Dubai Seismic Hazard Investigation

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Probabilistic seismic hazard analysis for rock sites in the cities of AbuDhabi, Dubai and Ra's Al Khaymah, United Arab EmiratesG. Aldama-Bustosa; J. J. Bommera; C. H. Fentona; P. J. Stafforda

a Department of Civil & Environmental Engineering, Imperial College London, South Kensingtoncampus, London, UK

To cite this Article Aldama-Bustos, G. , Bommer, J. J. , Fenton, C. H. and Stafford, P. J.(2009) 'Probabilistic seismic hazardanalysis for rock sites in the cities of Abu Dhabi, Dubai and Ra's Al Khaymah, United Arab Emirates', Georisk:Assessment and Management of Risk for Engineered Systems and Geohazards, 3: 1, 1 — 29To link to this Article: DOI: 10.1080/17499510802331363URL: http://dx.doi.org/10.1080/17499510802331363

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Probabilistic seismic hazard analysis for rock sites in the cities of Abu Dhabi, Dubai and Ra’s Al

Khaymah, United Arab Emirates

G. Aldama-Bustos, J. J. Bommer*, C. H. Fenton and P. J. Stafford

Department of Civil & Environmental Engineering, Imperial College London, South Kensington campus, London, UK

(Received 18 December 2007; final version received 8 July 2008)

The United Arab Emirates (UAE) is undergoing very rapid development with one of the highest construction

rates in the world. A number of studies of the seismic hazard in the UAE have been published in recent years,presenting diverse interpretations of the earthquake threat in this country of relatively low local seismicity,creating confusion regarding appropriate seismic design levels. Although there is inevitably considerableuncertainty associated with the assessment of seismic hazard in such a region, those studies indicating rather high

levels of ground motion associated with a 475-year return period are found to be the result of inappropriateseismic source zonations that spread seismicity from the Zagros region of Iran into the Arabian Peninsula. A newprobabilistic seismic hazard analysis is performed within a logic-tree framework, and the results displayed as

uniform hazard spectra for rock sites in the cities of Abu Dhabi, Dubai and Ra’s Al Khaymah in the UAE. Theresults support the UBC 1997 classification of the two former cities in Zone 0 (no seismic design required)whereas in Ra’s Al Khaymah Zone 1 would be appropriate.

Keywords: United Arab Emirates; seismic hazard; earthquakes; uniform hazard spectra; Abu Dhabi;

Dubai; Ra’s Al Khaymah

Introduction

The United Arab Emirates (UAE) has been under-

going accelerated development in recent years, and

the UAE currently has one of the highest rates of

construction in the world. During the period from

2000 to 2004, the construction sector GDP in the

UAE increased by 166%, with an annual growth rate

of 27.7% (DCCI 2006). The main focus of construc-

tion activity is Dubai, where almost half of the

construction in the UAE is centred. Together with

Abu Dhabi, Dubai accounts for more than 60% of

the real estate market of the Gulf Cooperation

Council. One of the clearest indications of the

remarkable level of construction activity in Dubai is

the fact that of the estimated 125,000 travelling

cranes in operation worldwide, between 15 and

25% are working in Dubai (Landais 2006, Nicolson

2007).Clearly, if there is an appreciable level of seismic

hazard in the UAE, it would be important for

seismic actions to be considered in the design of

structures built in this country, but the available

information on this issue is contradictory. The

clearest example of this is the fact that for buildings

of 5 storeys or more in height, the Dubai Munici-

pality requires design to the seismic loads specified

for Zone 2A of the 1997 edition of the UniformBuilding Code, UBC97 (ICBO, 1997). Interestingly,UBC97 itself specifies Zone 0 � which implies thatno seismic design is required � for both Dubai and

Abu Dhabi; Ra’s Al Khaymah is not classified in thecode. For rock sites, Zone 0 in UBC97 correspondsto a horizontal peak ground acceleration (PGA)significantly lower than 0.075g, which is the value

assigned to Zone 1.There are at least seven studies, two published in

the 1990s and covering most of the ArabianPeninsula and five specifically for the UAE pub-lished in recent years, which have presented seismic

hazard estimates for the Emirates. These studieshave drawn very different conclusions regarding thelevel of seismic hazard in the UAE, with estimates of

the 475-year peak ground acceleration (PGA) inDubai ranging from less than 0.05g to 0.16g.Although even the larger value is not particularlysevere, the cost implications of the consequent

seismic design requirements, especially for high-riseconstruction that is prevalent in the UAE, aresignificant.

The purpose of this study is to obtain a reliableestimate of the seismic hazard in the Emirates,

expressed as uniform hazard spectra (UHS) on rocksites in three key cities in the country: Abu Dhabi,

*Corresponding author. T: �44-20-7594-5984, F: �44-20-7594-5934, Email: [email protected]

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Dubai and Ra’s Al Khaymah. The paper starts withan overview of the tectonic setting of the UAE,including major active structures in the regionproducing earthquakes that could affect the Emi-rates. The local and regional seismicity is alsoreviewed within the context of compiling an earth-quake catalogue to form the basis of the probabilisticseismic hazard analysis (PSHA) performed withinthis study. This review includes discussion of someminor earthquakes reported in the UAE during thelast few years. In the light of the tectonic setting andregional seismicity, published hazard studies for theUAE are then reviewed in the third section of thepaper, with particular focus on the source zonationsthat have been used and also the ground-motionprediction equations (GMPE) that they employed.

The fourth section of the paper then describes thenew PSHA performed for the three selected sites inthe UAE, including disaggregation to identify thesource zones contributing most significantly to thehazard. The fifth section then discusses the results interms of PGA values and UHS, and how thesecompare to previous hazard estimates and to theelastic design spectra from UBC97. The paper closeswith brief conclusions regarding recommendationsfor seismic design in the UAE and for additionalstudies regarding potential earthquake hazard in theEmirates.

Regional tectonics and seismicity

Given the divergent stances that previous researchershave adopted when developing seismic source modelsfor hazard analyses within the UAE it is important tobegin by independently outlining the findings of thepublished literature on the regional tectonics andseismicity of the UAE and surrounding areas.

Tectonic setting of the UAE

The UAE is situated in the northeast of the Arabianplate which is bounded by a series of well definedtectonic margins. The regional tectonic setting isportrayed in Figure 1. In the southeast, the Africanand Arabian plates diverge across the Gulf of Adenwhile the Red Sea spreading boundary defines theinterface between these two plates in the southwest(Johnson 1998, Vita-Finzi 2001). In the northwest,the Dead Sea transform faults skirt the Mediterra-nean Sea and run through to the Taurus Mountainsat the east of the Turkish plate (Vita-Finzi 2001). Thenorthern margin of the Arabian plate is definedprincipally by the Zagros thrust and fold belt thatterminates to the north of the eastern limit of thePersian Gulf (Jackson and McKenzie 1984). The

remainder of the north-eastern margin of the Arabianplate is defined by the Makran subduction zonewhere the Arabian plate subducts beneath the Eur-asian plate (Farhoudi and Karig 1977, Bayer et al.2006). The final boundary defining the Arabian plateis the Owen fracture zone that is a transformboundary separating the Indian and Arabian platesin the east (Johnson 1998, Vita-Finzi 2001, Fournieret al. 2007). Aside from these major boundaries theArabian plate is a stable landmass that does notexhibit any discernable trace of interior deformationduring the late Tertiary (Al Kadhi and Hancock1980, Vita-Finzi 2001). The interior of the Arabianplate is also not known to have experienced anysignificant seismic events over the past 2000 years orso (Reches and Schubert 1987, Vita-Finzi 2001) andmay be regarded as a stable cratonic region (Johnsonet al. 1994, Fenton et al. 2006).

At longitudes near the UAE, the Arabian plate iscurrently moving northwards (on a bearing of N88958E) at a rate of approximately 2292 mm/year withrespect to the Eurasian plate (Vernant et al. 2004).This convergence is accommodated by a combinationof intra-continental shortening throughout Iran (overthe Zagros fold and thrust belt) and by the subduc-tion of the Arabian plate beneath the Eurasian plateeastwards of about 588E (the Makran subductionzone in the north of the Gulf of Oman) (Farhoudiand Karig 1977, Bayer et al. 2006).

Although the boundaries outlined above definethe regional tectonic setting for the Arabian plate, themajority of the mentioned margins are too distant tobe significant contributors to the seismic hazard atsites within the UAE. However, Figure 1 alsoindicates the presence of some active tectonic struc-tures in the Oman Mountains close to the UAE(Johnson 1998). The deformation associated with thismountain range in addition to the boundaries of theZagros fold and thrust belt and the Makran subduc-tion zone constitute the major seismic sources that arelikely to influence the seismic hazard for sites withinthe UAE.

Understanding the regional tectonic setting allowsone to identify zones within which certain modes ofdeformation are to be expected. However, for thepurposes of compiling a seismic source model for usewithin a PSHA it is necessary to consider a higherlevel of resolution and to identify individual faultingstructures that have the potential to generate earth-quakes that may affect sites within the UAE.Unfortunately, the published literature on the geolo-gical features of the UAE and the north-easternArabian Peninsula are relatively scarce. This scarcityof information makes it difficult to constrain thenature of structural features in the Oman Mountains

2 G. Aldama-Bustos et al.


and within the Arabian plate at locations close to the

sites of interest for this study. In contrast, there are

numerous publications related to the specific struc-

tural features of southern Iran; in particular the

Zagros and Makran regions (e.g., Berberian 1995,

Hessami et al. 2003, Farhoudi and Karig 1977). The

seismotectonic characteristics of each of these three

key regions is briefly described in the following.

The Zagros fold and thrust beltThe NW-SE trending Zagros fold and thrust belt

extends for a distance of more that 1500 km from

eastern Turkey to the Zendan-Minab fault system of

southern Iran (Jackson and McKenzie 1984, Berber-

ian 1995, Hessami et al. 2003). Deformation com-

menced during the Pliocene and the region is

currently undergoing approximately 10 mm/year

shortening in the southeast and 5 mm/year in the

northwest as a result of the continental collision

between the Arabian and Eurasian plates (Jackson

and McKenzie 1984, Berberian 1995, Allen et al.

2004, Vernant et al. 2004).

Owing to the presence of several ductile sedimen-

tary layers in the Zagros, decoupling of the Phaner-

ozoic cover from the Precambrian metamorphic

basement has occurred along the Lower Cambrian

Hormoz Salt and above the Eocene-Oligocene

Asmari Limestone along the Miocene Gachsaran

Evaporites (Berberian 1995). Consequently, large-

magnitude earthquakes tend not to rupture the

near-surface deposits in the Zagros. Rather, the 6�15 km thick Phanerozoic sedimentary cover is folded,

producing active anticlinal uplift and synclinal sub-

sidence (Berberian 1995). This surface deformation

may be used to infer the locations of the dominant

blind thrust faults underlying the sedimentary cover

but there will necessarily be a large degree of

uncertainty associated with any such inferences.The Zagros region has been responsible for the

generation of large Ms�7 earthquakes in the past.

However, events of this magnitude are likely to be

approaching the maximum seismogenic potential of

the blind faults that characterise this region. Most of

the seismicity in the folded belt occurs on reverse

faults in the basement (dipping 408�508 and with NW

Figure 1. Cenozoic tectonic setting of the Arabian plate; from Johnson (1998). Inset, from Lippard et al. (1982), shows thelocation of the Dibba Zone in Oman.

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to WNW trending strikes) and with slip vectorstypically oriented with azimuths of 308�408 (Jacksonand McKenzie 1984, Hessami et al. 2003). Teleseismicbody-wave modelling demonstrates that most of thelarger earthquakes to have occurred in this regionnucleated at depths of 10�20 km, below the sedimen-tary layers (Jackson and Fitch 1981, Jackson andMcKenzie 1984). There is no evidence for subcrustal(focal depths greater than �30 km) seismicity in theZagros (Jackson and McKenzie 1984).

Berberian (1995) discusses the Zagros fold andthrust belt in detail and should be consulted forfurther details regarding this seismotectonic domain.However, for the purposes of the present study itsuffices to state that this domain consists of a series ofmajor blind thrust faults with significant seismogenicpotential interspersed with regions of distributedseismicity associated with numerous blind thrustsand associated folding. As the precise locations ofthe blind faults are unknown it is prudent to regardthe various subdomains of the Zagros as arealsources.

The Makran subduction zoneThe Makran region, bounding southern Pakistan andsouth-eastern Iran, is a 1000 km section of theEurasia-Arabian plate boundary where northwardsubduction of the oceanic Arabian crust has occurredcontinuously since the Early Cretaceous (Byrneand Sykes 1992). There is some uncertainty regardingthe rate of subduction in the Makran with Vernantet al. (2004) estimating a value in the range of 19.592to 2792 mm/year; the latter value being the relativevelocity of the Arabian margin of the Gulf of Omanwith respect to Eurasia. The Makran subduction zoneis unusual in that the zone has no recognizedbathymetric trench and the eastern and westernhalves of the zone exhibit very different patterns ofseismicity. These patterns of seismicity, with the eastand west having historical records with and withoutgreat events respectively, suggest a possible segmen-tation of the subduction zone. Such segmentation isfurther suggested by the offsets in the volcanic arcand by the large-scale two-block structure of theoverriding plate (the Lut and Helmand blocksdiscussed by Byrne and Sykes 1992). The boundarybetween the segments appears to occur near 618E,coincident with the Sistan suture zone (Byrne andSykes 1992). On the other hand, many geologic andtectonic features show no segmentation along theMakran: the margin remains nearly straight for itsentire 1000 km length; present marine geophysicaldata show no significant offsets anywhere offshorealong the margin; and the age of the subducting plate

lies between approximately 70 and 100 Ma along theentire arc (Quittmeyer 1979, Byrne and Sykes 1992).

The largest earthquake to have been recorded inthis region was an event in 1945 with a surface-wavemagnitude, Ms, of 8.0 (Quittmeyer and Jacob 1979).The distribution of intensities and the long-termaftershock activity suggest that the length of therupture zone was between 100 and 200 km, and thatthe rupture propagated to the east of the epicentre(Byrne and Sykes 1992, Quittmeyer 1979). Usingteleseismic recordings, Quittmeyer (1979) and Jacoband Quittmeyer (1979) define a shallow dippingseismic zone � at about �68 according to Byrneand Sykes (1992) � that extends to depths of about80 km just south of the volcanic arc of the over-ridingblocks and locate the trench about 150 km south ofthe Iranian coastline (at a latitude of �248N). Thedip increases to approximately 198 beyond a latitudeof 26.58N. Byrne and Sykes (1992) suggest anaseismic zone at the toe of the subducting plate thatis also corroborated through plotting the EHBseismicity catalogue (Engdahl et al. 1998) updatedto include events to 2004 (Engdahl, personal com-munication 2006). The down-dip extent of theprimary seismogenic portion of the Makran subduc-tion zone is uncertain. The seismicity distributionsuggests that the majority of activity occurs over theuppermost half of the shallow-dipping portion of thesubducting plate. However, the presence of at leasttwo events beyond this zone in addition to seismo-genic depths observed in other subduction zonesthroughout the world (Tichelaar and Ruff 1993)suggest that this interface may extend down fartherto where the dip of the subducting plate steepens,thus increasing the seismogenic potential of thissource.

Although great earthquakes have not been ob-served in the western Makran, evidence of coseismicdeformation in this region exists in the form of asequence of uplifted coastal terraces (Page et al. 1978,Quittmeyer 1979). It is therefore possible that theobserved segmentation of the Makran is simply aresult of a limited period of observation and thatfuture large events may occur in this region. Whetheror not the Makran subduction zone is segmentedclearly has a significant influence upon the seismo-genic potential of the zone.

Complex faulting occurs in the region where theZagros fold and thrust belt and the Makran subduc-tion zone meet. The faults in this region thataccommodate this significant change in tectonicregime are known as the Zendan-Minab fault system.This system of north to northwest trending faultsplays two major roles: (1) to accommodate theoblique plate convergence, and (2) to transform



Zagros collision processes into the Makran subduc-tion (Regard et al. 2004, 2005). This deformation isnot solely accommodated through the Zendan-Minabsystem as a contribution is also made by the moreeasterly located Sabzevaran-Jiroft Fault system (Re-gard et al. 2005). Being sympathetic fault systems, theseismogenic potentials of these faults are not as greatas those comprising the major margins of the Zagrosand the Makran. However, their proximity to siteswithin the UAE (particularly the northern UAE)means that these fault systems will inevitably con-tribute to estimates of seismic hazard and musttherefore be included within any seismic source modeldeveloped for the region. Moderate-to-large eventsare known to have occurred within these faultsystems in the past and are inevitable given thedifferent modes of deformation that exist to the eastand west of the region.

Oman Mountains and the Dibba LineThe Oman Mountains, also known as the HajarMountains, are located along the north-easternmargin of the Arabian plate, in northern Oman (seeFigure 1). They reach over 3 km in height and exhibitmany features consistent with active tectonics (Kuskyet al. 2005). The present height and ruggedness of theOman Mountains is a product of Cretaceous ophio-lite obduction, Tertiary extension, and rejuvenateduplift and erosion, which began at the end of theOligocene and continues today (Kusky et al. 2005).

In addition to field evidence of active faultingthere is also historical evidence of earthquake activityin the region (Kusky et al. 2005). Furthermore, on 11March 2002, an earthquake of Mw�5 occurredwithin the Oman Mountains and was felt overmuch of northern UAE and Oman. It had a normalmechanism with a slight right-lateral strike-slipcomponent, which is consistent with the large-scaletectonics of the region. The normal componentsuggests relaxation of obducted crust of the Semailophiolite, while the right-lateral strike-slip compo-nent is consistent with shear across the Oman Line(Rodgers et al. 2006).

Little research has been conducted on the neotec-tonics of northern Oman. The British GeologicalSurvey recently carried out a detailed geologicalsurvey of the northern part of the UAE that goessome way to improving this situation (Ellison andSykes 2006). As a result of this project, previouslyknown structures were better identified; among thesestructures are the Dibba Line, the Wadi Shimal andthe Wadi Ham faults that lie within the Dibba-Masafi-Fujairah area of the northern UAE. TheDibba Line is almost parallel to the Zendan-Minab

fault system with a NE-SW strike and dextral strike-slip motion (Lippard et al. 1982, Kusky et al. 2005,Rodgers et al. 2006, Styles et al. 2006). Observedpatterns of uplift over this region led Kusky et al.(2005) to suggest that the transform boundary con-sisting of both the Zendan-Minab fault system andcontuining down to the Dibba line may penetrateboth the over-riding and subducting plates.

The proximity of the Oman Mountains and theassociated fault structures within this range dictatethat a seismic source should be assigned to this regionwhen conducting a PSHA for sites in the UAE.Characterisation of this source is complicated bythe fact that very little seismicity information isavailable to constrain activity rates. The activity ofthis source must, therefore, be based primarily upongeological considerations of rates of uplift andaverage regional deformation.

Regional seismicity

For the purposes of characterising the activity ratesof the identified seismic sources a seismicity cataloguewas compiled using information from several sources:

. the on-line bulletin of the United States Geo-logical Survey (USGS 2003), which includesinformation from the National Ocean andAtmospheric Administration (NOAA) andthe Preliminary Determination of Epicenters(PDE) provided by the National EarthquakeInformation Center (NEIC);

. the International Seismological Centre on-linebulletin (ISC 2003);

. two regional catalogues compiled by Ambra-seys and Melville (1982) and Ambraseys et al.(1994);

. the EHB catalogue (Engdahl et al. 1998)updated to consider events up to 2004 (En-gdahl, personal communication 2006);

. the Earthquake Data Bank of the InternationalInstitute of Earthquake Engineering and Seis-mology (IIEES 2003); and,

. the first earthquake catalogue of Iran byBerberian (1994).

The initial catalogue was compiled for a spatialregion spanning 478 to 668E and 218 to 318N andincluded all events having an assigned magnitude of 4and above on any magnitude scale. For events priorto 1900, even those events without reported magni-tude values were considered with the aim of compar-ing locations and dates provided by the differentagencies. The catalogue covers the time period from3000 BC up to 1 October 2003. This latter date waschosen as only events reported prior to this date were

Georisk 5


regarded as being definitive by the International

Seismological Centre (ISC 2003) at the date the

catalogue was compiled. During the compilation of

the dataset a thorough check was conducted to ensure

that any duplicated events were removed. A particu-

lar effort was made to gather additional information

regarding events with magnitude of 6.5 and greater

(on any reported scale) from more detailed studies

that can be found in the literature, either for

particular events or regions (e.g., Berberian 1973,

1979, Melville 1978, Quittmeyer 1979, Jackson and

Fitch 1981, Jackson and McKenzie 1984, Baker et al.

1993, Berberian 1995, Berberian and Yeats 1999,

Berberian et al. 2001, Maggi et al. 2000, 2002,

Ambraseys and Bilham 2003a,b, Talebian and Jack-

son 2004, Walker et al. 2005).The compiled seismicity catalogue may be re-

garded as spanning three time periods of distinctly

different quality: (1) historical seismicity, prior to

1900, and consisting almost exclusively of macro-

seismic information; (2) the early instrumental per-

iod, from 1900-1963 inclusive, consisting of a

combination of macroseismic information and rela-

tively low-quality instrumental data; and (3) the

modern instrumental period, from 1964 onwards

and consisting of good quality instrumental data.

Many of the studies cited previously relate to

significant events that have occurred during the

modern instrumental period and enhance the quality

of this portion of the catalogue. The uncertainties in

location improve over these three time periods with

events reported prior to the modern instrumental

period having lateral position uncertainties com-

monly in excess of 30 km. Depths over this periodare also very poorly constrained.

The most common scale upon which magnitudes

have been reported for the region is surface-wavemagnitude, Ms. This is especially the case for theevents assigned magnitudes in the historical and earlyinstrumental parts of the catalogue with alternativescales becoming more common during the moderninstrumental period. In order to ensure that theseismicity catalogue was compiled in terms of a

homogeneous magnitude scale, all events for whichsurface-wave magnitudes were not reported wereconverted to this scale from Mb and Mw using therelationships of Ambraseys and Bommer (1990) andAmbraseys and Free (1997) respectively. The spatialdistribution of the compiled seismicity catalogue,using a consistent Ms magnitude scale, is plotted in

Figure 2.In addition to the checks that were made to

identify any duplicate events and the conversions thatwere made in order to achieve homogeneous magni-tude assignments, the catalogue was further processedto remove dependent events and to identify relevantcompleteness levels. A hybrid approach consisting ofthe temporal window from Reasenberg’s (1985)

algorithm and the spatial window from Knopoff’s(2000) algorithm was adopted for obtaining thedeclustered catalogue.

Although any event having an assigned magni-tude greater than 4.0 was initially included into thebase catalogue, for most of the period spanned by thecatalogue the level of completeness will be signifi-cantly greater than this level. The procedure used toidentify the completeness levels of the catalogue are

Figure 2. Homogenous seismicity catalogue in Ms for the UAE including all foreshocks and aftershocks. Diamonds indicatethe locations of the sites considered in this study: AD�Abu Dhabi, D�Dubai, and RAK�Ra’s Al Khaymah. All events ofMs]4.0 known to have occurred since 658 A.D. are plotted. The completeness levels vary with magnitude as discussed in thetext.



based on that originally proposed by Stepp (1972)and implemented into the software Wizmap II(Musson 2001). On the basis of this approach thedeclustered catalogue may be regarded as beingcomplete above Ms4.0 from 1967 onwards, Ms5.0from 1925 onwards, Ms6.0 from 1910 and Ms7.0 from1800. Various other levels of completeness wereadopted for other magnitudes and periods on thebasis of the chosen algorithm.

As previously stated, the compiled seismicitycatalogue includes events occurring prior to October2003. According to the online bulletin of the ISC(2006), a series of events have occurred within or nearto the UAE and the sites of interest in this study sincethat time. In fact, when conducting a search forevents to have occurred within a 150 km radius ofDubai the ISC catalogue returns 49 events over theperiod 1924�1999 (0.65 events per year) 17 of whichdo not have reported magnitudes and only three ofwhich are located inland within the Arabian Penin-sula. However, when considering the time span from2000-2006 the same search returns 18 events (3 eventsper year), six of which are inland and two justoffshore (Figure 3).

Given the current construction rates in the UAE,assessing the reliability of this apparent increase inthe rate of seismicity is very important and influencesthe manner in which activity parameters are derivedfor seismic sources when conducting PSHA. Theobserved increase may be explained in three obviousways: (1) the regional seismicity has genuinely

increased; (2) many of the reported events (16/18)currently have preliminary locations and later qualitychecks will reveal these events to be mislocated; or (3)many events that have legitimately occurred in thepast have not been detected or reported. Even thoughthe reported events all have small magnitudes, theimplications of each of these explanations are sig-nificant and the numbered events in the right-handside of Figure 3 were therefore scrutinised on anindividual basis.

In order to corroborate the locations currentlyreported by the ISC (2006), alternative online agen-cies were queried and literature searches of local andregional newspapers were conducted. These agenciesincluded the USGS online bulletin (USGS 2006), theearthquake databank of the International Institute ofEarthquake Engineering and Seismology (IIEES2006), and the earthquake databank of the Eur-opean-Mediterranean Seismological Centre (EMSC2006). A broadened search area was selected in orderto allow identification of possible mislocations, i.e.,events occurring with the same, or very similar, timesbut reported at different locations.

On the basis of the comparisons of the eventlocations of the various agencies it was possible toconfirm the magnitudes and locations of events 51and 52 (Figure 3) using the location from the USGSin addition to news reports of these events being feltin Dubai (Kazmi 2002, Shaghouri 2002) and recentlypublished literature regarding the focal mechanismand depth of the events (Rodgers et al. 2006). Events

Figure 3. Queries of the ISC online bulletin for events within 150 km of Dubai. Left: events occurring between 1924�1999;right: events occurring between 2000�2006. N/M�no magnitude. The events were listed chronologically and the numbersassigned consecutively for simple identification.

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53 and 57 are only reported by the IIEES (2006) andthe information provided in the ISC on-line bulletin(2006) has not yet been reviewed by that agency.Owing to the low magnitudes of these events, theabsence of felt reports does not preclude the possibi-lity that these events genuinely occurred within theArabian Peninsula. Additionally, their location closeto the Hajar Mountains in Oman supports thepossibility that these events have genuinely occurredwithin the Arabian Peninsula but that they have beenmislocated and may well have occurred closer to-wards, or within, the mountains.

For event 64, a similar event was located by theEMSC (2006) but with a slightly different origin time.On the basis of such limited data it is difficult toassert that both events are the same. For event 65, noreport of a similar event from any other agency wasfound that could confirm whether or not the ISClocation is accurate. However, the agency reportingthis event is responsible for other mislocations andthe absence of any felt reports for this event, whichapparently occurred just 32 km from Abu Dhabi,casts suspicion on the legitimacy of the reportedlocation for this event. For events 66 and 67, reportsof the same event but with different locations to thosereported by the ISC (2006) were found in the USGS(2006), IIEES (2006) and EMSC (2006) catalogues,implying errors of up to 400 km in the ISC’spreliminary location. This evidence would suggestthat these events occurred in southern Iran (withinthe Zagros fold belt), rather than within the ArabianPeninsula.

Therefore, while recent events such as events 51and 52 have occurred inland within the ArabianPeninsula in recent years, other events that arecurrently reported to have occurred within the areaare most likely to be mislocated events that will beremoved upon revision. The apparent recent increasein seismicity is therefore most likely to be a combina-tion of natural variation in seismicity rates andmislocated events. The seismicity catalogue that hasbeen compiled for the purpose of conducting thePSHA for this study may therefore be used withconfidence when deriving activity parameters for theidentified seismic sources.

Review of previous hazard studies

The earliest published hazard study covering theUAE of which the authors are aware is that carriedout by Al-Haddad et al. (1994), which focused onderiving seismic design criteria for the Kingdom ofSaudi Arabia but mapped hazard over the entireArabian Peninsula. The study defined seismic sourcezones along the western and southern boundaries of

the Peninsula, as well as in southwest and southernIran, incorporating the Zagros and Makran regions.No source zones were defined in the vicinity of theUAE, the closest source of events � apart frombackground activity � being a source zone thatcombines the southernmost part of the Zagros andthe western edge of the Makran subduction zone(Figure 4). A single ground-motion prediction equa-tion, reportedly adapted from a western NorthAmerican equation for application in western SaudiArabia, was employed to map PGA for a 475-yearreturn period; the assumed site classification for themapping is not stated but is probably rock or stiffsoil. The highest acceleration contour in the UAE, atthe very northernmost limit of the country, is 0.10g,but within the cities of Abu Dhabi, Dubai and Ra’sAl Khaymah the PGA values are below 0.05g.

The hazard estimates for UAE in the Al-Haddadet al. (1994) study are consistent with the classifica-tion of seismic hazard in Abu Dhabi and Dubaipresented in UBC97, which as noted previouslyclassifies both cities as Zone 0 whence seismic designis not required. This contrasts starkly with the hazardmapped in UAE as part of the Global SeismicHazard Assessment Project (GSHAP), which pro-duced a map of PGA with a return period of 475years in Europe, Africa and the Middle East(Grunthal et al. 1999). The values of PGA mappedin the UAE were significantly higher than thoseimplied by UBC97 and mapped by Al-Haddadet al. (1994), with peak accelerations in Ra’s AlKhaymah, Dubai and Abu Dhabi being 0.40g, 0.32gand 0.24g, respectively, which would correspond toUBC97 classifications of Zones 4, 3 and 2B. How-ever, these estimates can be disregarded because theyare not the result of actual hazard calculations, as thenortheast corner of the Arabian Peninsula was notcovered by any of the regional sub-projects inGSHAP and as a result, in order not to leave blankareas within the global map, ‘‘the hazard was mappedby simulating the attenuated effect of the seismicactivity in the Dead Sea fault area (Near East) andin the Zagros province of Iran’’ (Grunthal et al. 1999).In other words, the hazard estimates in the UAE weresimply inferred from the hazard in surroundingregions without any consideration of local seismicity.

Abdalla and Al-Homoud (2004) presented thefirst PSHA specifically for the United Arab Emirates.Their study begins by dismissing the GSHAP hazardestimates for the UAE � citing, ad verbatim butwithout reference, a report prepared by the secondauthor of this paper for an engineering consultancymaking a case for reduced seismic design loads to theDubai Municipality � and they go on to produce anew hazard map. Abdalla and Al-Homoud (2004)



defined seismic source zones not dissimilar to those of

Al-Haddad et al. (1994) within Iran, but also defined

two smaller zones (Regions III and VII) that effec-

tively link the stable UAE with the active areas of the

Zagros and Makran regions (Figure 4). In particular,

Region III � for which a maximum magnitude of 6

was specified � smoothes the seismicity from the

southern limit of the Zagros across the Gulf and

assumes that these earthquakes could equally occur

along the coast of the Emirates. No clear rationale is

given for the configuration of this source zone, indeed

the authors state that, with regard to the absence of a

seismic source zone in the northeast of the Arabian

Peninsula within the Al-Haddad et al. (1994) study,

‘‘ . . . there is no clearly defined tectonic structure in

that area and no significant earthquake activity. It

would be very difficult to define a seismic source to

capture the very limited earthquake [data] in that

area.’’ There is little doubt that the geometry of

Region III, in which it is assumed that the earth-

quakes, which are clearly clustered along its northern

boundary, have equal probability of occurring any-

where within the source zone, will have appreciably

inflated the hazard along its southern boundary

within the UAE. Similarly, Region VII � for which

a maximum magnitude of 7.5 is assigned � combines

parts of the Arabian stable craton, the Zagros

compression zone and the Zagros-Makran transition

zone (Minab-Zendan fault system); this source zone

will also have contributed to inflating the seismic

hazard estimates within the UAE.An additional factor that is likely to have

contributed to the rather high estimates of hazard

in the Abdalla and Al-Homoud (2004) study is the

use of the ground-motion prediction equation derived

from Iranian strong-motion data by Zare (2002). The

Figure 4. Seismic source zones defined for the PSHA studies of the regions by (a) Al-Haddad et al. (1994), (b) Abdalla andAl-Homoud (2004), (c) Peiris et al. (2006), and (d) Musson et al. (2006).

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standard deviation of the residuals of log10(PGA) forthis equation is 0.334, which is very high whencompared to other predictive equations for PGA(c.f., Douglas 2003) and this large aleatory variabilitywill have contributed to high hazard estimates (e.g.,Bommer and Abrahamson 2006). The resultinghazard map for the UAE for PGA with a 475-yearreturn period indicates values of 0.16g, 0.15g and0.10g for Ra’s Al Khaymah, Dubai and Abu Dhabirespectively, which would lead to all three cities beingclassified as UBC97 Zone 2A.

More recently, Sigbjornsson and Elnashai (2006)presented a new hazard study focused only on Dubai.The results were presented in terms of uniform hazardspectra for return periods of 974 and 2475 years, aswell as synthetic accelerograms for use in dynamicanalyses. The paper presents neither a map of theseismic source zones considered nor tabulates therecurrence parameters used in the hazard calcula-tions, but indicates that the zonation was based onthe work of Tavakoli and Ghafory-Ashtiany (1999)for Iran but also included the Dibba fault and a faultalong the western coast of the UAE. The latter sourcewas also considered by Wyss and Al-Homoud (2004)and is present in the tectonic map of Saudi Arabiaand adjacent areas by Johnson (1998). The inclusionof this fault is a controversial issue that is discussedlater in Section 5. Sigbjornsson and Elnashai (2006)use the ground-motion prediction equations of Am-braseys et al. (1996) and Simpson (1996), extendingthe coefficients from a maximum period of 2 s up to 4s. These equations, derived from recordings ofshallow crustal earthquakes in Europe, North Africaand the Middle East, are applied to all source zones,including the subduction earthquakes in the Makran.The results are presented in the form of a hazardcurve for PGA and UHS for Dubai for the two returnperiods considered.

The hazard levels indicated by the Sigbjornssonand Elnashai (2006) study are actually higher thanthose found by Abdalla and Al-Homoud (2004),which themselves are considered to be conservative.

In contrast with the relatively high hazard esti-mates presented by Abdalla and Al-Homoud (2004)and by Sigbjornsson and Elnashai (2006), two otherrecent studies have indicated much lower levels ofseismic hazard in the UAE. However, whereas thetwo previous studies have appeared in a peer-reviewed journal, these other studies have beenpublished in conference proceedings and as an inter-nal report respectively, so their impact has beenreduced. The first of these is by Peiris et al. (2006)who present UHS for rock sites in Dubai and AbuDhabi for return periods of 475 and 2475 years. Theirseismic source zonation (Figure 4) is consistent with

those of Al-Haddad et al. (1994) for the ArabianPeninsula and of Tavakoli and Ghafory-Ashtiany(1999) for Iran, and also with the regional tectonics.However, the recurrence parameters, for example forthe Zagros region, differ from those given byTavakoli and Ghafory-Ashtiany (1999), with muchhigher b-values in the Peiris et al. (2006) study. Thepaper does not explain how the recurrence para-meters for the Arabian stable craton are obtained �which according to their own disaggregation is thedominant seismic source for hazard in Dubai � whichis an important issue given the very sparse earthquakedata in this area.

Peiris et al. (2006) used the ground-motion pre-diction equations of Ambraseys et al. (1996) andSadigh et al. (1997) for active crustal regions as wellas for earthquakes in the Makran subduction zone.For areas of tectonic extension in the Red Sea and theIndian Ocean they employed the equations of Spu-dich et al. (1999) and for the stable craton of theArabian Peninsula the equations of Atkinson andBoore (1997) and Dahle et al. (1990), even though thelatter are now generally considered to be obsolete.

For Dubai and Abu Dhabi, Peiris et al. (2006)estimate the 475-year PGA values as 0.06g and 0.05grespectively, which would correspond to Zone 1UBC97 classification for both cities. The uniformhazard spectrum for rock sites in Dubai for a returnperiod of 2475 years is a factor of 5 or more lowerthan that of Sigbjornsson and Elnashai (2006) in theshort-period range.

A more recent study of the seismic hazard of theUAE is that presented by Musson et al. (2006), whichwas performed by the British Geological Survey onbehalf of the government of the UAE. The studyproduced maps of PGA in rock for return periods of475, 1000 and 10 000 years, as well as UHS for thecapital cities of the seven Emirates up to responseperiods of 2 s for the same three return periods.

The seismic source zonation defined by Mussonet al. (2006) � reproduced in Figure 4 � is generallyconsistent with the tectonics and seismicity of theregion. One interesting observation, however, is thatthey effectively conclude that the western portion ofthe Makran zone is almost aseismic, relocating the1483 Hormuz earthquake to a position 250 kmnortheast of where it was located by Ambraseysand Melville (1982). Musson et al. (2006) support thisdecision by performing sensitivity analyses to showthe influence of the seismic activity in the westernMakran region is very small for response periods upto 1.0 s and a return period of 475 years.

In terms of ground-motion prediction equations,Musson et al. (2006) employ the Ambraseys et al.(1996) equations for spectral ordinates and the



Ambraseys (1995) equation for PGA, for all seismicsource zones including the stable craton of theArabian Peninsula and the Makran subductionzone. The PGA map at 475 years is used to assignan equivalent UBC97 zonation to the country (Figure5); the peak accelerations in Ra’s Al Khaymah,Dubai and Abu Dhabi are respectively 0.08g, 0.05gand 0.035g, leading to classification of Ra’s AlKhaymah as Zone 1 and the other two cities asZone 0.

The most recent study is that by Husein Malkawiet al. (2007), which presents hazard curves for 15major cities in the Emirates. The study treats allearthquakes in the Makran, Zagros and stableregions as a single source, and applies a singleground-motion prediction equation for all events,the eastern North American equation of Atkinsonand Boore (1997) for PGA on very hard rock sites.Response spectra are then constructed by findingmagnitude-distance pairs consistent with the PGAvalues � corresponding to different return periods �and applying the western North American equationsof Joyner and Boore (1988). The hazard results arenot reported herein because there are sufficientshortcomings in the study to make its findings highlyquestionable.

In summary, the existing published studies ofseismic hazard in the UAE present an incomplete andcontradictory image. Although there are clear reasonsto treat the higher hazard estimates from GSHAP,

Abdalla and Al-Homoud (2004) and Sigbjornssonand Elnashai (2006) as being suspect, the studies ofPeiris et al. (2006) and Musson et al. (2006) cannot beconsidered definitive in terms of their treatment of theregional seismic sources and the selection of appro-priate ground-motion prediction equations. There-fore, there is scope and justification for a new PSHAfor the Emirates, accounting for uncertainty in theseismicity and ground-motion models, and makinguse of the latest available data.

Seismic hazard analysis

This section describes the calculations performed toestimate the seismic hazard in the cities of AbuDhabi, Dubai and Ra’s Al Khaymah in terms ofPGA and spectral ordinates on rock sites.

Source zones

On the basis of the literature review of the seismo-tectonic setting briefly summarised in section 2, 20distinct seismic sources were identified and includedwithin the seismic source model. These sourcesprincipally relate to the Zagros fold and thrust belts,with the work of Berberian (1995) heavily used topartition this overall region into various sub-sources:the Makran subduction zone, with alternative rupturehypotheses regarding the segmentation of this struc-ture; the Minab-Zendan transform region forming

Figure 5. Hazard zonation of the UAE in terms of UBC97 zones by Musson et al. (2006).

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the interface of the two previous regions; and theOman Mountains. In addition to these key seismo-tectonic structures, the stable craton that constitutesmost of the Arabian Peninsula was treated as abackground source. The full list of identified seismicsources is given in Table 1. In this table sources 5 and15 have three alternative designations (I, II & III)representing alternative source zonations that havebeen considered for these sources. These alternativesare considered in order to account for the epistemicuncertainty associated with the location of theboundary between sources 5 and 15. Of course, thereis also uncertainty associated with the boundarylocations of all sources. However, the boundarybetween these particular sources is offshore and isrelatively poorly constrained as a consequence.Furthermore, the location of this boundary is criticalfrom a source zonation perspective as it dictates how

far south the seismicity of the southern Zagros ispresumed to occur, with obvious implications forhazard in the UAE.

In Figure 6 the preferred seismic source model (I)is shown together with the alternative locations of theboundaries between sources 5 and 15, with thedeclustered seismicity catalogue used for the deriva-tion of the activity parameters, as discussed in thenext section. The preferred source model shown inFigure 6 includes a distinct source 4 corresponding tothe Zagros Foredeep, a zonation that is consistentwith the cross section of Berberian (1995). However,the plot of the seismicity shown in Figure 6 indicatesthat events occur either side of this boundary betweensources 4 and 5. Although we are confident about theexistence of the blind Zagros Foredeep fault thatdefines the boundary between sources 4 and 5, it ispossible that this boundary may be located farther

Table 1. Seismic sources identified for the PSHA of this study.

Source number Source name Source number Source name

1 High Zagros thrust belt 11 Makran Intraplate2 Simple Fold belt 12 Makran background3 Dezful Embayment 13 Sabzevaran-Jorift fault

4 Zagros Foredeep 14 Minab-Zendan fault5 Persian Gulf (I, II & III) 15 Stable craton (I, II & III)6 Kazerurn fault 16 Owen fracture zone

7 Borazjan fault 17 Oman mountains8 Aliabad zone I8 Makran interplate9 Nek south fault 19 Makran Inter east

10 Gowk fault zone 20 Makran Inter west

Figure 6. Seismic source models employed for the PSHA presented in this study. The numbers relate to the sources listed in

Table 1. The dashed lines indicate alternative source boundaries for the southern Zagros and the declustered seismicitycatalogue used to derive activity parameters for the seismic sources. WCF�West Coast Fault. Within sources 6, 7, 13 and 14dotted-dashed lines indicate the traces of the faults.



south than shown in Figure 6 or that the seismicitythat is observed to the south of this boundary isdriven by a similar mechanism to that within theZagros Foredeep itself. For this reason we hypothe-size two additional source zonations: Zone II com-bines the seismicity of sources 4 and 5 into a singlesource and shifts the preferred boundary betweensources 5 and 15 slightly northwards; Zone III alsocombines sources 4 and 5 but retains the preferredsouthern position of the boundary between sources5 and 15.

The implications of these alternatives is that in thepreferred model, using Zone I, the seismicity of theZagros Foredeep is constrained to occur within orclose to the southern coast of Iran while the seismicityof the Persian Gulf is encompassed within a separateseismic source. For the second model, correspondingto Zone II in Figure 6, most of the seismicity that hasbeen observed within the Persian Gulf is combinedwith the seismicity of the Zagros Foredeep and themore northerly boundary between sources 5 and 15means that more earthquakes are located within thestable craton (source 15). The final model ensuresthat no seismicity is inadvertently pushed into thestable craton, but the seismicity of the ZagrosForedeep will govern the activity rates for thecombined sources 4 and 5 under this zonation. Inthis case the southern boundary of the combinedsource passes relatively close to the cities of Ra’s AlKaymah and Dubai with the implication that rela-tively high rates of earthquake activity are modelledto occur within the Persian Gulf at distances that mayaffect the hazard of the cities just mentioned.

In Figure 6 one may observe that sources 6, 7, 13and 14 are shown as dotted-dashed lines that indicatefault sources. This is also implied by the namingconvention used in Table 1. In these cases, the linesthat surround the dashed lines indicate regions forwhich the observed seismicity is assumed to beassociated with the fault sources. In these locationsit is known that dominant fault sources exist but thatthere is also distributed seismicity around these faultsources. By modelling these sources as fault sources,but assigning seismicity to these faults within an areaaround the faults, we are assuming that this seismicityis directly related to the deformation accommodatedby the fault sources.

The remaining clarification that must be maderegarding the adopted source model relates to thepartitioning of the Makran subduction zone intoMakran interplate, inter east and inter west sources(Table 1). These alternatives have been selected so asto entertain the possibility that the interface of theMakran subduction zone may be segmented asmentioned in section 2. For source 18, the entire

subduction interface is allowed to rupture in a singleevent while sources 19 and 20 correspond to thesituation where the zone is segmented and the eastand west sections of the interface rupture indepen-dently.

Recurrence parameters

With the exception of the Makran subduction zone(sources 18, 19 and 20) all seismic sources wereassumed to generate earthquakes according to adoubly bounded exponential distribution (Cornelland Vanmarcke 1969). Sources 18 to 20 weremodelled using the characteristic earthquake distri-bution of Youngs and Coppersmith (1985). Often,fault sources are assumed to act in a characteristicmanner and this may suggest that sources 6, 7, 9, 13and 14 may be more appropriately modelled using thecharacteristic distribution. However, for these sourcesthere was insufficient geological data to adequatelyconstrain the recurrence rates of the largest events.Furthermore, the seismicity data that was availabledid not indicate any significant departures from thepredictions of the doubly bounded exponential dis-tribution and for this reason this latter model wasadopted owing to its simplicity.

The seismicity catalogue has varying levels ofcompleteness and these must be accounted for whenderiving activity parameters for the seismic sources.For this reason the parameters of the doublybounded exponential distribution were obtainedusing the maximum likelihood estimation procedureof Weichert (1980). This procedure allows the activityparameters b and nmin to be determined where theseparameters are related to the a- and b-values ofthe original Gutenberg-Richter (1944) distribution(see Weichert 1980). For the sources whose magni-tude-frequency distributions were modelled with thecharacteristic earthquake distribution, seismic-mo-ment release rates were determined from estimatesof fault-slip rates and the assumed fault geometries.These moment release rates were then used inconjunction with the model of Youngs and Copper-smith (1985) in order to quantify the magnitude-frequency characteristics of the sources. The activityparameters for the Oman Mountains (source 17) werederived in a similar manner but using the doublybounded exponential distribution instead of thecharacteristic model. Rates of uplift were used toinfer slip rates, and moment release rates, that thenenabled activity parameters to be determined.

For the Arabian stable craton, different values ofb were used on the basis of two publications. The firstof these is Fenton et al. (2006) who propose a b valueof 1.84 as a world average for seismicity in stable

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cratonic cores and an annual long-term rate of 0.004events per 106 km2 for events of Mw ] 6. The secondstudy is that of Johnson et al. (1994) who report a b

value of 2.26 as an average over all stable continentalregions and an annual occurrence rate for events ofMw ] 6, essentially the same as that of Fenton et al.(2006). For this source the nmin values were calculatedby fixing the b values within Weichert’s (1980)procedure and fitting the curves to the observedArabian stable craton seismicity. Sensitivity analyseswere conducted assigning a weight of 1 to each of thetwo b values is turn (and zero to the other one), andthe influence on the resulting hazard curves wasfound to be low.

For estimation of the maximum magnitude foreach source, the relations proposed by Wells andCoppersmith (1994) were used when consistent dataregarding fault type and total length of the faultswere able to be retrieved. When this was not possible,the maximum magnitude was estimated using thestatistical procedure proposed by Kijko (2004),specifically by making use of equations (4) and (6)of that study. In any remaining case the maximummagnitude was obtained by adding 0.5 units to themaximum observed magnitude. While this practice isnot ideal, the paucity of data related to these sourcesdictated that it was difficult to justify the use of moreelaborate approaches. The seismicity parametersassumed for each seismic source are given in Table 2.

Ground-motion prediction equations

Just as alternative seismic source models wereadopted in section 4.1 to account for the epistemicuncertainty associated with not knowing the truelocations of seismic source boundaries, alternativeground-motion models were selected for use withinthe various tectonic provinces considered in thisstudy. In total, seven different ground-motion modelswere considered, selected following the guidelinesproposed by Cotton et al. (2006). All of these modelsare either recent updates of previous, well regardedand commonly implemented models, or are oldermodels that have been used extensively in hazardanalyses throughout the world.

The models of Youngs et al. (1997) and Atkinsonand Boore (2003) were used to model the groundmotions from subduction zone earthquakes occurringwithin the Makran sources. For earthquakes occur-ring within the active shallow crustal sources theground-motion models of Abrahamson and Silva(1997), Ambraseys et al. (2005), Boore and Atkinson(2007), and Akkar and Bommer (2007) were used.For earthquakes occurring within the stable craton(source 15) the model of Atkinson and Boore (2006)

was used in conjunction with the equations for the

active shallow crustal sources. The largest weight

(0.55) is assigned to the model for stable regions, but

it is acknowledged that the definition of the Arabian

Peninsula as a stable craton is not unambiguously

confirmed and given the proximity of the Emirates to

actively deforming regions, branches are included in

the logic-tree that allow for the fact ground motions

may be more similar to those from active crustal

regions. For the active regions such as the Zagros,

two of the equations are derived from datasets of

European and Middle Eastern strong-motion data

that include records from Iran, and there is increasing

evidence that motions in western North America are

broadly similar to those from this region (e.g.,

Stafford et al. 2008).All of these models were incorporated within a

logic tree framework (described in detail in section

4.4). The various models that are used require the

specification of different input parameters, adopt

different site classification schemes and estimate the

distribution of ground motions in terms of different

horizontal component definitions. A summary of the

properties of the suite of selected ground-motion

models is given in Table 3.In order to implement these models within a logic

tree these alternative inputs and outputs must be

transformed into common metrics (Bommer et al.

2005). As may be appreciated from Table 3, all of the

models use the same magnitude scale (Mw) and this

scale is different to that used to characterise the

seismic sources. For this reason, the Ambraseys and

Free (1997) relationship was used to transform the

ground-motion models into the surface-wave magni-

tude scale. Fortunately, the software used to perform

the hazard calculations, Crisis 2007 (Ordaz et al.

2007), uses tabulated ground-motion values to define

the level of ground-motion corresponding to a

particular magnitude-distance scenario. This frame-

work enables this magnitude conversion to be im-

plemented very easily. Likewise, the conversions

among distance metrics are handled in a straightfor-

ward manner using this software package (in actual

fact, conversions are not required). Although the

model of Abrahamson and Silva (1997) adopts a

slightly different site classification scheme, the differ-

ence is not considered great enough to warrant

making any adjustments for this aspect of this model.

The only significant adjustment that was required in

order to implement the models in a consistent manner

was to adjust the predictions of the Ambraseys et al.

(2005) from the larger horizontal component to the

geometric mean using the relationships of Beyer and

Bommer (2006).



Hazard calculations

All of the hazard calculations were performed using

the Crisis 2007 software (Ordaz et al. 2007). Thesoftware is essentially an implementation of theCornell (1968) PSHA framework, with the aleatoryvariability in the ground-motion explicitly included

(Bommer and Abrahamson 2006). All sources wereassumed to behave in a Poissonian manner and onlyearthquakes of Ms]4.0 were deemed to be ofengineering significance. This is a rather low mini-

mum magnitude but was selected so as to be

conservative. Events as small as Ms 4.0 are very

unlikely to cause damage to engineered structures

(Bommer et al. 2001).The alternative seismic source zonations, activity

rates and ground-motion models that have previously

been discussed are all incorporated into the hazard

calculations through the use of the logic tree for-

mulation shown in Figure 7. In this figure, all of the

parameter values and the weights assigned to each of

Table 2. Seismicity parameters for all of the seismic sources listed in Table 1. Where alternative models of the source

boundaries are relevant the seismicity parameters corresponding to each of the alternatives are provided.

Doubly-bounded Exponential Seismicity

Zone/Region Mmax s(Mmax) Mmax(Obs) Mmin

N ofevents b s(b) b s(b) vmin s(vmin) a

All Catalogue 8.0 4.0 1290 1.66 0.037 0.72 0.016 24.10 0.019 4.26

Zagros 7.2 0.12 7.1 4.0 840 1.86 0.055 0.81 0.024 17.24 0.021 4.47High Zagros 6.8 0.20 6.6 4.0 55 1.52 0.203 0.66 0.088 1.07 0.019 2.67Simple Fold 7.3 0.17 7.1 4.0 456 1.91 0.075 0.83 0.033 9.56 0.021 4.31Dezful Embayment 5.8 0.14 5.7 4.0 122 1.77 0.186 0.77 0.081 2.56 0.021 3.51Zagros Foredeep 6.9 0.14 6.8 4.0 83 1.40 0.157 0.61 0.068 1.56 0.019 2.63Persian Gulf I 6.1 0.23 5.9 4.0 52 1.78 0.261 0.77 0.113 1.08 0.021 3.14Persian Gulf II 6.9 0.12 6.8 4.0 129 1.57 0.133 0.68 0.058 2.52 0.020 3.14Persian Gulf III 6.9 0.12 6.8 4.0 135 1.59 0.130 0.69 0.057 2.65 0.020 3.18Kaserum Fault 6.8* 0.23* 6.0 4.0 24 1.45 0.301 0.63 0.131 0.45 0.019 2.18Borazjan Fault 6.9* 0.23* 5.5 4.0 27 2.10 0.348 0.91 0.151 0.58 0.021 3.41

Makran 8.2 0.24 8.0 4.0 401 1.76 0.070 0.76 0.031 8.02 0.070 3.96Aliabad zone 6.5 0.16 6.4 4.0 90 1 95 0.187 0.85 0.081 1.89 0.021 3.66Nek south fault 8.0*;

8.0�0.28* 7.0 4.0 7 0 99 0.363 0.43 0.158 0.11 0.016 0.77

Gowk fault zone 8.1*;8.0�

0.28* 7.0 4.0 62 1 76 0.179 0.77 0.078 1.24 0.000 3.16

Makran Interplate LT 0.00 8.0 4.0 85 1 83 0.162 0.79 0.070 1.64 0.020 3.40Makran Interplate East LT 0.00 8.0 4.0 62 1 74 0.182 0.76 0.079 1.18 0.020 3.10Makran Interplate West LT 0.00 7.7 4.0 22 2 06 0.358 0.89 0.155 0.44 0.021 3.22Makran Intraplate 6.8 0.24 6.6 4.0 54 1 63 0.212 0.71 0.092 1.07 0.020 2.87Makran Background 7.5 0.50 7.0 4.0 87 1 66 0.149 0.72 0.065 1.71 0.020 3.11Jorift-Sabzevaran fault 6.7* 0.28* 5.7 4.0 21 2 42 0.448 1.05 0.195 0.47 0.022 3.87Minab-Zendan fault 6.5* 0.28* 5.8 4.0 11 1 25 0.455 0.54 0.198 0.20 0.018 1.49

Stable craton I & III 7.0** 0.50 6.5 4.0 13 LT LT LT LT LT LT LTStable craton II 7.0** 0.50 6.5 4.0 19 LT LT LT LT LT LT LT

Owen fracture zone 6.0 0.20 5.8 4.0 26 1.57 0.364 0.68 0.158 0.52 0.020 2.47Oman Mountains LT LT 5.1 4.0 5 1.00 LT 0.43 LT LT LT LT

Characteristic seismicity

Zone/Region Option Mmax Mmin Mch s(Mch)

OccurrenceInterval(yr)

Makran Interplate WSZ1 8.5* 7.8 8.2 0.25* 203WSZ2 8.6* 7.9 8.3 0.25* 428

Makran Interplate East WSZ1 8.3* 7.5 8.0 0.25* 139WSZ2 8.5* 7.8 8.2 0.25* 422

Makran Interplate West WSZ1 8.2* 7.4 7.8 0.25* 121WSZ2 8.4* 7.6 8.0 0.25* 356

Notes:

� Proposed by Berberian & Yeats (1999).

* Calculated using Wells & Coppersmith’s (1994) relationships.

** Maximum magnitude observed plus 0.5.

LT-See logic tree.

WSZ 1 and WSZ 2 are the two likely widths considered for the seismogenic zone in Makran Interplate.

All magnitudes are in Ms scale. When not specified, maximum magnitude was calculated using the procedure proposed by

Kijko (2004).

Bold numbers are the regional earthquake recurrence parameters for the whole catalogue, Zagros and Makran regions.

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Table 3. Summary of the characteristics of the ground-motion models used in this study.

EquationMagnitude

scale Mmin*** Mmax*** Site conditions*Distancedefintion

Dmax**(km)

Horizontalcomponentdefinition Faulting mechanism Tectonic enviroment

Abrahamson & Silva

[1997]

Mw 4.4 7.4 Rock [Vs�600 m/s] Rrup 220 Geometric mean Reverse/ Reverse-

Oblique/Others

Active regions,

shallow earthquakesAkkar & Bommer[2007]

Mw 5.0 7.6 Rock [Vs�750 m/s] RJB 100 Geometric mean Normal/Reverse Active regions,shallow earthquakes

Ambraseys et al[2005]

Mw 5.0 7.6 Rock [Vs�750 m/s] RJB 100 Larger horizontalcomponent

Normal/Thrust/Odd

Active regions,shallow earthquakes

Atkinson & Boore[2003]

Mw 5.0 8.3 Rock [Vs�760 m/s] Rrup 550 Randomhorizontal

component

Interface/ In-Slab Active regions,subduction zones

Atkinson & Boore[2006]

Mw 3.5 8.0 Rock [Vs�760 m/s] Rrup 1000 Unspecified Unspecified Stable continentalregions

Boore & Atkinson[2007]

Mw 4.2 7.9 Rock [Vs�760 m/s] RJB 300 GMRotD50;GMRotI50

Unspecified/ Strike-slip/Normal/Reverse

Active regions,shallow earthquakes

Youngs et al. [1997] Mw 5.0 8.2 Rock [Vs�750 m/s] Rrup 500 Geometric mean Interface/ In-Slab Active regions,

subduction zones

Notes:

*-Site condition considered for this study. Other site conditions are reported by the modelers.

**-Maximum distance source-to-site distance m data set.

***-Maximum and minimum magnitudes in data set.

16

G.Aldama-Busto

set

al.


Figure 7. Components of the logic tree used for the hazard calculations showing all of the alternative branches and theirassociated weights.

Georisk 17


the various alternatives are shown. There are four

components of the tree that relate to source zonations

and activity rates and one component that relates to

the ground-motion models. For the uncertainty

associated with the location of the boundaries

between sources 4, 5 and 15 in the southern Zagros

and the Persian Gulf weights of 0.6, 0.25, and 0.15

were allocated to options I, II and III respectively. In

all cases, the weights that have been assigned to

branches have been done so on a subjective basis

through discussion among the authors. In most cases

the extent of the uncertainty is such that we saw little

option other than to allocate equal, or very similar,

weighting to the various options. In other cases

however, such as for the source boundaries just

mentioned, or for the segmentation of the Makran,

we have greater confidence in a particular model and

have allocated a weight that reflects this. However,

where possible we have sought to acknowledge

alternative legitimate viewpoints and to include these

as alternatives into our formulation.

The activity rates of the stable craton from the

studies of Johnson et al. (1994) and Fenton et al.

(2006) have been allocated similar weights, with

slightly higher weight assigned to the latter study

reflecting the more recent nature of this work. The

activity rates for the Oman Mountains involve a

series of branches that reflect the large degree of

uncertainty associated with the rate of coseismic

deformation in this region. The complexity of this

component of the logic tree is justified given the

relatively close proximity of this source to the sites of

interest in this study. The scenario involving segmen-

ted rupture of the Makran is strongly favoured,

primarily on the basis of recorded earthquake activ-

ity. Additional uncertainty is also incorporated into

this component of the logic tree to account for the

unknown extent to which the subduction interface

extends down-dip. For this component the shorter

rupture width is preferred on the basis of the

observed seismicity, but only slightly so, and is given

a weighting of 0.6.

Figure 8. Disaggregation results for Abu Dhabi for a return period of 500 years and for PGA and spectral accelerations of

0.2, 1.0 and 3.0 s.



For the ground-motion models in the stable

craton the model of Atkinson and Boore (2006) is

strongly favoured with a weight of 0.55 being

allocated to this model and 0.45 spread among the

remaining four crustal models. The active shallow

crustal models are all given equal weight when used

for modelling ground-motions from earthquakes

occurring in the shallow crustal sources. Finally, the

more recent subduction zone model of Atkinson and

Boore (2003) is favoured quite strongly over the older

Youngs et al. (1997) model.The net result of accounting for the various model

alternatives was to require 5184 hazard curves, and

corresponding hazard disaggregations, to be gener-

ated. Annual rates of exceedance were determined for

spectral accelerations (including peak ground accel-

eration, PGA) ranging from 0.001g to 1.0g. To enable

comparison with previous studies, the suite of hazard

curves were summarised by taking the mean of the

rates of occurrence corresponding to each consideredlevel of ground motion.

Disaggregation of the hazard

For every hazard curve that was generated during thehazard calculations the level of ground-motion cor-responding to return periods of 500, 1000, 2500, 5000,and 10 000 years was determined and the associatedrate of occurrence disaggregated in order to identifythe magnitude-distance scenarios contributing mostsignificantly to the hazard (Bazzurro and Cornell1999). As we adopt the weighted mean of the hazardcurves in order to summarise the hazard results wemay also determine the weighted mean of thedisaggregation results in order to summarise therelative contributions made by the various magni-tude-distance scenarios. The disaggregated results forthe 500-year return period for the three sites and for5%-damped spectral accelerations at periods of 0

Figure 9. Disaggregation results for Dubai for a return period of 500 years and for PGA and spectral accelerations of 0.2, 1.0and 3.0 s.

Georisk 19


(PGA), 0.2, 1.0 and 3.0 s are shown in Figures 8

to 10.From Figure 8 to 10 one may readily appreciate

that as one moves to spectral accelerations at longer

response periods the main contribution to the hazard

increasingly comes from larger magnitude events

located at greater distances. Such a result is to be

expected as the seismic sources that are very close to

the sites in question tend to have relatively low

activity rates and one is therefore rather unlikely to

observe large magnitude events at short distances. As

larger magnitude earthquakes are far more likely to

generate strong long-period components of ground

motion than their small-to-moderate counterparts the

contribution to the hazard expectedly moves to larger

magnitude events as the response period increases.One may also appreciate from Figure 8 to 10 that

as we move from Abu Dhabi in the south to Ra’s Al

Khaymah in the north the relative contributions of

the Zagros and Makran regions to the hazard at short

periods increases. Again, this is a result that may be

anticipated. For short-period ground-motion mea-

sures such as PGA and spectral acceleration at 0.2 s,

the rate of attenuation with distance is relatively high.

This attenuation rate precludes the possibility of Abu

Dhabi being strongly affected by seismicity associated

with the Zagros and the hazard is consequently

dominated by events occurring within the stable

craton. As one moves to Ra’s Al Khaymah the

distance to the Zagros becomes small enough that

the hazard begins to become affected by seismicity

from this source. This effect is clearly demonstrated

through contrasting the top panels of Figures 8

and 10.One should also note that, particularly for spec-

tral accelerations of 1.0 and 3.0 s, the magnitude

distance scenarios contributing most significantly to

the hazard correspond to scenarios that are well

beyond the ranges of applicability of the ground-

motion models used for the analysis (at Abu Dhabi,

the dominant scenario at 1.0 and 3.0 s is at about

400 km). For these scenarios the hazard values must

Figure 10. Disaggregation results for Ra’s Al Khaymah for a return period of 500 years and for PGA and spectral

accelerations of 0.2, 1.0 and 3.0 s.



be interpreted with a considerable degree of caution,but as few, if any, current empirical equations arevalid for such large magnitudes and such longdistances, this is a common, and almost unavoidable,problem.

For all sites the largest contribution to the seismichazard for short-period ground-motion measurescomes from the stable craton. Modifying the activityrates for this source will therefore have a significantimpact upon the calculated hazard. The activity ratesfor the stable craton were determined on the basis ofpublished studies that consider large compositecatalogues of stable continental regions and stablecratonic cores; they were not derived specifically forthe Arabian Peninsula itself. This point is worthemphasising as there is significant uncertainty asso-ciated with the rates of events within such stableregions (Fenton et al., 2006) and one must be awareof the sensitivity of the results to the assumptionsmade regarding the activity of this backgroundsource. However, it is also clear that these sourcesonly contribute significantly to the seismic hazard asa result of the relatively low seismic activity of theregion. The fact that the hazard is dominated by verysmall earthquakes close to the sites of interest, andthat these events correspond to a seismic source oflow activity, reinforces the view that the seismichazard for sites within the UAE is low.

Discussion of results

Figure 11 shows seismic hazard curves for rock sitesin the three cities in terms of PGA and in terms of 1.0-

s spectral acceleration. The values are very similar atthe three sites although as expected there is anincrease in the hazard level from south to northowing to the increasing proximity of the Zagros zone.However, at long return periods, for which the localseismic activity becomes dominant, particularly forthe short-period motions, the hazard curves converge.Figure 12 shows the uniform hazard spectra on rockfor the three cities at return periods ranging from 500to 10 000 years.

Direct comparisons between the results of thisnew study and those of previous hazard estimates forthe UAE are limited by the incomplete informationregarding definitions of site conditions and horizontalcomponent definitions provided in the earlier studies.All of the studies state that the hazard estimates arefor rock sites, with the exception of Sigbjornsson andElnashai (2006) who do not specify the assumed siteclassification; for the purpose of the comparisonsmade herein, it is simply assumed that the siteconditions are uniformly rock and therefore noadjustments are needed. In terms of the definitionof the horizontal component of motion, differentmeasures can give appreciably different results (Beyerand Bommer 2006), but none of the studies explicitlystate the definition used. The current study has usedthe geometric mean horizontal component, whereasby virtue of the ground motion equations employedthe studies of both Sigbjornsson and Elnashai (2006)and Musson et al. (2006) are based on the largerhorizontal component. Abdalla and Al-Homoud(2004) use a ground motion equation based on bothhorizontal components of each accelerogram, which

Figure 11. Seismic hazard curves for PGA and 1-s spectral acceleration at rock sites in the three cities.

Georisk 21


can broadly be considered equivalent to the geometric

mean component (Beyer and Bommer 2006). Peiris

et al. (2006), however, use a number of different

ground-motion prediction equations that use differ-

ent component definitions, and state neither whether

adjustments were made for compatibility nor which

component is used for the output. For comparative

purposes, it is assumed that their results represent the

geometric mean component as this is the definition

employed in most of the ground-motion prediction

equations used in their study.Figure 13 compares the 475-year UHS for Dubai

from the current study with those from Peiris et al.

(2006) and Musson et al. (2006), whereas Figure 14

compares the 2,475-year UHS for Dubai from this

study with those from Peiris et al. (2006) and

Sigbjornsson and Elnashai (2006).These figures show generally good agreement

amongst the results of this study and those from

Peiris et al. (2006) and Musson et al. (2006), and as no

major flaws were identified in either of those studies

this could be favourably interpreted as corroboration

of the new results. Comparisons with the results of

Musson et al. (2006) for all three sites and longer

return periods show consistently close agreement,

although the spectral ordinates from Musson et al.

(2006) are generally slighter higher. However, this

could largely be the result of the use of different

component definitions: Beyer and Bommer (2006)

found ratios of the larger component to the geometric

Figure 12. Uniform hazard spectra for rock sites in the three cities.



mean component increasing with period from about

1.1 to 1.2 at periods of 1.0 second and greater. These

correction factors cannot be applied directly to the

UHS but do allow one to suppose that much of the

observed disagreement would be removed had these

studies used the same horizontal component defini-

tion.The spectrum from Sigbjornsson and Elnashai

(2006), however, is clearly very different from the

other studies, resulting in spectral accelerations asmuch as 6 times higher at some response periods.Even accounting for the use of equations based on thelarger horizontal component, this study is still pre-senting a fundamentally different picture of theseismic hazard in the UAE. The information pro-vided in the paper by Sigbjornsson and Elnashai(2006) is not sufficient to be able to determine exactlywhy the calculated hazard is so much higher, but thedefinition the fault along the western coast of theUAE as an active seismic source seems to be one ofthe key contributing factors (see discussion below).Given the absence of any convincing evidence for theactivity of this source, the spectral ordinates ofSigbjornsson and Elnashai (2006) shown in Figure14 could be considered extremely conservative.

Figure 15 compares the 475-year uniform hazardspectra for the three cities with the spectrum for rock(class B, which corresponds to shear-wave velocitiesbetween 760 and 1,500 m/s consistent with the siteclassification assumed in the hazard calculations)sites in Zone 1 from UBC97.

Figure 15 indicates that all three cities couldconceivably be classified as UBC97 Zone 0, althoughcogent arguments could be made for considering Ra’sAl Khaymah, and possibly even Dubai if one wishesto err on the conservative side, as Zone 1. To raiseseismic design actions to Zone 1 in Abu Dhabi wouldbe difficult to justify given how far below the spectralordinates the UHS is across the entire period range.These results are very much in line with the zonationmap of Musson et al. (2006) presented in Figure 5.

The possible existence of a major active geologicalfault running along the west coast of the UAE isclearly an issue of concern, and a definitive assess-ment of the seismic hazard in the Emirates will needto resolve this matter. The seismic hazard studies thathave included this fault, which we have called theWest Coast Fault (WCF), as an active seismic source,are based on the Tectonic Map of Saudi Arabia andAdjacent Areas by Johnson (1998). That study drewheavily on an earlier work by Brown (1972) whichpresented ‘‘selected tectonic elements of Saudi Arabiaand, in lesser details, elements in adjacent parts of theArabian Peninsula’’ (Johnson 1998), whence it is notclear how reliable the information is for the UAE.Amongst the publications on the geology of thisregion that were reviewed, including Al-Hinai et al.(1997), Glennie (2001) and Lippard et al. (1982), onlyHancock et al. (1984) refer to a fault near the westcoast of the Emirates, but the mapped trace isannotated with a question mark and no details arepresented in the text.

Notwithstanding the weak evidence for theexistence of this fault, for the purposes of this

Figure 13. Uniform hazard spectra for rock sites in Dubaifor a 475-year return period from this study and those ofPeiris et al. (2006) and Musson et al. (2006).

Figure 14. Uniform hazard spectra for rock sites in Dubaifor a 2475-year return period from this study and those of

Peiris et al. (2006) and Sigbjornsson and Elnashai (2006).

Georisk 23


sensitivity analysis we assume that it is a 322-km

vertically dipping strike-slip fault running from

23.878N, 53.598E to 25.768N, 56.038E (Figure 6),

based on the map of Johnson (1998). This trace

crosses the cities of Dubai and Abu Dhabi, and

passes very close to Ra’s Al Khaymah, whence if it

were an active source the risk implications would be

very serious. Assigning activity to the fault is more

difficult because there is no instrumental seismicity

that appears to be directly associated with this

structure, and the historical record for the Emirates

is almost null because of sparse population and the

absence of important towns and cities where seismic

damage could have been recorded. Instead, the fault

is assumed to behave as a characteristic earthquake

source and the slip rate is estimated indirectly from

the maximum rate that could pass undetected from

the available information. The data used for this

purpose are contours of the base of the Tertiary and

the approximate base of the Mesozoic rocks that are

overlain by sediments known as sabkhas, which are

composed of sand, silt or clay covered by a crust of

halite (salt). These deposits were formed by post-

glacial flooding between 10 and 15 Ma ago, so it is

conservatively assumed that they are 10 Ma old. The

map of Brown (1972) is at a scale of 1:4 000 000 and

it was assumed that any offset in the contours

resulting from accumulated slip on the fault would

be discernible if at least 1 mm in length on the map,

implying a total slip of 4 km and a slip rate of

0.4 mm/year. Additional constraint on the slip rate

could be inferred from the GPS measurements from

two stations in Oman presented by Vernant et al.

(2004); if one makes the very conservative

Figure 15. 475-year UHS for rock sites in the three cities compared with the spectrum from UBC97 for class B sites in Zone 1.

Figure 16. Hazard curves for Dubai, in terms of PGA and 1-second spectral acceleration, with the seismic sources considered

in this study (thick black line) and including the West Coast Fault as an active source (thin grey line) with a slip rate of0.4 mm/year.



assumption that all of the relative displacement is

taken up by the WCF, this would yield a slip rate of

2.06 mm/year, but most of this displacement is

actually owing to the rotational behaviour of the

Arabian plate and any remaining displacements are

probably accommodated by the Dibba Line. For the

maximum magnitude, the empirical relationship of

Wells and Coppersmith (1994) for strike-slip faults

would suggest a value of Mw 8, but such an event

seems very unlikely in view of the lack of discernible

offset. Therefore, a value of 7.090.5 was assumed;

as the same moment rate is assumed, this may be

considered a conservative assumption as the char-

acteristic event therefore has a much shorter recur-

rence interval.Adopting the slip rate of 0.4 mm/year, the

hazard calculations were re-run, and the mean

hazard curves for Dubai, in terms of two ground-

motion parameters, are shown in Figure 16. The

inclusion of the WCF increases the hazard for

annual exceedance frequencies below 10�3 but

even at 10�6 the increase in the ground-motion

amplitude is less than a factor of two. This is

confirmed in Figure 17, which compares the UHS

for Dubai at three return periods with and without

the contribution from this source. At the return

periods that would normally govern the engineering

design of non-critical structures (i.e., 500 and 2500

years), the increase in the spectral ordinates is

modest. To obtain UHS at all three sites that

matched the UBC97 zone 1 spectrum would require

a slip rate on the fault of 2.5 mm/year, and to

produce a 475-year PGA for Dubai that would

match that obtained by Sigbjornsson and Elnashai

(2006), a slip rate on the fault of 6.0 mm/year would

be required.

Conclusions

This paper presents a new probabilistic assessment ofseismic hazard, in terms of ground motions in rock,for three cities in the United Arab Emirates. Thestudy has been performed within a logic-tree frame-

work to account for uncertainties in the models forseismic sources and ground-motion prediction. Theresults support the conclusions of several previousstudies that the hazard levels in the UAE are low andthat for structures of normal occupancy seismic

design should not be required, except perhaps inmore northerly areas such as Ra’s Al Khaymah. Thethree published studies that suggest much higherhazard levels in the Emirates can be discounted: the

GSHAP map has no technical basis in the UAE;Abdalla and Al-Homoud (2004) use an inappropriateand unjustifiable source zonation that spreads seis-micity associated with the Zagros to the UAE; and

Sigbjornsson and Elnashai (2006) obtain higherhazard estimates largely as a result of including as aseismic source a major active fault running along thewestern coast of the UAE. The seismic potential ofthis source is not supported by instrumental, histor-

ical or paleoseismic evidence, and its inclusion inPSHA calculations is difficult to justify. However, adefinitive seismic hazard study for the UAE shouldinclude a thorough examination of this inferred

structure, using at least geomorphic indicators and,if necessary, paleoseismological investigations as well.If this inferred geological fault were shown to existand a degree of activity were proven, the implications

would be serious and seismic design considerationscould then become warranted. However, to include itas a seismic source in PSHA calculations on the basisof existing evidence is excessively conservativealthough Sigbjornsson and Elnashai (2006) actually

Figure 17. Uniform hazard spectra for rock sites in Dubai at three different return periods as produced by this study (thickblack line) and as modified by the inclusion of the West Coast Fault as an active fault with a slip rate of 0.4 mm/year (thin

lines).

Georisk 25


state that ‘‘as limited information is currently availableon the seismic activity of these faults, it is possible thatthe current study has underestimated their impor-tance’’; our suspicion is the opposite, i.e., that theimportance of the Dibba fault and the fault along theUAE coast has been exaggerated.

The hazard calculations and disaggregations pre-sented in this study demonstrate that the hazard isactually dominated by the sparse local seismicity,particularly at longer return periods. It is importantto bear in mind, however, that the study has notconsidered the effect of surface soil deposits, whichcould significantly amplify long-period motions gen-erated by large-magnitude, distant earthquakes in theZagros and Makran regions, which in turn couldaffect the high-rise structures dominating the skylineof Dubai. Therefore, the conclusions of the currentstudy should not be treated as definitive and before afinal decision can be made regarding whether or notseismic design considerations are required in Dubai,in particular, site response studies considering localsoil effects should be performed.

Acknowledgements

Our thanks to Dr Dirk Hollnack of Munich Re for bringing

to our attention reports of recent seismic activity in andaround the UAE, and also for encouragement in producingthis study. We are also grateful to Dr Matthew Free ofArup Geotechnics for providing a copy of the recent

University of Sharjah seismic hazard study for the Emi-rates. The paper was significantly improved by twoinsightful and constructive reviews and we express our

gratitude to the anonymous referees for their very helpfulcomments on the manuscript. Peter Stafford is a Fellow ofthe Willis Research Network and that support is gratefully

acknowledged.

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