too weak to sustain large local stress. Large earthquakes will therefore be associated principally with the unsticking of the wedge from the backstop or from the downgoing slab along the main subduction thrust and with relative lateral movement between locked and unlocked segments of the forearc. Events of both types appear to have occurred in June 2000, with the movement between segments of the Indian Ocean plate increasing the stress and triggering failure along the subduction thrust (Abercrombie et al. 2003).

The results of future GPS measurements in the Enggano-Beng- kulu area (there have, unfortunately, been no measurements on Enggano since the earthquake) are thus likely to be very different from those obtained between 1991 and 1993. Amongst other things, they can be expected to provide insights into the highly controversial question of the extent to which trench-parallel motion is accommodated by the Mentawai Fault. It seems prob- able that the new vectors will resemble the vectors shown in Figure 2.4 for the islands north of Siberut, i.e. they will show almost entirely trench-parallel motion, implying a primarily trans- current long-term function. The characteristics of both the main earthquake and the extensive aftershock sequence suggest that effects of the Enggano Great Earthquake are unlikely to be seen in the forearc north of Bengkulu (Abercrombie et al. 2003), and in fact no such effects have been observed in post-earthquake GPS studies in the Central Domain (Bock et al. 2003). If this is the case, then dangerous levels of stress must be accumulating in the region from South Pagai to Siberut.

The June 2000 Enggano earthquake was completely oversha- dowed by the December 2004 Simeulue event, information on which was posted on the National Earthquake Information

14 CHAPTER 2

Center website within a few days (NEIC 2005). The suggested

maximum displacement was 15 m, in a region where convergence

is more nearly orthogonal to the trench than it is further south (see

Figs 2.1 and 2.4). Bizarrely, in view of this latter fact, the results

from the only GPS site NW of the change of strike, on Pulau Babi

(PB on Fig. 2.4), suggest that during the 1989-1993 period the

forearc moved slightly further in a direction parallel to the

trench than did the Indian Ocean, the supposed driver of the

forearc motion. It also seems that about half of the Indian Ocean

trench-normal motion was accommodated between Pulau Babi

and Sumatra, which is less than at Enggano, but much more

than predicted by simple sliver-plate models. The motion of

Simeulue, a few tens of kilometres to the NW, might, of course,

have been different but there is no bathymetric or other evidence

for the placement by NEIC (2005) of an extensional (or any other)

boundary to a 'Burma Plate' immediately east of Pulau Babi.

Fault-plane solutions for the Simeulue earthquake are consist-

ent with either SW-directed thrusting dipping at about 10 ~ to the

NE or NE-directed reverse faulting dipping at about 80 ~ (NEIC

2005). The first of these is much the more likely, but thrusting

on a surface so nearly horizontal, when the Benioff Zone dips at

about 30 ~ in the vicinity of the hypocentre, raises some questions.

The Harvard Centroid Moment Tensor solution, however, places

the centroid west of the forearc ridge and beneath the eastern

wall of the trench (at 3.09~ 94.26~ cf. the NEIC epicentre at

3.30~ 95.96E~ Since, subject to errors introduced by faulty vel-

ocity models, hypocentres correspond to points of rupture intiation

whereas centroids represent weighted average locations of

moment release (Meredith Nettles pers. comm. 2005), the results

can be interpreted as describing an event initiated in the vicinity of the Mentawai Fault and propagating oceanwards and also

NW along the forearc. The complexity of stress patterns in the epi-

central area is indicated by the multiplicity of previous smaller

shocks, some of which had strike-slip solutions and others sol-

utions similar to that of the December 2004 event (see Newcomb & McCann 1987, Fig. 2). The fact that the region

around the Mentawai Fault appears to respond to stress in different

ways at different places and at different times is consistent with the

fault itself being the expression of a fundamental geological dis-

continuity rather than a simple break through an essentially homo-

geneous rock mass.

The Simeulue event also spectacularly confirmed the extreme

segmentation of the forearc. Aftershocks occurred along 1200

km of the arc, from the site of the main shock as far as the northern tip of the Andamans, but there was virtually no activity to the SE

(NEIC 2005). The bathymetric high northwest of Simeulue where

the epicentre was located may therefore be the surface expression

of a discontinuity similar to those associated with the Banyak and

Batu highs further south. The extents of Great Earthquake ruptures

are strongly correlated with the extents of deep marine basins

between Sumatra and the forearc ridge and, given that the NW

limit of the rupture zone of the 1861 event was not at Simeulue

but at the Banyak Islands (Newcomb & McCann 1987), it seems

possible that stress is still building up in a 'Simeulue Basin'

segment, to be catastrophically released at some time in the not

too distant future.

Vertical movements

It is more difficult to monitor vertical movements with GPS than

horizontal movements, both because of the generally smaller

displacements and because the accuracy is inherently lower.

At present, more reliable estimates of rates of vertical motion

are being obtained by observing short-term changes in relative

sea level. Natawidjaja et al. (2000) studied the submergence and

emergence of corals and deduced a pattern of progressive

landward tilting of the forearc ridge, with uplift within about

115 km of the trench axis and subsidence at all greater distances.

Instantaneous vertical movements of tens of centimetres associ-

ated with large earthquakes were superimposed on this pattern. Individual islands in the northern part of the forearc often record

similar tilting. Islets shown on Dutch colonial maps as protecting

Sinabang harbour, at the eastern end of the north coast of Simeulue

(S in Fig. 2.4), are now permanently submerged, and palm trees

are dying along much of the coast as salt water invades the soil

around their roots. Muara Siberut, the main town on Siberut (MS on Fig. 2.4), is regularly flooded at high tide and some

nearby offshore 'islands' consist entirely of mangroves with

their roots submerged even at low tide.

On Nias the situation is more complicated, since the west coast

can be divided into two very different sectors. In the north the

coastal region is flat and swampy and the beach is broad and

gently sloping, but in the south there are cliffs 50-100 m high

and the sea floor shelves steeply. This section of the coastline is

concave seawards and appears to be a scarp created by failure of

an unstable slope (see Fig. 2.1). The relatively low gravity field

along the coast and offshore (see Fig. 3.5) suggests loss of mass

from this region and also supports the concept of failure of a

slope that has been uplifted to unsustainable elevations. On

the opposite (eastern) side of the island, rivers have been incised

in narrow valleys to depths of 5 - 1 0 m within a broad coastal

plain east of the Mentawai Fault, suggesting recent and rapid

uplift, but further north there is evidence of both uplift and

subsidence. The uplift of the coastal plain on Nias could have been associ-

ated with great earthquakes. Zachariasen et al. (1999) interpreted

the results of a detailed study of coral heads exposed around

the Mentawai Islands of Sipora and North and South Pagai, south of Siberut, as recording aseismic subsidence followed by

co-seismic uplift related to the great earthquake of 1833. In this

area, and in contrast to areas further north, both aseismic and

co-seismic movements appear to have involved tilting towards

the trench. Deducing long-term regional displacement patterns

from measurements of movements over a few years, or even

over tens of years, is clearly never going to be a simple exercise.

Note added in proof

The earthquake activity in the central Sumatra forearc between 26 December 2004 and the end of April 2005 is summarized in Figure

2.8. The first four plots show how the seismicity associated with the 26 December event gradually died away during the succeeding

three months. It is clear that even as late as March 2005, the

majority of events were part of the aftershock sequence.

However, on 28 March 2005 there was a further Great Earthquake,

with an epicentre just west of the Banyak Islands and an estimated magnitude of 8.6. The distribution of aftershocks to this event indi-

cated that rupture extended throughout the whole of the region

between the Banyaks and the December 26 epicentre. It was, in

fact, being quite widely predicted in the first few months of

2005 that this would be where the next break would occur.

However, and unexpectedly, the zone of aftershocks also extended

south as far as the Batu Islands (Fig. 2.8e). It seems therefore that

not only had the last remaining segment that had no historic record

of Great Earthquakes failed, but that the segment that ruptured in

1861 moved with it. Fault plane solutions by both the NEIC and the Harvard group

indicated a shallow thrust, at an even smaller angle of dip than

had been the case the previous December. Once again, movement

seems to have been initiated close to where the Mentawai Fault

(assumed to be near vertical) would reach the subduction fault at

depth, and once again there was a significant displacement

between the calculated positions of the epicentre and the centroid.

In this case, however, the centroid lay south rather than west of the

SEISMOLOGY & NEOTECTONICS 15

6 ~ ~ ~lc ~ : December 26 - Decemb'er 31, 2004 I

o O ~ O - o o-,\ , o ~ %., ~ ...... ~I

, o . F2" ( ~ ) .....

o -w=. \ I

0o : : ; : ..................................................... ............ .....

a : % :

. . . . ~ o " - , a,,,i,,,,~., 20osl

. o \ , ........... ...............

. . . . . . . . . . . . . . . . . . . . . . . . . . .

o o % ~ ............... ~ .............. ........... \ \ ............. : February, 2005

0 .................. . ~ ................. - ~ , . , ........

. . . . . . . , ~' . . . . . . .................... .................. ............ ............... + .................. ~ . 3

. . . . . . . . . . . . . . . . . . . .

~ ~ ~ March ! - March 27. 2005

o .~ , . ~ o

. . . . . . . . . . O ~ . . . . . . . . . . . R .................. ? ....... ~ ,'4"N

' " "l

. . . . . . . . . . . . . . . . . . . . . . . f ........................... " 2

. . . . . . C) ~ . . . . . ~ .............. Apr i l 10 - April;30, 2005

d

2 . . . . . . . . . . . . . . . , ......................... ~ , . . . . ; ............ ; . : ........ i o~

' ' . . . . o' o .................................. 2~

I ~) ~ ~ ...... ~ .................... , ,~ , ~ ~: ,

Fig. 2.8. Central Sumatra seismicity,

December 26 2004 to April 30 2005. Epicentres plotted from catalogues available

on the Internet from http:// www.ngdc.noaa.gov. Note that the periods

covered (shown in the top fight hand corner of each diagram) are not of uniform length,

being dictated in part by the dates of initiation of significant earthquake swarms.

The circles corresponding to the NEIC epicentres of the two Great Earthquakes (in plots a and e) are shaded and the locations of

the centroids of their Harvard CMT solutions are indicated by fault-plane

solution 'beachballs'.

epicentre, and was still a considerable distance from the trench.

Also, and as might have been expected in view of the smaller mag-

nitude of the shock, and hence the probable smaller width of the

slip zone, the displacement between centroid and hypocentre

was considerably less than in December. The greater distance of

the centroid from the trench, together with the smaller magnitude,

may be sufficient explanation for the much smaller associated

tsunami, which was only about 3 metres high on exposed coasts

of Nias and Simeulue and decreased rapidly in amplitude at

more remote locations. It is also possible that submarine slides,

which may have contributed to the destructive power the Decem-

ber wave, did not occur in March because of the absence of any

remaining potentially unstable slopes. The aftershock sequence

(Figures 2.8e and f) was notable for being much more tightly con-

strained to the region immediately beneath the forearc ridge than

had been the case following the December event.

A new train of events began still further south and just seaward

of Muara Siberut in the following weeks. There were a few rela-

tively weak shocks in this area in the period immediately after

March 28 (Figure 28e), but the first major event (Mw=6.7) took

place on April 10, and was followed three quarters of an hour

later by another strong (Mw=6.5) shock. Once again, the Menta-

wai Fault appears to have controlled the location at which

failure was initiated. Both events were compressional but, in con-

trast to the two Great Earthquakes, the slip planes were much

steeper (from 30 ~ to 60~ There followed numerous weaker

events in the same area but, again in contrast to the pattern associ-

ated with the Great Earthquakes, there was no significant rupture

propagation (Fig. 28f). It is to be hoped that the earthquakes in

this isolated cluster will prove to be the last major events in the

current phase of southward-propagating unzipping of subduction

west of Sumatra.

Chapter 3

The gravity field

JOHN MILSOM & ADRIAN WALKER

Data sources

The gravity field of Sumatra and the surrounding marine areas is

shown in Figure 3.1. Contours in the onshore area of Bouguer

gravity, but offshore are of free-air gravity. Terrain corrections

have not been applied. Although marine gravity measurements

have been made in the forearc basin and elsewhere on a number

of research cruises (e.g. Kieckhefer et al. 1981), the data from

these generally widely spaced lines have not been used in pre-

paring the maps because free-air gravity values obtained from

inversion of satellite radar altimetry provide more systematic

coverage and can resolve anomalies with widths of as little as

7 km (Sandwell & Smith 1997). The onshore and satellite-

derived offshore data were matched at coastlines without undue

difficulty, as should be the case because both free-air and

Bouguer corrections are zero at sea level. However, gradients

tend to be steep at the coasts in the forearc region, partly

because of the change from free-air gravity, which is strongly

correlated with local bathymetry, to Bouguer gravity, which is cor-

rected for local topography. Figure 3.2 shows the locations of the onshore stations used in

preparing Figure 3.1, but not of the offshore estimates, distributed

on a regular 2 minute grid. Onshore data were obtained from a

variety of sources, but unfortunately the results of the many detailed gravity surveys carried out by oil companies remain

confidential. The largest single available data set was assembled

as part of the collaboration between the British Geological

Survey (BGS) and the Geological Research and Development

Centre (GRDC) during the period 1988-1995. Almost all of

Sumatra south of the equator was covered at a reconnaissance

level, although there are significant gaps in a few areas where

access would have been especially difficult. In addition to the

Sumatra mainland, measurements were made on Bangka and

Billiton islands in the northeast and the Mentawai islands in the

west (Fig. 3.2). GRDC have published numerous Bouguer maps at 1:250 000 scale showing contours, generally at 2 mGal

intervals, and station locations. There are also two summary

maps at 1 000 000 scale (Padang and Palembang sheets), con-

toured at 5 mGal intervals and without station positions. Terrain

corrections, of up to 12 mGal, were applied in preparing the

summary maps but were not used for any of the 1:250 000 detailed maps. The two versions of Bouguer gravity are therefore slightly

different in the mountainous areas close to the Sumatran Fault

but gradients in these areas are in any case steep, and overall

patterns are very similar. Coverage north of the equator, principally by GRDC and

LEMIGAS (the Indonesian Petroleum Research Institute), is less

complete than in the south but is progressing rapidly. Moreover,

Japanese universities working between 1977 and 1979 obtained

data along many of the more important roads in the Lake Toba

area (Fig. 3.2). In the northern forearc LEMIGAS collaborated with the University of London in surveys of all of the major

islands (Milsom et al. 1991). Stations were mainly along the

coasts, except on Nias. LEMIGAS/UofL stations on Siberut

were restricted to the southeastern corner, but the island was

subsequently covered at a reconnaissance level by GRDC.

In 1991 and 1992, stations were established along major roads

throughout Sumatra by BAKOSURTANAL, the Indonesian

geodetic survey authority. A map showing the locations of the

BAKOSURTANAL stations and Bouguer gravity contours after

the application of a severe high-cut filter has been circulated

on a very limited basis, but these stations are not included in

Figure 3.2. An unfiltered but very small scale version of the

BAKOSURTANAL Bouguer map was published by Kadir et al.

(1996), and the data may also have been used by GRDC in pre-

paring the 1:10000000 Bouguer anomaly map of Indonesia

(Sobari et at. 1993). BAKOSURTANAL Bouguer values around

the Toba caldera are generally 10-20mGal higher than those

reported by the Japanese groups, a difference probably due to

the lack of terrain corrections in the Japanese work.

The onshore contours in Figure 3.1 are based on actual point

gravity data where available, supplemented where necessary by

values estimated at known BAKOSURTANAL station positions

using the contours of Kadir et al. (1996). Accuracy is inevitably

low where this has been done, and even so some significant gaps

remain. The problem of making full use of good regional coverage

where this exists and at the same time displaying in an acceptable way the results of interpolation across larger gaps has been

addressed by overlaying the map based on a relatively fine

(0.1 ~ grid, which is blank in areas of inadequate coverage, on a

map produced using a much coarser grid and a greater degree of

interpolation. This is obviously unsatisfactory as a quantitative method, but Figure 3.1 is intended to be used only qualitatively

and the general patterns can be considered sufficiently well estab-

lished to support regional interpretation. It is just possible on

Figure 3.1 to identify discontinuities in the colour patterns at the

edges of areas where the coarse grid has been used.

Extending Figure 3.1 to include Billiton has brought western

Java within the boundaries of the map. The data used were

obtained in 1970 by the BGS, working in conjunction with the

Geological Survey of Indonesia. The results of recent more

detailed work on Java by GRDC are not shown but are generally

compatible with the BGS survey.

Regional gravity patterns

The most prominent features in Figure 3.1 are offshore. Gravity

highs with north-south or N N E - S S W trends are associated

with fracture zones and seamount chains on the Indian Ocean

Plate and these control the positions of individual culminations

on the broad flexural high at the outer margin of the Sumatra

Trench. Two deep NW-SE-trending free-air lows, associated

respectively with the trench and the forearc basin, intervene

between this oceanic domain and the Sumatran mainland and

are separated from each other by a high along the forearc ridge.

The low over the trench exists because the mass deficit of the

water column is not in local isostatic equilibrium but is balanced

elastically by the offset mass of the subducting slab.

Although the available gravity coverage is much less complete

north of the equator than in the south, there can be no doubting

the existence of fundamental differences between SE and NW

Sumatra. In the south the Barisan mountains are associated with

a narrow, discontinuous and rather weak Bouguer low that,

where it exists, coincides quite precisely with the axis of the

mountain range, but in the north the low deepens and expands to

GRAVITY FIELD 17

Fig. 3.1. The gravity field of Sumatra and the surrounding seas, based on data from sources discussed in the text. Contours are of free-air gravity offshore and Bouguer

gravity onshore. The Bouguer reduction density is 2.67 Mg m -3. Faint white contours are bathymetry, at 200 m and at intervals of 500 m thereafter, from the GEBCO digital

atlas prepared by the British Oceanographic Data Centre. The continuous black line running the length of Sumatra marks the approximate surface trace of the Sumatran Fault.

The yellow line crossing the forearc basin near the equator marks the location of the interpreted profile of Figure 3.6. The black outlines enclosing the letters 'O' and 'B'

indicate the locations of the gravity surveys of the Ombilin and Bengkulu basins shown in Figures 3.3 and 3.4 respectively. The letter B also indicates the approximate

position of the town of Bengkulu. TS and T indicate, respectively, Lake Toba (including Samosir Island) and Lake Tawar. The letters 'IFZ' at about 97 ~ 30'E mark the central

trough of the Investigator Fracture Zone. The inset shows the GEM-T3 long wavelength gravity field in the Sumatra region (see Lerch et al. 1994).