ORIGINAL PAPER

Seismicity and seismotectonics of the Gulf of Aqaba region

Zuhair H. El-Isa

Received: 9 October 2011 /Accepted: 23 May 2012 /Published online: 8 June 2012# Saudi Society for Geosciences 2012

Abstract The historical seismicity of the last ten centuriesand the instrumental data that occurred in the Gulf of Aqabaregion during the period 1982–2008 are evaluated. It isfound that 12 historical earthquakes have occurred withaverage recurrence periods 70–90 and 333–500 years forM≥6.0 and 7.0, respectively. Those with M≤6.5 appear tobe incomplete and require further investigation. More than98 % of the instrumental data has occurred in the form ofswarms and sequences. The first have released about 32 %of the total energy and are most likely related to subsurfacevolcanic activities. Their epicentral distribution indicatesthat all regional faults of the gulf area are active in thepresent, but with clear concentration within the area boundby latitudes 28.2°–29.8° and longitudes 34.4°–35.2°.Regional strike-slip faults of the northern two basins appearto be as twice active as the normal, or more. An appreciablelevel of seismic hazard is envisaged as the “a” value is 6.0–6.2 while the “b” value shows a temporal variation, mostlyin the range 0.8–1.05. More than 95 % of the seismic energywas released from earthquakes shallower than 22 km. Thisindicates a brittle upper crust and a ductile lower crust andupper mantle. Tectonic movements at depths>22 km appearto be aseismic. The epicentral distribution of the fiveswarms indicates that the lengths of the causative faultsvaried in the range 45–70 km. The maximum expectedmagnitude is Mw06.8–7.2. This implies a seismic slip rateof about 0.54–0.8 Cm/year and some 20–30 % of aseismictectonic movements. This and the sequence nature of theseismicity of this region result in a noticeable hazard reduc-tion. Combining the seismicity data of the Gulf of Aqaba

region with other geophysical, geological, tectonic, andenvironmental data, clearly indicate that the seismicity ofthis region is as old as the initiation of the gulf itself. Noapparent southward or northward migration of activity isobserved.

Keywords Gulf of Aqaba . Seismicity . Seismotectonics .

Seismic hazard

Introduction

The seismicity of the Gulf of Aqaba has been mistakenlydescribed by some researchers as only “recent” (e.g.,Salamon et al. 1996; Klinger et al. 1999). Contrary to thislots of geological, geophysical, tectonic, volcanic, environ-mental, historical, and other justifications are available insupport of the fact that the seismicity of this region is as oldas the initiation of the gulf. Historical information asobtained from old Arab literature confirms that more thanten destructive earthquakes (M≥6.0–6.5) were felt in thisregion during the last ten centuries (Poirier and Taher 1980;Maamoon et al. 1984; Ambraseys et al. 1994). Some ofthese were felt quite strongly and were thus epicenteredclose to the gulf if not within it. Two more strong earth-quakes that are known to have occurred in the northern partof Wadi Araba have also been felt in the Aqaba region.Furthermore, all historic earthquakes of the Gulf of Suezwere also felt in the Gulf of Aqaba region as most of theseare quite close. Other lower magnitude earthquakes are alsoreported to have been felt in this region in historic times.Elderly people living in villages and towns of the gulf andother close cities, namely Haqel and Tabuk have reportedthat for many years during the twentieth century they havefelt small-to-moderate earthquakes, some of which appear

Z. H. El-Isa (*)Department of Geology, University of Jordan,Amman 11942, Jordane-mail: elisaz@go.com.jo

Arab J Geosci (2013) 6:3437–3449DOI 10.1007/s12517-012-0604-8

from their description to have occurred as sequences. Due tothe scarcity of earthquake stations in this region prior to the1960s, all earthquakes with magnitudes≤4.0–4.5 are notlikely to have been recorded and located by distant existingstations at the time.

It is clear that the seismicity of the Gulf of Aqaba hasincreased in the last two to three decades, but at the sametime earthquake monitoring in this region has started onlythree decades ago (El-Isa 1983). Within the Gulf of Aqabaregion in particular, earthquake monitoring has also in-creased more within the last two decades, as many stationswere installed in Sinai, west of the gulf, others were in-stalled within northwestern Saudi Arabia, east of the gulf,and before that, more stations were installed within Jordan,north of the gulf. The relatively large number of stationsrunning in the Aqaba Gulf region nowadays is capable oflocating all local earthquakes with magnitudes Ml≥1.5 witha high accuracy.

Both historical and instrumental seismicity data that oc-curred within the Gulf of Aqaba region are compiled fromall available sources. Detailed analyses are made to theseincluding frequency magnitude, seismic moment, and otherseismotectonics calculations. Final results are presented inthis article.

Regional tectonics

The Gulf of Aqaba forms the southernmost segment of the1100 km long Jordan Dead Sea transform that links the RedSea floor spreading in the south to the plate convergence inthe north. With its 180 km length and 20–30 km width, itforms a major plate boundary that separates between theArabian plate in the east and the Sinai-Palestine microplatein the west. Its maximum depth is 1,850 m in its middle part(Garfunkel 1981; Garfunkel et al. 1981). With a total of107 km multi-stage left-lateral shear along this transform(Quennell 1959; Freund et al. 1970; Bartov 1974), minorcomponents of extension, compression, and upwarping oc-cur in many localities including the Gulf of Aqaba. Normalfaulting is quite common along both sides of the gulf withdisplacements that may reach a few kilometers in certainlocalities (Said 1962; Garfunkel 1981). Changes in the trendof the regional strike-slip faults which resulted in the crea-tion of large basins along this transform, such as the DeadSea ( Quennell 1959), have also produced three marinebasins within the Gulf of Aqaba ( Garfunkel 1981). Theseare from north to south Aqaba-Haqel, Nuabie, and Tiran-Maqna basins, see Fig. (1). Extensive uplifting along thewhole transform is a characteristic feature that increasessouthwards towards the Gulf of Aqaba and the Red Sea.This is believed to have started in Late Tertiary. Its

continuation in the present is witnessed along the gulf byraised beaches that can be seen in many localities.

Continental breakup was accompanied by widespread,predominantly basaltic volcanism that was mostly restrictedto the eastern margin. Radiometric dating of these indicatethat this activity began in the Oligocene 30–20 Myr ago andcontinued to Recent times in some localities (Meneisy andKreuzer 1974; Barberi et al. 1975; Blank 1977; Steinitz etal. 1978). On either side of the gulf, long early Neogenedykes trending NW parallel to each other and the Gulf ofSuez are believed to have accompanied the initial stage ofthe development of the Red Sea–Gulf of Suez (Said 1962;Bender 1974; Garfunkel et al. 1974). Faulting along thisdirection (NW) is also present in this area. This volcanismwas followed by the shear along the Gulf of Aqaba. Asystem of faults subparallel to the gulf exists within a zonea few tens of kilometers wide on either side. Said (1962)mapped some of these faults and reported that these werecontemporaneous with the faults of the Gulf of Suez, andolder than those NE faults that determine the location ofAqaba Gulf. Later, these were found to be of left-lateralshear with displacements that range from several meters tocumulative amounts of 24 km in the west and some 20 km inthe east (Freund et al. 1970; Garfunkel 1970).

From a study of the tectonic development of the westernmargin of the Gulf of Aqaba. It is suggested that the wholeshear along the Jordan transform could have happened with-in the last 5–10 Myr. Other geological studies confirm thatthe oldest movements along this transform are surely youn-ger than those in the Suez basin, suggesting the end of theextension of the Gulf of Suez and the transfer of the motionalong the Aqaba Gulf to have occurred some 10–14 Myrago (e.g., Steckler et al. 1988; Bayer et al. 1988).

The seismicity data

All available historical and instrumental seismicity sourceshave been investigated in an attempt to get as complete aseismicity record as possible. These include catalogues andseismicity maps of historical data made by many researchers(e.g., Poirier and Taher 1980; Maamoon et al. 1984; El-Isa1988, 1990, 1992; El-Isa and Shanti 1989; Ambraseys andMelville 1989; Ambraseys et al. 1994; Al Amri and Al-Amri1999; Zilberman et al. 2005; Marzouk 2008). It is well-knownthat the historical seismicity of the Arab and Islamic region isone of the best documented, particularly for the period thatstarts with Islam, more than 14 centuries ago. Therefore it isintended to compile the historical earthquakes which haveoccurred during the last 14 centuries in and around the Gulfof Aqaba. Instrumental data were compiled from a number ofsources and checked against each other. These include cata-logues from the Jordan University Seismological Station

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(UNJ), the Jordan Seismological Observatory (JSO), theInternational Seismological Center (ISC), the NationalEarthquake Information Center (NEIC) of the USGS, andthe American Incorporated research Institutions forSeismology (IRS). The obtained seismicity data are presentedand evaluated in the following sections.

The historical seismicity data

Search in the different seismicity sources revealed that forthe Gulf of Aqaba region in particular, historical seismicityinformation appear to be scarce for the period preceding theyear 1000 AD. It is found that during the last 10 centuries,12 historical earthquakes have affected the Gulf of Aqabaregion (Poirier and Taher 1980; Maamoon et al. 1984;Klinger et al. 2000; Zilberman et al. 2005; Marzouk 2008;Ferry et al. 2011). Six of these were epicentered closer than0.3°–1° from the nearest margin of the gulf, see Fig. (1a)and Table (1). These have occurred in the years 1068, 1212,1312, 1588, 1778, and 1900. The magnitudes of four ofthese are most probably not less than 6.5, but not more than7.0 as their intensity compiled information may indicate.The magnitudes of the other two are most probably about

6.0, but not more than 6.5. Four of these earthquakes areepicentered very close to the northern end of the gulf, two ofwhich may be related to the southern part of Araba fault,namely the 1068 and 1212 earthquakes. It is very likely thatat least some of these four earthquakes, if not all wereepicentered within the gulf proper as firstly, the reporteddamage in the cities of the Arab region, in Egypt, Palestine,Syria, and Saudi Arabia can support this; secondly, two ofthese earthquakes, namely the 1068 and 1588 were locatedby Ambraseys and Melville (1989) in northwestern SaudiArabia, more than 100–220 km east of Gulf of Aqaba, butwithout solid justification; and thirdly, the recent instrumen-tal seismicity data of the following section indicate thatregional strike-slip faults of the gulf are of potential hazardand have produced the Mw07.1 magnitude earthquake ofthe year 1995, see Fig. (1a) and Table (1). It is possibletherefore that at least some of the historical earthquakes, ifnot all have initiated along some of the gulf’s faults. The1312 earthquake is epicentered in southern Sinai but closeenough to both gulfs. The 1778 earthquake is epicentered inRas Mohammad area, i.e., almost within the Gulf of Suez.But it is epicentered along a regional NNE trending strike-slip fault that extends to the Gulf of Aqaba, and thus cantectonically be related to the Gulf of Aqaba. In every case,

Fig. 1 a Regional tectonics of the Jordan Dead Sea transform. Red starsrepresent possible epicenters of all historical earthquakes that have affectedthe Gulf of Aqaba region during the last 10 centuries. The two blue starsrepresent the epicenters of the 1969 Gulf of Suez (M06.9) and 1995 Gulfof Aqaba earthquakes ( M06.1) (b) Fault system and Miocene dykes of

the Gulf of Aqaba region and the three basins resulting from the arrange-ment of the en-echelon strike-slip faults within the gulf. The circlesrepresent the epicentral locations of all instrumental earthquakes thatoccurred in the gulf region with Ml≥4.9 during the period 1982–2008

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its epicenter is very close to both gulfs and thus will equallyaffect both. The 1293 and 1458 earthquakes are epicenteredalong the northern end of Araba fault, close to the southernpart of the Dead Sea. These are about 160 and 180 kmdistant from Aqaba city and their assigned magnitudes arenot less than 6.5, see Fig. (1a) and Table (1). Future similarearthquakes in this region will certainly affect the northernpart of the Gulf of Aqaba. The three Gulf of Suezearthquakes of 1425, 1814, and 1879 are 150–200 kmdistant from the Gulf of Aqaba margins. Future earth-quakes along the Gulf of Suez can be much closer andof more hazard. The same applies to any future earth-quakes that may occur anywhere in the northern part ofthe Red Sea.

Considering that all the 12 historical earthquakes of Table(1) were felt in the Gulf of Aqaba region with intensities≥VI–VII, i.e., Ml≥6.0, and accounting for the two instrumen-tal earthquakes of Table (1), then the overall average recur-rence period of feeling such earthquakes in this region isabout 71 years. Ignoring the three earthquakes of northernGulf of Suez and accounting for the two instrumental ones,the average recurrence period for the Ml≥6.0 earthquake is91 years. Considering the earthquakes with Ml≥6.5 thatmay be confidently related to the Gulf of Aqaba and/or veryclose to it, i.e., the 1068, 1212, 1588, 1312, and 1969earthquakes, their average recurrence period is 200 years.The Ml07.0 magnitude may be assigned to the 1068 and1212 earthquakes only. If this is the case, then the averagerecurrence period for the 7.0 magnitude in this region isabout 500 years. If the magnitude of the 1995 Nuwabieearthquake is considered to be Mw07.2 rather than Ml06.2, then the overall average recurrence periods for the 6.5

and 7.0 magnitudes are reduced to 167 and 333 years inrespective order.

The instrumental seismicity data (1982–2008)

All available local and international seismological bulletinshave been searched for the instrumental seismicity of thelast few decades. These include catalogues from the JordanUniversity Seismological Station (UNJ), the JordanSeismological Observatory (JSO), the InternationalSeismological Center (ISC), the National EarthquakeInformation Center (NEIC) of the USGS, and the AmericanIncorporated Research Institutions for Seismology (IRS). It isfound that the IRIS catalogues appear to include the mostcomplete set of seismicity data for this study region and thusthe following calculations and analyses were based mainly onthe IRIS data. Cross-checking with other bulletins was con-tinuously made. The seismicity of the period before the year1981 is poorly documented in all sources including the IRIS.Therefore it was decided to concentrate on the data for theperiod 1982–2008. It is found that during this period the Gulfof Aqaba region bounded by latitudes 28°N–30°N and longi-tudes 33.9°E–35.5°E has experienced some 4784 earthquakeswith magnitude Ml≥1, some 1848 and 339 earthquakes withmagnitudes Ml≥2.5 and 3.5, respectively. Only nine earth-quakes occurred with Ml≥5 including the largest which oc-curred on November 22, 1995 with Ml06.2, Mw07.2, seeTable (2), Fig. (2) for epicentral distribution of earthquakeswith Ml≥3.5 and Fig. (3) for time distribution of earthquakeswith Ml≥2.5.

Table 1 Historical earthquakes that were felt in the Gulf of Aqaba region during the last ten centuries

Date Latitude (N) Longitude (E) Intensity (Io) Magnitude (M) Epicentral region

3/18/1068 29.8 35.0 IX 6.5–7.0 Gulf of Aqaba

5/2/1212 29.3 34.6 VIII–IX 6.5–7.0 Gulf of Aqaba

1293 31.0 35.5 VIII 6.0–6.5 Northern Wadi Araba

1312 28.5 34 VIII 6.0–6.5 Southern Sinai

6/23/1425 29 33 VII 6.0 Gulf of Suez

1458 31.2 35.5 VIII 6.0–6.5 Northern Wadi Araba

1588 28.5 34.6 VII–VIII 6.0–6.5 Southern Wadi Araba-Gulf of Aqaba

6/23/1778 27.5 34.1 VII 6.0 Ras Mohammad, Gulf of Suez

6/27/1814 29 33 VII 6.0 Gulf of Suez

7/11/1879 29 33 VII 6.0 Gulf of Suez

1887 25.5 34.5 VIII 6.0–6.5 Northern Red Sea

3/6/1900 29.3 34.6 VII 6.0 Gulf of Aqaba

3/31/1969 27.6 34 6.9 Ras Mohammad, Gulf of Suez

11/22/1995 28.8091 34.7951 6.2 Gulf of Aqaba

The largest two instrumentally recorded earthquakes of 1969 and 1995 are also included, see Fig. (1a)

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Frequency-magnitude analysis were made to all the seis-micity data of Table (2) which also includes the deduced aand b values and the calculated recurrence periods for Ml06, 6.5, and 7.0. These analyses show that the threshold

magnitude of the instrumental seismicity data of the Gulfof Aqaba for the period 1982 onward is 2.5. Magnitudesbelow this show a clear deficiency. It is therefore concludedthat the background a value for this study region is in the

Table 2 The number and mag-nitudes of earthquakes thatoccurred in the Gulf of Aqabaregion during the period January1, 1982–December 31, 2008, thecalculated a and b values of thefrequencymagnitude analysis andrecurrence periods for Ml06, 6.5,and 7

Magnitude (Ml ≥) Number Calculated a value Calculated b value Recurrence period in years formagnitude (Ml)

6.0 6.5 7.0

1.0 4784 4.97±0.29 0.7793±0.05 14 34 83

1.5 3738 5.27±0.22 0.847±0.04 18 46 123

2.0 2735 5.60±0.16 0.918±0.03 22 63 181

2.5 1848 5.95±0.11 0.989±0.018 26 82 256

3.0 937 6.24±0.09 1.048±0.015 30 101 336

3.5 339 6.50±0.10 1.098±0.017 33 117 415

4.0 178 6.90±0.10 1.173±0.017 37 143 553

4.5 46

5.0 9

5.5 2

6.0 1

Fig. 2 The seismicity of theGulf of Aqaba region Ml≥3.5as compiled from the IRIS filesfor the period 1982–2008

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range 5.95–6.24 and the background b value is in the range0.99–1.05. This implies that the recurrence periods for the6.0, 6.5, and 7.0 earthquakes are in the ranges 26–30, 82–101, and 256–336 years. Such recurrence periods are lowerthan those deduced from the historical seismicity, particu-larly for the magnitudes 6.0 and 6.5. This implies that either(1) Historical seismicity is incomplete for earthquakes withmagnitudes 6.0≤M≤6.5 or (2) The seismicity fluctuateswith time, and the instrumental data of the period 1982–2008 does not represent the long-term seismicity of the gulfregion, and/or (3) The assigned magnitudes and locations ofhistorical earthquakes require further investigations.

Further frequency-magnitude analyses were made to theseismicity data and the b values were calculated from theseismicity data of each year of the whole period 1982–2008.The results are plotted on Fig. (4) which also shows the totalnumber of earthquakes that occurred in each year withmagnitudes≥2.5 and the highest magnitude of that year.The b value shows a clear temporal variation where itrecords a minimum value (0.48) in the year 1987 and amaximum value (1.49) in the year 2002. It varies for mostyears in the range 0.8–1.1. The seismicity data for the years1989 and 1992 showed minimum number of earthquakesthat did not allow precise b calculations. Total yearly num-ber of earthquakes (Ml≥2.5) show clear fluctuation withtime. Two maxima of 189 and 314 earthquakes are seenfrom the 1993 swarm and the 1995 sequence, respectively.Two smaller maxima are seen from the 1983 and the 1999–2003 swarms. These data show that the Gulf of Aqaba

region was seismologically quite during the periods 1985–1992 and 2003–2008, though two swarms have occurredduring these years, namely the 1986 and the 1990–1991swarms, see also Table (3) and Figs. (3 and 5). The highestmagnitude/year is represented by the 6.2 earthquake of the1995 sequence, followed by the 5.8 earthquake of the 1993swarm, followed by the 4.9 earthquake of the 1983 swarm.A highest magnitude of≤4.4 (non-destructive) is observedfor 16 years, i.e., about 60 % of the study period. During 12of these years, i.e., about 44 % of the study period, thehighest observed Ml is≤3.9.

Considering the total number of earthquakes that haveoccurred within each month of the whole study period(324 months) with magnitudes Ml02.5 and 3.0, the averagedaily numbers of these earthquakes were calculated andplotted on Fig. (5). The equivalent magnitude of all earth-quakes of each month and its seismic moments were alsocalculated and plotted. No single earthquake is reported withMl≥2.5 for the year 1982. Apart from this, the seismicitydata shows clear clusters of earthquake activities repre-sented mainly by swarms and/or sequences. To differentiatebetween swarms and sequences, the following criteria wereconsidered (Utsu 1971; Hill 1977; Ogata et al. 1995; Scholz2002; Fischer et al. 2003): (1) Swarms denote groups ofearthquakes clustering in time and space, none of whichmay be classified as a main shock; (2) Origin times ofearthquakes belonging to a swarm are mutually similar anddiffer from the origin times of other earthquakes in the area;(3) Clustering in time is the most striking feature and

Fig. 3 The time distribution of the seismicity ( Ml≥2.5 ) of the Gulf of Aqaba region during the period 1982–2008

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probably displays a common origin of the swarm events; (4)Significant decrease of the inter-event (waiting) time isobserved for swarms, while it remains large for backgroundactivity; and (5) The largest aftershock in the sequence is atleast 1 unit smaller than the mainshock (Utsu 1971), thoughother researchers lower the difference to 0.45, i.e., if thelargest aftershock is lower than the mainshock by less than0.45, then this will be a swarm (e.g., Ogata et al. 1995).

Based on these criteria, the clusters of the seismicity of theGulf of Aqaba, Figs. (3 and 5), were evaluated and found toconsist of five typical swarms, namely the 1983, 1986, 1990–1991, 1993, and the 1999–2003 activities. During the periodsMay 1983–March 1990 and April 2003–December 2008,relatively low seismicity is observed to be scattered all alongthe gulf region. The normal background seismicity level in theGulf of Aqaba area appears to be represented by the average0.67 earthquake/day with Ml≥2.5. Higher rates indicate ab-normal activities represented mostly by sequences and

swarms. The 1995–1998 activity is the only cluster that quali-fies for a sequence. The total calculated seismic moment forthe whole seismicity amounts to 4.7835×10e25 dyne.cm,which is equivalent to an earthquake with Ml06.4. The seis-mic moment of all swarms and one sequence amounts to4.6803×10e25 dyne.cm, which represents about 97.8 % ofthe total. This indicates that at least during the study period,the seismic energy was released from the Gulf of Aqabaregion mainly in the form of sequences and swarms. Theseismic moment of the whole 1995 sequence is 3.1377×10e25 dyne.cm which represents about 66 % of the totalmoment, see Table (3). The focal mechanism of the largestshocks of this sequence is known to be mainly of strike-slipwith minor dip-slip components (Pinar and Turkelli 1997;Abdel-Fattah et al. 1997, 2006 ; Badawy and Horváth1999a, b). This sequence included some 857 earthquakes(Ml≥2.5) with their epicentral distribution scattered all overmost of the gulf between latitudes 28.3° and 29.5° and

Table 3 The main characteristics of the five earthquake swarms and the one sequence that occurred in the Gulf of Aqaba region during the studyperiod (1982–2008)

Date Mlargest Number ofshocks Ml≥3

Duration Area oflargest shocks≥3

Focal mechanismof largest shocks

Seismic moment ×10e25 dyne.cm

Mequivalent b value of thewhole swarm

Jan.–Apr. 1983 Swarm 4.9 94 4 months Lat 28.8°–29.4° Strike-Slip 0.1559 5.4 1.05Long 34.4°–35.1°

Mar.–June 1986 Swarm 3.2 1 4 months Lat 28.5°–29.6° 0.00021 3.5 1.0Long 34.6°–35.0°

Apr 1990–May 1991 Swarm 4.3 32 14 months Lat 28.8°–29.3° 0.00039 4.3 1.05Long 34.3° 34.85

July 1993–July 1994 Swarm 5.8 206 13 months Lat 28.5°–28.85° Dip-Slip 1.3463 6.0 1.08Long 34.55°–34.9°

Nov.1995- Mar. 1998Sequence

6.2 480 28 months Lat 28.75°–29.3° Strike-Slip 3.1377 6.3 1.01Long 34.65°–35°

July 1999–Mar. 2003 Swarm 4.5 163 45 months Lat 28.1°–29.5° 0.0398 5.0 1.26Long34.4°–35.1°

-35

15

65

115

165

215

265

315

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1983 1988 1993 1998 2003 2008

Nu

mb

er o

f ea

rth

qu

akes

/yea

r

Cal

cula

ted

b-v

alu

e

Time in years

4.9 4.8

3.1

5.86.2

3.9

2.6

4.4

Fig. 4 Temporal b-variation(blue line), the total number ofearthquakes that occurred peryear with magnitudes Ml≥2.5(red) and the highest Ml in eachyear for the Gulf of Aqabaregion during the period1982–2008

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longitudes 34.4° and 35.1°. Most earthquakes with Ml≥4.0are epicentered in the region between latitudes 28.6° and 29.4°and longitudes 34.5° and 35°, many of which appear to beepicentered along the two strike-slip en-echelon faults ofNuwabie basin including the largest 4 earthquakes with Ml≥5.0, see Figs. (1 and 2). The epicentral distribution of thisactivity indicates that the three major NNE trending strike-slipfaults of the Gulf of Aqaba were active during the period ofthis sequence, but most of the activity concentrated along themargins of Nuwabie basin, Fig. (1), see also (Pinar andTurkelli 1997; Abdel-Fattah et al. 1997, 2006). The seismicmoment of the five mentioned swarms amounts to 1.5426×10e25 dyne.cm representing about 32 % of the total activity.These are of three types: large, medium, and small. The 1993–1994 swarm is of the first type. It is the largest swarm thatoccurred during the study period and lasted for more than ayear. Its seismic moment amounts to 1.3463×10e25 dyne.cmwhich represents about 28 % of the total, see Table (3) andFigs. (3 and 5). This activity is equivalent to one earthquakewithMl06.0. The swarm included five earthquakes with Ml05–5.8 and 47 with Ml≥4.0, all of which were epicenteredwithin the area bounded by latitudes 28.5° and 28.8° andlongitudes 34.6° and 34.8°. The focal mechanism of thelargest shocks of this swarm is mainly dip slip with minorstrike-slip components (Abdel-Fattah et al. 1997, 2006; Pinarand Turkelli 1997; Badawy and Horváth 1999a, b). Thismechanism and their location indicate that the normal faultsthat bound the gulf west of Nuwabie basin are the source of

this large swarm. The second (medium) type is represented bythe 1983 and 1999–2003 swarms. The first lasted about4 months and included 20 earthquakes with Ml04.0–4.9.These were epicentered mainly between latitudes 28.9° and29.4° and longitudes 34.6° and 35.2°. Their epicentral distri-bution and strike-slip mechanism of their largest (El-Isa et al.1984) strongly relate this swarm to the northeast trendingstrike-slip fault of Aqaba–Nuwabie that forms the westernborder of the Aqaba–Haqel basin, see Figs. (1 and 2). Theseismic moment of this swarm is 0.1559×10e25 dyne.cmwhich forms about 3.3 % of the total moment of the studyregion, see Table (3) and Figs. (3 and 5). Its Mequivalent is5.4. The 1999–2003 swarm lasted for more than 45 monthsthrough which some 163 earthquakes were recorded withMl03.0–4.5. The total seismic moment of this swarm was0.0398×10e25 dyne.cm representing less than 1.0 % of thetotal seismic moment of the study region, see Table (3) andFigs. (3 and 5). Its Mequivalent is 5.0. The epicentral distri-bution of some 457 earthquakes of this swarm with Ml≥2.5cover most of the gulf between latitudes 28.1° and 29.5° andlongitudes 34.4° and 35.1° which may indicate that all region-al faults of the gulf were active, but slightly during the periodof this swarm. Careful examination to epicentral distributionshows that during the first 24 months of this swarm, thenorthern half of the gulf between latitudes 28.8° and 29.5°and longitudes 34.4° and 35.1° was more active than thesouthern part. As from November 2002, the activity becamehigher in the southern part between latitudes 28.1° and 28.8°

Fig. 5 The average daily occurrence of all earthquakes with Ml≥2.5 (blue) and 3.0 (red) based on monthly calculations for the 324 months of thestudy period. Calculated equivalent magnitude (Meq.) of the seismicity of each month and its seismic moments (yellow) are also plotted

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and longitudes 34.6° and 34.8°. The third (small) type isrepresented by the other two swarms of Table (3), namely,the 1986 and 1990–1991 swarms, see also Figs. (4 and 5). Thefirst lasted 4 months and included only one earthquake withMl≥3. With its limited seismic moment (0.00021×10e25dyne.cm) and 3.5 Mequivalent, it was epicentered mainlybetween latitudes 28.5° and 29.6° and longitudes 34.7° and35°. This may indicate that both en-echelon faults of theAqaba–Haqel basin form the source of this swarm where thesecond extends farther south towards Nuwabie basin, see Fig.(1). The 1990–1991 swarm appears to have lasted some14 months, though much of the activity occurred within oneweek during April 1990. It included only three earthquakeswith Ml≥3.0, had an Mequivalent04.3, and a seismic mo-ment of 0.00039×10e25 dyne.cm. This swarm was epicen-tered mainly between latitudes 28.85° and 29.35° andlongitudes 34.4° and 35°, i.e., within the northern part of themiddle Nuwabie basin and the southern part of the northernAqaba–Haqel basin, see Figs. (1) and (2). This may thereforeindicate that the faults of the northern part of the gulf wereactive during the period of this swarm. The focal depths of all339 earthquakes that occurred in the Gulf of Aqaba regionduring the 27-year study period with Ml≥3.5 are plotted onthe cross section of Fig. (6a) with respect to their latitudes.Only one of these is reported with a depth h060 km, thoughseven other smaller magnitude earthquakes are reported withfocal depths in the range 50–120 km. Such anomalous depthearthquakes for this region remain questionable, particularly ifthese are of low magnitudes. Apart from these, only six otherearthquakes are reported with depths slightly higher than30 km. Only 43 earthquakes had 10 km≤h≥22 km and 38others had h≥22 km, i.e., about 90 % of all earthquakes withMl≥3.5 have occurred at depths≤22 km, i.e., within the uppercrust including the largest of the 1993–1994 swarm and the1995–1998 sequence. More than 80 % of earthquakes isreported with depths h≤15 km, i.e., within the upper part ofthe upper crust. The focal distribution of Fig. (6a) indicatesthat most of the earthquakes are located between latitudes28.2° and 29.4°, i.e., over a total length of about 130 km.Most earthquakes are epicentered between latitudes 28.7° and29.4°, i.e., within a length of about 80 km, an area that coversboth Nuwabie and Aqaba–Haqel basins of Fig. (1).

Some 85 % of all earthquakes with Ml≥4 of the 1983swarm had focal depths h≤21 km. All earthquakes of thisswarm with Ml≥4 are bound between latitudes 29.1°–29.3°and longitudes 34.6°–35°. This means that the length of thecausative fault of this swarm is mostly≤50 km. Only fiveearthquakes of the 1986 swarm had reported focal depths,all were shallower than 20 km, three of which were shal-lower than 12 km. All reported earthquakes of the 1990–1991 swarms had focal depths shallower than 15 km. Morethan 80 % of these occurred within the uppermost 5 kmdepth of the uppermost crust. Most earthquakes of this

swarm were bound between latitudes 28.8° and 29.2° andlongitudes 34.5° and 34.9° which imply that the length ofthe causative fault is mostly≤50 km. More than 90 % of theearthquakes of the 1993–1994 swarm occurred at depthsshallower than 22 km. Not less than 60 % of the activityincluding all with Ml≥5 occurred at h≤12 km and some40 % at depths h≤3 km. This implies that more than 95 % ofthe seismic energy of this swarm was released from theupper part of the upper crust. The swarm concentratedwithin the area bound by latitudes 28.5°–29.1° and longi-tudes 34.4°–35° which implies that the length of the causa-tive fault is mostly≤70 km. More than 95 % and 90 % of theearthquakes of the 1995–1998 sequence had focal depthsshallower than 22 and 15 km, respectively, including thelargest (Ml06.2), see Fig. (6b). Some 80 % and 70 % of theearthquakes of this sequence had focal depths h≤10 and5 km, respectively. More than 95 % of the seismic energyof this sequence has been released from the uppermost 10–15 km of the crust, see Fig. (6b). More than 85 % of theactivity of the 1999–2003 swarm had focal depths≤15 km.The focal depth distribution of all instrumental seismicitydata of the study region for the period 1982–2008 clearlyindicate a brittle upper crust and a ductile lower crust anduppermost mantle. This is in agreement with previous con-clusions made for the whole Jordan Dead Sea transformregion (e.g., El-Isa 1988; Mechie and El-Isa 1988). Withinthe upper crust itself, it appears that the uppermost half is themost brittle.

Discussion

The seismicity data of the Gulf of Aqaba for the period1982–2008 indicate that all its regional faults are active inthe present including both normal and strike-slip types.Contrary to the belief of some workers that the seismicityof this region is only recent (e.g., Salamon et al. 1996 ;Alamri et al. 1991; Klinger et al. 1999), both historical andinstrumental seismicity data strongly indicate that the earth-quake activity of this region is as old as the initiation of thegulf. This is particularly true when all geological, tectonic,geophysical, and environmental factors are considered.Collected information from elderly people in the cities ofAqaba, Haqel, and Tabouk confirm the occurrence of rela-tively big earthquakes that occurred in this region during thetwentieth century prior to the year 1982. Some are reportedto have occurred in sequences and swarms. The fact thatinstrumental seismicity of this region appears to start onlytwo to three decades ago is simply a matter of seismologicalmonitoring. A clear increase of activity with time correlateswith the construction of more seismological stations east,west, and north of the gulf. The compiled data of the period1982–2008 indicate a threshold magnitude of about Ml0

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2.5, though many lower magnitudes are routinely reportedin recent years. The normal background seismicity level ofthe Gulf of Aqaba is represented by the occurrence of onytwo earthquakes per month with Ml≤2.5.

Previous studies on historical seismicity of this regionduring the last ten centuries (e.g., Poirier and Taher 1980;Maamoon et al. 1984; Klinger et al. 2000; Zilberman et al.2005; Marzouk 2008; Ferry et al. 2011) indicate the occur-rence of only two to three earthquakes, one of the largest,namely the 1068 earthquake of Table (1) has been located

some 160 km east of the gulf by Ambraseys and Melville(1989). The historical data presented in this article indicatethat on the average, the Gulf of Aqaba area has been affectedby one earthquakeM≥6.0 once every about 70 years. Some ofthese were probably located within the gulf or at least veryclose to it. This is evidenced by: (1) the local and regionalintensity effects of these earthquakes, (2) the epicentral distri-bution of the instrumental seismicity of the above section, and(3) the geotectonic evolution of the study area. When thecalculated recurrence periods of Table (2) are correlated with

28 28.2 28.4 28.6 28.8 29 29.2 29.4 29.6 29.8 30-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Latitude

28 28.2 28.4 28.6 28.8 29 29.2 29.4 29.6 29.8 30-40

-35

-30

-25

-20

-15

-10

-5

0b

a

Latitude

Fig. 6 a The focal depth distribution of all earthquakes (Ml≥3.5) that occurred in the Gulf of Aqaba region during the period 1982–2008. b Thefocal depth distribution of all earthquakes ( Ml≥3.5 ) that occurred in the Gulf of Aqaba region during the 1995–1998 sequence

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those obtained from historical data, it is noted that for earth-quakes with magnitudes≥7.0, the deuced periods appear tocorrelate as both are in the ranges 256–336 and 333–500 years,respectively. Those recurrence periods for both magnitudes≥6.0 and 6.5 of the instrumental data are almost half those of thehistorical: 26–30 years compared to 71–91 and 82–101 yearscompared to 167–200 years in respective order, see Table (2).This discrepancy can be caused by one or more of the follow-ing: (1) historical seismicity is incomplete for earthquakeswith magnitudes 6.0≤M≤6.5 or (2) the seismicity fluctuateswith time, and the instrumental data of the period 1982–2008does not represent the long-term seismicity of the gulf region,and/or (3) the assigned magnitudes and locations of historicalearthquakes require further investigations. It is more likelythat the historical seismicity is incomplete for the 6.0–6.5magnitude range.

The seismicity of the study area occurs almost totally inthe form of sequences and swarms. During the 27 years ofthe study period, five swarms were recorded which resultedin the release of about 32 % of the total seismic energy.These are in three types that vary in their time duration,focal mechanism, causative sources, and seismic energy.The 1983 swarm was reported to be related to possiblesubsurface volcanic activities (El-Isa et al. 1984). It is notunlikely that the other swarms might also be related tosimilar cause as the volcanic activities in the study areaand the Jordan Dead Sea transform region in general, arewell documented and known to continue to recent times(e.g., Horowitz et al. 1973; Meneisy and Kreuzer 1974;Barberi et al. 1975; Blank 1977; Steinitz et al. 1978). Theoccurrence of earthquakes as swarms and sequences resultsin a considerable hazard reduction. The 1995–1998 se-quence was a major activity that resulted in releasing about66 % of the total seismic energy of the study period, but itshazard was limited as firstly, it was a long period sequencethat lasted about 28 months, secondly, its largest shockreleased only about half of the total energy of the sequence,and thirdly, this main shock is reported to has occurred asthree successive shocks that had some 5 to 11 s betweeneach two ( Abdel-Fattah et al. 1997, 2006 ; Klinger et al.1999 ; Pinar and Turkelli 1997).

The a and b results of Table (2) show that the first variesin the range 5–6.9, while the second varies in the range0.78–1.17. The frequency magnitude analysis shows a nicelinear relationship starting from magnitude≥2.5, i.e., numb-ers of lower magnitude earthquakes are incomplete. Thismagnitude is therefore the threshold for this study region.As numbers of earthquakes with magnitudes≥3.5 or 4.0 arealso reduced, it is concluded that the best a and b values forthis study region are represented by earthquakes with mag-nitudes≥2.5 and/or 3.0 of Table (2). Their values are 6.0–6.2and 0.99–1.05 in respective order. The a value indicates arelatively high hazard in this region as the recurrence

periods for the magnitudes 6.0, 6.5, and 7.0 are 26–30,82–101, and 256–336 years in respective order. Such peri-ods are lower than the deduced values for the Jordan DeadSea transform as a whole (El-Isa 1988, 1992). The calculat-ed b value after the occurrence of each swarm is mostly inthe range 1.0–1.05, typical for continental transformregions, see Table (3). On the other hand, the b values showa considerable temporal variation in the range 0.48–1.49,mostly in a lower range 0.8–1.1. Such variations appear tobe a common criterion preceding and following the earth-quake occurrence and are mostly related to changes in thestress system in the concerned study region (El-Isa 2011).Such variations will also affect the level of the hazard as itwill change the deduced recurrence periods.

The epicentral distribution of the instrumental seismicitydata indicates that all regional faults of the Gulf of Aqabaregion are active in the present. A major concentration forall earthquakes≥3.5 is observed between latitudes 28.2°–29.8° and longitudes 34.4°–35.2°. This includes events fromall mentioned swarms and sequence, see Fig. (6a, b). Anorthward or southward migration of activity with time(e.g., Abdel-Fattah et al. 2006; Klinger et al. 1999) is notevident from this data. It is noted however that the regionalfaults of the Aqaba–Haqel and Nuwabie basins were themost active during the study period including both normaland strike-slip types of faults, the later being more active.Accepting the fact that the major events of the 1993–1994and one of the two or three subevents of the major 6.2magnitude earthquake of the 1995 sequence that representsabout 18 % of its total released energy had a dip-slipmovement while the other subevents and other big after-shocks of this sequence and the 1983 swarm, all had strike-slip movements (Abdel-Fattah et al. 1997, 2006; Pinar andTurkelli 1997; El-Isa et al. 1984), it is concluded that about40 % of the seismic energy was released by dip-slip move-ments. If the 6.2 earthquake is considered totally of strike-slip movement (Klinger et al. 1999), then the dip-slip move-ment is reduced to about 28 % only. In every case theinstrumental seismicity data strongly indicate that the re-gional tectonics of the Gulf of Aqaba is characterized byboth vertical and horizontal movements, the second beingabout twice the first, or more.

Considering that the fault length is normally defined bythe extent of the aftershock zone (Benioff 1962; Mogi1968), the epicentral distribution of four of the swarms,namely the 1983, 1990–1991, 1993–1994, and the 1999–2003 indicate that the lengths of their causative sourcesvaried in the range 45–70 km. The aftershocks of the1995–1998 sequence indicate that the length of its causativesource is about 80 km. This is the approximate length of anyof the regional en-echelon strike-slip faults of the gulfregion, see Fig. (1). This indicates that during the studyperiod, tectonic movements were restricted mostly to only

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portions of the individual faults at a time. The only possibletime that movements might have involved the total length ofa fault is during the first day of the 1995–1998 activity, andthen not at its full depth. Such length is slightly higher thanpreviously determined from seismological data (50 km) andgeodetic and interferometry (65 km) as reported by Klingeret al. (1999) and Abdel-Fattah et al. (2006). More than 90 %and 80 % of all earthquakes with Ml≥3.5 have occurred atdepths shallower than 22 and 15 km, respectively. Thelargest shocks of the 1993–1994 swarm had focal depths≤12 km and that of the 1995–1998 sequence and its subeventshave occurred at depths≤13–15 km. In the Aqaba regionand farther north, a transitional zone occurs in the depthrange 18–21 km that separates between the upper and lowerparts of the crust. The depth to this zone decreases slightlyto the south beneath the Gulf of Aqaba region (Ginzburg etal. 1979; El-Isa et al. 1987). It is therefore concluded thatearthquake activity appears to be restricted mostly to thebrittle upper crust. Very little and small earthquakes occur inthe ductile lower crust. A clear concentration of seismicity isobserved in the upper part of the upper crust which seems tobehave more brittle. It is concluded therefore that tectonicmovements at depths below the upper crust do occuraseismically. Within the upper crust itself, the seismic be-havior appears to decrease slightly with depth.

Conclusions

1. Both historical and instrumental sets of data indicatethat the seismicity of the Gulf of Aqaba is not recent.When these data are combined with other geophysical,geological, tectonic, and environmental data, it becomesvery clear that the seismicity of this region is as old asthe initiation of the gulf itself. No apparent southward ornorthward migration of activity is observed.

2. The Gulf of Aqaba region has been affected by some 12historical earthquakes within the last 10 decades withaverage recurrence periods 70–90, 167–200, and 333–500 years for M≥6.0, 6.5, and 7.0, respectively. Atleast, the largest four of these have occurred very closeto the gulf or within it. Correlating historical with in-strumental seismicity data, it is concluded that the firstappears to be incomplete and require further investiga-tions, particularly for earthquakes with M≤6.5.

3. More than 98 % of the instrumental seismicity of thestudy period 1982–2008 has occurred in the form ofswarms and sequences. The first have released about32 % of the total energy and are most likely related tosubsurface volcanic activities.

4. The epicentral distribution of the instrumental seismicitydata indicates that all regional faults of the gulf area areactive in the present. Most of the activity of the study

periodwas concentratedwithin the area bound by latitudes28.2°–29.8° and longitudes 34.4°–35.2°. This implies thatthe regional strike-slip and normal faults of the northerntwo basins Aqaba–Haqel and Nuwabie were the mostactive during this period. Their activity is at least twicethat of the normal faults.

5. Frequency-magnitude analysis indicate an appreciablelevel of seismic hazard in this region as the a value is6.0–6.2 and the b value shows a temporal variation,mostly in the range 0.8–1.05, typical for continentaltype of transform regions. The fact that more than98 % of the seismic energy was released in the formof swarms and sequences results in a considerable re-duction of the hazard level.

6. The focal depth distribution indicates that more than95 % of the seismic energy was released from earth-quakes shallower than 20–22 km with a major concen-tration at depths≤15 km. This indicates a brittle uppercrust and a ductile lower crust and upper mantle.Tectonic movements at depths≥22 km appear to bealmost totally aseismic.

7. The epicentral distribution of the five swarms indi-cates that the lengths of the causative faults varied inthe range 45–70 km. The maximum observed faultlength (80 km) is that associated with the 1995–1998sequence. Considering such fault lengths and depths,it is concluded that the maximum expected magni-tude from the Gulf of Aqaba faults is in the rangeMw06.8–7.2.

8. Considering the calculated recurrence periods and seis-mic moment and the deduced fault lengths and depths, itis concluded that seismic slip rate along the Gulf ofAqaba region is in the range 0.47–0.70 Cm/year if thefault length and depth are 80 and 15 km, respectively. Ifthe length is reduced to 70 km, the slip rate increases to0.54–0.8 Cm/year. This indicates that not less 20–30 %of the tectonic movements of the study region are aseis-mic which results in a further hazard reduction.

Acknowledgments This work has been completed during a sabbaticalyear the author spent at the University of Michigan, Ann Arbor, withfinancial support from the University of Jordan, Amman. Thanks are dueto Larry Ruff of UM for many useful discussions through the preparationof this work and his critical reading of the manuscript.

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