A new evaluation of seismic hazard for the northwesternpart of Saudi Arabia

Nassir S. Al-Arifi • R. E. Fat-Helbary • Ahmad R. Khalil •

Aref A. Lashin

Received: 11 September 2012 / Accepted: 23 May 2013� Springer Science+Business Media Dordrecht 2013

Abstract A new evaluation of seismic hazard in the northwestern part of Saudi Arabia area

has been carried out, for the minimization and mitigation of earthquake losses. The study area

controlled by a number of major regional tectonic features including the Red Sea, Gulf of

Suez, and Gulf of Aqaba—Dead Sea fault system and a larger number of recorded earth-

quakes. A probabilistic seismic hazard analysis is performed within a logic-tree framework.

The analysis started with an exhaustive revision of the seismic catalogues of the area and

vicinity from which a catalogue has been declustered and homogenized at moment magni-

tude, Mw. According to the tectonic setting and spatial distribution of earthquakes in area and

vicinity, twelve seismic zones are defined. The seismic hazard parameters, namely the b-

parameter (of the Gutenberg–Richter relation), Mmax (the upper-bound magnitude), and, k(the annual rate of occurrence of earthquakes with minimum magnitude Mmin C 3), are

calculated for each seismic zone. The ground-motion prediction models have been selected

according to the tectonic regime associated with the earthquakes in each source zone. The

results are displayed in the form of contour maps of peak ground acceleration and five ground-

motion spectral periods for a return period of 475 years. The maximum peak ground accel-

eration values are found around the Gulf of Aqaba and it is 325 gal for 475-year return period

(equivalent to 90 % probability of non-exceedance in 50 years), while the lowest values in

the eastern region is 75 gal. This is the first study developed at the northwestern part of Saudi

N. S. Al-Arifi (&) � A. A. LashinDepartment of Geology and Geophysics, College of Science, King Saud University, Riyadh, SaudiArabiae-mail: nalarifi@ksu.edu.sa

R. E. Fat-HelbaryAswan Earthquake Research Center, NRIAG, Aswan, Egypt

A. R. KhalilPrince Sultan Research Center for Environment, Water and Desert, King Saud University, Riyadh,Saudi Arabia

A. A. LashinGeology Department, Faculty of Science, Benha University, P.O. Box 13518, Benha, Egypt

123

Nat HazardsDOI 10.1007/s11069-013-0756-1

Arabia in terms of peak ground acceleration and different spectral acceleration—SA (T). As a

result, the new generation of maps will be useful in the revision of seismic codes of the area.

Keywords Seismic hazard � Saudi Arabia � Spectral acceleration � Ground motion

1 Introduction

The Saudi Arabia has been undergoing accelerated development in recent years and cur-

rently has one of the highest rates of construction in the world. The investigated area is

located between Lat. 26.5�–31.5�N and Long. 34�–38�E northwestern part of Saudi Arabia

(Fig. 1). Due to their proximity of the study area, the seismotectonic of the Dead Sea Rift,

Red Sea and Gulf of Suez, and a larger number of recorded earthquakes, special emphases

were placed on the study of seismic hazard in the selected area. This study is concerned

with obtaining a reliable estimate of the seismic hazard in terms of peak ground accel-

eration (PGA) and different spectral acceleration—SA (T) for the purpose of minimization

and mitigation of earthquake losses, and for the implementation of planning and socio-

economic development.

Different methods of approach and distribution equations have been developed and

applied in the evaluation of seismic hazard, e.g., Kawasumi (1957), Cornell (1968), Al-

germissen and Perkins (1976), Hattori (1979), Schenkova et al. (1981), Boore and Joyner

(1984), Kijko and Sellevoll (1990), and the applications of Poisson distribution and

Gumbel’s theory (1958) of extremes. The methods usually take into account the source-

propagation-site effects using the free or arbitrary zone method to obtain a frequency

relation; the seismic hazard or expected maximum value of ground motion for a given

interval of time is then estimated using probability distribution.

Some of these methods have been applied in some related studies of western Saudi

Arabia and adjacent areas (Ali 1984; Al-Noury and Ali 1986; Yucceman and Qa’dan 1987;

Thenhaus et al. 1989; Al-Haddad et al. 1994; Al-Amri 1995; Al-Amri et al. 2003) based on

arbitrary zonation of the seismic sources. These studies have drawn very different con-

clusions regarding the level of seismic hazard in NW Saudi Arabia area, with estimates of

the 475-year PGA in the study area ranging from \0.1 to 0.325 g.

Authors used different data catalogue that were gathered and cross-checked from dif-

ferent sources and networks and for deferent time spans that make the final product

(catalogue) as complete as possible. We also used different seismogenic zones, test each,

and conclude the best zones that lead the study to new results considered as an addition to

the seismic hazard of the study area. The study area will have an economical and geo-

strategic importance for the near future of the Middle East.

In this study, the probabilistic seismic hazard assessment (PSHA) is conducted because

it is widely considered as seismology’s most valuable contribution to earthquake hazard

assessment (Reiter 1991; Frankel 1995; Fat-Helbary 1995; Woo 1996; Giardini 1999;

Bommer et al. 2004; Deif et al. 2011). This approach is based on the assumption that

seismicity is Poissonian process, with event magnitude exponentially distributed and

equiprobability of occurrence over a seismogenic zone. The theoretical assumption of this

approach was first introduced by Cornell (1968), and it has been computerized by various

authors (Algermissen and Perkins 1976; McGuire 1976) by considering the seismic sources

as wide area or as linear faults. For each seismic source, the geometry, recurrence

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relationship, and attenuation relation are needed. PSHA is performed utilizing CRISIS

2007 software (Ordaz et al. 2007). CRISIS 2007 accommodates uncertainty in the seis-

micity parameters and ground-motion scaling model, and has a user-friendly interface.

The effect of the aleatory uncertainty is taken into consideration in the hazard equation

and handled by CRISIS 2007 by specifying a mean value and bounds for the recurrence

parameters, the maximum magnitude of each seismogenic zone, and the predicted ground

motions. In this study, the epistemic uncertainties are treated by taking alternatives for the

seismotectonic models, and the ground-motion scaling relationship, which in turn impli-

cate several different estimates of the ground motion.

The output of a seismic hazard analysis using PSHA is expressed by the cumulative

probability distribution of a maximum ground-motion intensity parameter for individual

site. If a region with more than a single site is concerned, the seismic hazard analysis can

Fig. 1 Tectonic setting of the Dead Sea transform in the Middle East, compiled after Garfunkel et al.(1981); Salamon et al. (1996). Arrows indicate directions of relative motion at faults

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be systematically applied to a grid of point and constructing a regional seismic probability

maps for acceleration or intensity levels corresponding to a given return period.

A probabilistic seismic hazard assessed to produce 5 % damped spectral acceleration

values on rock for the PGA, in addition to five spectral periods (0.1, 0.2, 0.3, 1.0, and 2.0 s)

for a return period of 475 years to define approximate uniform response spectra at each

0.25 9 0.25 grid nods.

2 Tectonic setting

Saudi Arabia comprises the largest part of the Arabian plate, which is characterized by a

variety of seismotectonic environments. These seismotectonic environments result from the

motion of the Arabian plate relative to other neighboring plates along narrow zones or

tectonic boundaries. Red Sea and Dead Sea fault systems have been identified as significant

boundaries relative to the study area (Fig. 1). The Red Sea is a NNW–SSE-trending

depression about 2,000 km long, separating Arabia and Africa (El-Shazly 1982). Morpho-

logically, the floor of the Red Sea is built of elongated main trough (from 15�N to the Sinai

Peninsula, 28�N), bisected in the south by a deeper and narrower axial trough (between 15�N

and 20�N) where the seafloor spreading process took place (Cochran 1983). North of 20�N,

the axial trough becomes discontinuous forming a series of large deeps spaced at about

50 km. The region to the north of 23�300N is interpreted as representing the late stage of

continental extension and is characterized by the presence of an axial depression.

At the northern end of the Red Sea, the active extensional features produced the strike-slip

motion along the Gulf of Suez in addition to the sinistral shear along the Gulf of Aqaba–Dead

Sea rift system (Ben-Menahem et al. 1976). It is postulated that these depressions were

formed by the anticlockwise rotation of Saudi Arabia away from Africa about a pole of

rotation in the central or south-central Mediterranean Sea (36.5�N, 18.0�E) (Le Pichon and

Gaulier 1988) and the Sinai sub-plate has moved relative to both Africa and Arabia.

The Suez rift is a slightly arcuate, generally NW (30�N–40�W, parallel to the major

normal faults) striking depression, 69–80 km wide, and about 500 km long (Steckler et al.

1988). Its structure is dominated by faults. The birth of the Gulf of Suez graben dates back

to the late Oligocene (Said 1969).

The Aqaba rift is 30–40 km wide and 110 km long; it forms the southern part of the

1,100 km long Dead Sea Rift that separates the Arabian Plate from the Sinai sub-plate.

Hosney (1985) suggested a late Miocene–Pliocene age of the Aqaba Gulf. The main faults of

Aqaba Gulf trend are N–S to NNE–SSW. The Aqaba Gulf–Dead Sea fault system is a left-

lateral transform fault linking the Zagros–Taurus area of plate convergence, with the Red

Sea opening (Lyberis 1988). Tectonically, the Gulf of Aqaba faulting structure is dominated

by a N–S strike-slip system that resulted in the creation of three en-echelon rhomb-shaped

grabens, similar to that at the Dead Sea (pull-apart basin), where changes in the trend to

NNW and back to N–S occur. These grabens are filled with a few kilometers of sediments.

The Red Sea and its northern extension, the Gulf of Suez, and the Gulf of Aqaba are

considered to be the result of divergence of the lithospheric plates.

3 Seismicity

Historical seismicity is reported to have been felt in the study area (northwestern Saudi

Arabia during the period from 95 BC to 1910 AD at different localities with intensities in

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the range VI–IX). Some of these earthquakes have a macro-seismic magnitude determined

from the felt data and ranges from 5.4 to 6.7, while others have no defined magnitude. The

assigned intensity values are based on the reported damage that may be not homogeneous

either in time or space. From these historical activity, a conclusions can be made as

follows: (a) The only historical earthquake that has been reported in Gulf of Aqaba is the

March 18, 1068 earthquake (28.5 N and 36.7 E) that occurred in northern Hejaz and

caused destruction over hundreds of kilometers in northwestern Arabia and killed 20,000

people. While there are destructive earthquakes have been reported in the Gulf of Suez,

Jordan-Dead Sea transform, and Eastern Mediterranean regions (Ambraseys et al. 1994). In

addition, two earthquakes reported to have been felt in January 6, 1910 at 29.9 N and

33.8 E while the second one was felt in February 12, 1091 at 28.5 N and 34.0�E with an

intensity of VI–VII. This earthquake distribution may be due to the past population density

in the area. (b) The epicentral distribution of these historical earthquakes seems to correlate

with the general tectonics of the region. In the Gulf of Suez, four historical earthquakes

were reported in the southern part while in the northern part, there are six and there are no

reported earthquakes in the central part of the Gulf of Suez. Destructive earthquakes are

concentrated around Jordan-Dead Sea transform (El-Isa 1985; El-Isa and Hasweh 1988).

(c) At least nine historical earthquakes have occurred in the Jordan-Dead Sea transform

within the period 112–1546 AD (Ambraseys et al. 1994). Six of those are reported with

intensities in the range of VII–IX, i.e., with an overall average recurrence period of about

235 years. According to Ben-Menahem et al. (1976), twenty-six historical earthquakes

with magnitude ranging from 6.1 to 7.7 occurred in the south Jordan-Dead Sea transform in

the period 2150 BC to 1973 AD with an average recurrence period of 153 years.

Instrumental information of earthquakes in the study area during the period from 1900

to July 2012 was collected from a number of seismological catalogues and bulletins

including those of the International Seismological Centre (ISC), the National Earthquake

Information Service (NEIS). According to the analysis of instrumental earthquake distri-

bution (Fig. 2a), it is noticed that Sinai Peninsula is affected by number of earthquakes that

have been occurred within the period of 1900–2012 with wide range of magnitudes in the

range 2 \ M C 7.3. Strong earthquakes, as well as most of events with Ml B5, are con-

centrated into belts and extend along the geologically defined borders and margins of the

Sinai sub-plate. Low-moderate activity is noticed within the Sinai itself. The seismicity of

the region indicates that the Gulf of Aqaba has been the more active segment of the plate

boundary between Arabian plate and Sinai sub-plate. Since 1983, the Gulf region was

affected by four major earthquake sequences (Al-Arifi 1996; Al-Arifi et al. 2012): (a) the

first swarm was on January 21, 1983, which lasted within a few months. There were 181

earthquakes listed in the IPRG bulletin and 60 shocks from the swarm were recorded by the

Jordan University Seismological station. More than 500 shocks (Ml [ 3.5) were recorded

by Saudi local mobile network of 6 stations within a period of 72 days operation. The

largest shock had a magnitude of 5 and about 10 events had a magnitude greater than or

equal to 4.4 (El-Isa 1984). (b) The second swarm occurred in the southern part of Aqaba on

April 20–27, 1990 having a peak magnitude of Ml = 4.2. (c) The third swarm was on July

31, 1993 and continued until the end of August 1993. More than 600 events with 420 of

magnitude equal to or greater than 3. The main event (Ml = 5.8, Mw = 6.1) occurred on

August 3 and was felt on both side of the DST (Al-Arifi 1996; Al-Arifi et al. 2012). and

(d) the fourth swarm was on November 1995, which has been already described. On the

other hand, Wadi Araba segment of the Dead Sea transform has a lower activity level

during instrumental monitoring and this might be due to the lack of seismograph stations in

the vicinity of the fault. During this period, many earthquakes have occurred in the Gulf of

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Suez region between latitudes 27�N and 30�N. The largest earthquake (M = 6.9) occurred

on March 31, 1969 with an epicenter at the entrance of the Gulf (lat. 27.61�N, long.

33.91�E) at Shadwan island.

3.1 Earthquake catalogue

Preparing a reliable, long and homogenous earthquake catalogue represents the starting

point for any seismic hazard assessment study regardless of the approach adopted. The

initial catalogue was compiled for a spatial region spanning 30� to 40� E and Latitude 24�to 34� N in spans periods from 1900 up to 2012. Any duplication in the resultant catalogue

is removed, and events with magnitude 3.0 and above are included (Fig. 2a). The earth-

quake data comprise instrumental data from the ISC online bulletin for the period from

1900–2012 (http://www.isc.ac.uk/). ISC relies on contributions from seismological agen-

cies around the world. To date, a total of 488 agencies have contributed to the ISC Bulletin,

throughout its history. Several seismological agencies have been contributed to the cata-

logue of the study area in addition to IASOEI and NEIC such as:

• HLW, National research Institute of Astronomy and Geophysics (Egypt)

• RYD, King Saud University (Saudi Arabia)

• JSO, Jordan Seismological observatory (Jordan)

• SNSN, Saudi National Seismic Network (Saudi Arabia)

• NSSC, National Syrian Seismological Center (Syrian)

• DUSS, Damascus University (Syrian)

• IPRG, Institute for Petroleum Research and Geophysics (Israel)

• LAO, Large Aperture Seismic Array (USA)

3.1.1 Catalogue processing

To satisfy the spatial and temporal principles of independence of the earthquakes, dependent

events in the catalogue (i.e., foreshocks and aftershocks) were removed (catalogue declus-

tering) and then the relative completeness of each magnitude level was identified.

There is no unique way to decluster the seismicity catalogues. Two main declustering

algorithms are usually in use. The first approach was introduced by Gardner and Knopoff

(1974) and has been used in numerous hazard-related studies (e.g., Frankel 1995; Deif et al.

2009). It simply removes a space and time window after each main shock. The second one is

provided by Reasenberg (1985) who defines interaction windows in space-time in a some-

what more sophisticated way attempting to introduce physical properties behind triggering.

The spatial and temporal extent of a cluster is not fixed, as it is in the windowing method, but

depends on the development of an individual sequence. Several free parameters in Reasen-

berg’s algorithm determine the degree of clustering that should be applied.

Gardner and Knopoff (1974) technique was applied because of its simplicity using their

original parameters. The process consisted of the definition of a temporal and spatial

window adapted to each range of magnitude (the larger the main shock, the larger the

temporal and spatial window). All earthquakes situated inside the window defined fol-

lowing a main shock were considered as aftershocks and removed from the catalogue

(Fig. 2b).

The compiled catalogue comprises a total of about 6,658 earthquakes. After applying

the declustering method, a total of 4,755 (71.42 %) events out of 6,658 were removed. The

map in Fig. 2b displays the declustered catalogue containing 1,903 events.

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40° E30° E 39° E38° E37° E36° E35° E34° E33° E32° E31° E

34° N

33° N

32° N

31° N

30° N

29° N

28° N

27° N

26° N

25° N 250 0 250125

Kilometers

RED

SEA

MEDITERRANEAN SEA

Egypt

Saudi Arabia

Mw

2.8 to 3.73.7 to 4.64.6 to 5.45.4 to 6.3

40° E30° E 39° E38° E37° E36° E35° E34° E33° E32° E31° E

34° N

33° N

32° N

31° N

30° N

29° N

28° N

27° N

26° N

25° N 250 0 250125

Kilometers

RED SEA

MEDITERRANEAN SEA

Egypt

Saudi Arabia

3 to 3.83.8 to 4.64.6 to 5.55.5 to 6.36.3 to 7.1

(a)

(b)

6.3 to 7.1

Fig. 2 a The initial compiled catalogue in spans periods from 1900 up to 2012 for events with magnitude3.0 and above, b Declustered seismicity catalogue in spans periods from 1900 to 2012 and seismic sourcemodels employed for the PSHA presented in this study. The numbers relate to the seismic zones listed inTable 1

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3.1.2 Magnitude scale

Surface wave magnitude in the catalogue is the most common scale upon which magni-

tudes have been reported in the early instrumental period (before 1964). Body wave

magnitude scale becomes more common in the instrumental period. In addition, moment

and local magnitudes are also available for some recent events. Most magnitude scales

saturate with increasing earthquake size. This occurs because magnitudes scale, aside from

moment magnitude (Mw), is determined using a seismic wave of particular period and

wavelength. Seismic waves, whose wavelength is much smaller than the earthquake

source, do not increase in amplitude as the earthquake source size increases.

To ensure the catalogue magnitude homogeneity, all events for which MW were not

reported were converted into this scale. Conversion of multiple magnitude measures to a

single, representative magnitude is performed using empirical relationships developed

directly from the various magnitude measures in the compiled catalogue. Events for which

a surface wave magnitude (Ms) and body wave magnitude (mb) are reported, the following

conversion equations proposed by Scordilis (2006) have been used:

Mw ¼ 0:85 mb þ 1:03; 3:5�mb� 6:2 ðr ¼ 0:29Þ

Mw ¼ 0:67 Ms þ 2:07; 3:0�Ms� 6:1 ðr ¼ 0:17Þ

Mw ¼ 0:99 Ms þ 0:08; 6:2�Ms� 8:2 ðr ¼ 0:20ÞWhile for events for which local magnitude (Ml) and duration magnitude (Md) are

reported, the following conversion equations by Al Kathery (2010) have been adopted:

Mw ¼ 0:742 Ml þ 0:754; ðr ¼ 0:042Þ

Mw ¼ 0:843 Md þ 0:281; ðr ¼ 0:092Þ

4 Seismic hazard analysis

4.1 Probabilistic seismic hazard assessment (PSHA)

The PSHA was first introduced by Cornell (1968) and has since then been widely adopted

and modified (McGuire 1978; Bender and Perkins 1987). This technique uses the widest

possible amount of data, combining seismological, geological, and geophysical data to

build up a model of the earthquake-producing processes. The following is a very brief

theoretical background of the PSHA. The seismic sources are defined according to geo-

tectonic considerations; it is assumed that, within a seismic source, an independent

earthquake-occurrence process is taking place.

For each seismic source, magnitude exceedance rates are the number of earthquakes, per

unit time, in which the magnitude M is exceeded, and they characterize the seismicity of the

source. Seismic sources are usually lines, areas, or volumes, so a spatial integration process is

carried out to account for all possible focal locations. The acceleration exceedance rates due

to a single source, the ith source, are computed using the following expression:

viðaÞ ¼X

j

wij

ZMu

M0

�dkðMÞdM

� �PrðA [ a M;Rij

�� ÞdM;

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where M0 and Mu are, respectively, the minimum and maximum magnitudes considered in

the analysis, and Pr(A [ a M;Rij

�� Þ denotes the conditional probability that the chosen

ground-motion level a is exceeded for a given distance Rij and magnitude M. Rij are the

distances between the site and the sub-elements into which the source has been divided.�dkðMÞ

dM

h iis the probability density function of earthquake magnitude. A weight wij has been

assigned to each sub-element, and the expression above assumes that Swij = 1. Finally, the

contributions of all the sources—N- to earthquake hazard at the site are added:

vðaÞ ¼XN

i¼1

viðaÞ

Input parameters needed for performing a PSHA following the Cornell–McGuire

approach (Cornell 1968; McGuire 1976; Reiter 1991) are as follows.

1. An earthquake catalogue, which is used to derive recurrence rates and to estimate the

maximum possible earthquake for each seismic zone with potential hazard on the site

of interest.

2. A seismotectonic source model: this model identifies all the seismic sources that can

generate strong ground shaking at the site of interest. These seismic sources are shown

as map representations of lines (fault sources), and areal source zones that are defined

on the basis of a number of different types of geological, geophysical, and

seismological data. Within each seismic source, a single tectonic regime is assumed

to be responsible for the generation of earthquakes and all the points have the same

probability of being the focus of the future earthquake.

3. The seismicity recurrence characteristics for the seismic sources, where each source is

described by an earthquake recurrence relationship. A recurrence relationship indicates

the chance of an earthquake of a given size to occur anywhere inside the source zone

during a specified period of time. A maximum or upper-bound earthquake is chosen

for each source, which represents the largest event to be considered.

4. A predictive ground-motion model, which describes the attenuation of amplitudes of

ground motion as a function of distance and magnitude. Different models are

constructed for different frequencies and local site conditions. Below, we describe how

these input parameters were derived for the studied area.

Considering the short period for which earthquake data are available, there is insuffi-

cient information to calculate some of the parameters required precisely for PSHA. There

is therefore an element of uncertainty in what is known about the properties of any

seismogenic zone. Modern PSHA studies follow ideas proposed by Coppersmith and

Youngs (1986) to be able to deal with the uncertainty problem by incorporating into the

model what is known as a ‘logic tree,’ in which certain parameters are given a range of

possible values with weights as to the probability of each.

4.2 Earthquake source zones

Seismogenic zones are defined according to the present-day tectonic setting, recent and

historical seismicity, and the location of known faults in the area under study. The local

and regional seismicities of northwestern Saudi Arabia are divided into 12 seismogenic

zones (Fig. 3). These zones are related to the tectonic activity of the seismic trends that are

acting in the study area, Red Sea, Gulf of Suez also Gulf of Aqaba–Dead Sea rift trend.

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Fig. 3 Alternative seismotectonics models used in seismic hazard calculations, seismogenic zones areshown

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4.2.1 Northern Red Sea zones

The Red Sea and its two Gulfs Suez and Aqaba have experienced large damaging earth-

quakes; Archaeological evidences specify considerable earthquake activity and shear along

the Red Sea rift in the past times. Both of them has been examined by several authors;

Sieberg (1932), Maamoun (1976), Maamoun and Ibrahim (1978), El-Isa and Hasweh

(1989), Ambraseys et al. (1994), Abu El Enien (1997). These earthquakes frequently occur

in relation to the known active tectonic trends of the Red Sea–Gulf of Suez and of the East

Mediterranean Basin. The seismic activity in the northern Red Sea tends to follow the two

different structural trends and the shear zones of the Aqaba-Levant trend and the Gulf of

Suez zone (Maamoun et al. 1984).

The historical earthquakes that have been felt in the northern Red Sea (2200 BC to 1899

AD) were compiled by Badawy and Horvath (1999) from the Arabic documents and earlier

catalogue. Nine earthquakes believed to be occurred along the two Gulfs and Sinai during

that period. The most famous historical events are that of March 18 , 1068 (Levant Fault

area), May 20 , 1202 (Dead Sea), and July 11 , 1879 (Gulf of Suez) (Mahmoud 2003).

The recent seismicity of the Red Sea Rift clarifies the activity of the two Gulfs, the northern

and the southern Red Sea as well. They produce earthquakes of different magnitude levels

with moderate to high seismic activity. The seismicity of Red Sea axial trough clustered on or

near the transform faults of the deep axial trough in consistent with Alamri (1994) and

supports the interpretation of the magnetic anomalies in terms of seafloor spreading.

4.2.1.1 Gulf of Suez seismic zone It extends along the Gulf of Suez and represents the

broken arm of the northern Red Sea rift. Faulting of younger age was recognized in the

Gulf of Suez (Garfunkel and Bartov 1977), so the seismic expression of the current

information has to be expected. In the first half of the last century, it was believed that the

Gulf of Suez is aseismic and its activity may be related to the Red Sea rift transferred into a

shear on the Dead Sea transform (Le Pichon and Gaulier 1988). Recent seismic activity has

been proved along the gulf of Suez Ben-Menahem (1979) and Salamon et al. (1996). The

epicentral distribution of the micro-earthquakes shows clustering beneath Gobal Island,

southern Gulf of Suez, while the scattering events existed beneath the southern borders of

the gulf Daggett et al. (1986). The focal mechanism solutions of March 31, 1969 earth-

quake revealed the continuous activity.

High rate of seismicity at the southern end of the Gulf of Suez is attributed to the crustal

movements among the Arabian plate, African plate, and the Sinai sub-plate. The activity along

the Suez Gulf is supported by the recent seismic activity observed by the National Research

Institute of Astronomy and Geophysics (NRIAG-Bulletins 1994–2012, Helwan Egypt).

Based on the regional dip of the fault block, Meshref 1990 classified the Gulf of Suez

structurally into three zones: southern Gulf of Suez (SGS), middle Gulf of Suez (MGS),

and northern Gulf of Suez (NGS). SGS zone is characterized by intensive structural

deformation and as a result moderate to high earthquake activity and earthquake swarms as

well. The largest instrumental event recorded in SGS is Mw 6.9 on March 31, 1969 and 5.5

in 1972. The prevailing mechanism of this zone is extensional on the clysmic trend with

normal faulting. MGS is located in the middle of the Gulf, it is of low seismicity compared

to SGS. Focal mechanism solution showed strike-slip faulting with normal component;

therefore, it can be considered as a separate seismogenic zone. The largest event occurred

in this zone is Shukir 1983 mb = 5.3, it shows strike-slip faulting with minor normal

component (Maamoun 1985).NGS is located in the northern tip of the Gulf, it is of low

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seismic activity but according to NRIAG 1994–2012, it is still active. Historically, it

suffered a unique large earthquake in 742 AD with intensity VII. The focal mechanism

solution revealed normal faulting (El-Hefnawy et al. 2006).

4.2.1.2 The Aqaba–Dead Sea levant trend The Aqaba Gulf is documented as an active

seismic trend where many destructive earthquakes have been occurred. The historical

records of large and catastrophic earthquakes revealed that the Aqaba–Dead Sea Fault

System is of significant seismic hazard. The earthquakes of this trend are of shallow foci

and tend to occur along NE-trending faults. An earthquake with magnitude larger than

(Mw = 7.6) for the fault between the Dead Sea and the Aqaba Gulf (Wells and Copper-

smith 1994). The b value = 1.2 was determined by El-Sayed and Walhstrom (1996).

(a) The Dead Sea Zone This zone extends from Lebanon Mountains in the north (lat.

30.9–33.5N) to Arava Valley (Wadi Araba) in the south, including the Dead Sea and

Jordan River basins. Basically, the Dead Sea Rift is a pull-apart basin that formed as a

result of the shear movements along the Levant Faults System (Hatzor and Reches 1990).

The Dead Sea margins are asymmetrical with steeper bounding faults on the eastern

margins (Barazangi 1983). Several historical and instrumental earthquakes were docu-

mented. The largest historical earthquake occurred in January 1546 (Mw = 7.0). The

largest recent two events are those of March 1903 (Mw = 5.7) and July 1927 (Mw = 6.3)

where 500 were killed and 700 were injured (Al-Tarazi 1999).

(b) The Gulf of Aqaba zone This zone encompasses the Gulf of Aqaba and Arava Valley

to the north gulf, it extends between 28 and 30.9N. The earthquakes sequences of this zone

mainly characterized by left-lateral strike-slip mechanism along the main axis of the gulf

and normal slip along transverse faults. May 1212 (Mw = 6.7) is the largest historical

earthquake and the largest instrumental one is the November 22 , 1995 (Mw = 7.2).

Earthquake larger than 6 is predictable with an average return period 1,000–3,000 years

according to the paleoseismicity, Shapira and Jarradat (1995).

4.2.2 Eastern Mediterranean seismic zone

The seismic activity of this zone is relative to the movement of the surrounding plates. This

zone has experienced significant damage earthquakes. The eastern Mediterranean region is

known to be seismically active over a period of more than 2,000 years based on historical

and instrumental records. The tectonic and seismotectonic studies reflect highly compli-

cated tectonic setting. The area could be divided into seven major tectonic zones; in this

study, we consider only two of them, which can affect the area under study.

4.2.3 Volcanic zone

Harrat Lunayyir Volcanic area is located in the Arabian shield of northwestern Saudi

Arabia, immediately east of the Red Sea port of Umm Lajj. It is an area of extensive

basaltic lava flows and about fifty small volcanic cones. The center of the volcanic field is

about thirty miles inland from the Red Sea. Individual lava flows radiate out in all

directions from the center of the field. They follow stream valleys through the desert. Two

of the flows have reached the Red Sea, but they are now covered by sand along the coast.

The youngest lava flows at Harrat Lunayyir were produced by eruptions that occurred

about 1,000 years ago. Until 2009, the field was considered to be inactive because no

evidence of recent activity had been identified. Then, between April and June 2009, a

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swarm of over 30,000 earthquakes occurred beneath the volcanic field and a fissure about

five miles long opened to a width of about three feet. The enormous number of earthquakes

was a cause for concern. About 20 earthquakes were over magnitude 4 and a magnitude 5.4

earthquake on May 19, 2010 damaged buildings in the town of Al Ays.

4.2.4 Tabuk zone

This zone follows NW-trending faults that represent the northern extension of the Najd

Fault System. The seismicity of the zone is attributed to the reactivation movement of the

preexisting faults. It is characterized by low seismic activity concentrated along tabuk

Graben. It is located to the north of N–S Hail volcanic arc line of Cenozoic age. Histor-

ically, it is suffered from to large geological events, one is the volcanic eruptions of Harrat

Kishb, Khayber, Almadinah, and Rahat (Adams and Barazangi 1984) and the other one is

large earthquake with Magnitude 7.0 on March 1068, with intensity VII–IX, It caused

major damage and killed 20,000 people (Ambraseys et al. 1994). Another damaging

earthquake was documented on January 1, 1588 with intensity VII–IX, it is located at the

extension of structural trend starts from Tabuk parallel to the Red Sea up to Jordan.

4.3 Seismotectonic source models

The seismotectonic source model is always defined on the basis of available tectonic

setting, geophysical, and seismological data. The process of defining the seismotectonic

model is a matter of expert judgment; thus, it is clouded by some uncertainty. According to

the tectonic setting in the previous section and the spatial distribution of earthquakes in the

study area and the surrounding area, twelve seismic zones are identified in order to analyze

the seismic hazard according to area source model as in Fig. 3. Within each seismic source,

a single tectonic regime is assumed to be responsible for the generation of earthquakes and

all the points have the same probability of being the focus of the future earthquake. The

seismotectonic model shown on the map in Fig. 3 for two models tested in this study. Each

model comprises zones that show seismic activity and geological activity.

Because of the uncertainties in the seismological, geophysical, and geological data, i.e.,

because of existence of modeling uncertainties, we had to define several alternative seis-

motectonic models. The seismotectonic models tested in this study area are shown in

Fig. 3. For treating the epistemic uncertainties, model A is selected for the final solution.

Based on the spatial distribution of epicenters and reported active faults and focal

mechanism solutions, the seismogenic zones for model (a) are mapped into 12 seismogenic

zones as shown in Fig. 3 and Table 1. Mediterranean Sea zones part 1 and part 2; Dead Sea

zone; Jordan zone and East Jordan zone; Gulf of Suez zone; Sinai peninsula zone; Red Sea

1; Tabuk region zone; Zone 10 named as Red Sea 2 zone; Zone 11 named as Volcanic

Zone; and finally Zone 12 is the Gulf of Aqaba.

Alternative seismotectonic model B is defined based on the geological and structural

characteristic; therefore, zone 6 (Gulf of Suez) seismogenic zone is divided into three

subzones structurally and geologically; according to El-Hefnawy et al. 2006, he classified

the gulf of Suez into three zones such as northern Gulf of Suez (NGS), Middle Gulf of Suez

(MGS), and the southern Gulf of Suez (SGS). The Gulf of Aqaba Zone (12) also is divided

into three zones according to Garfunkel et al. (1981); Gulf of Aqaba is underlined by three

grabens filled with sediments, three basins are also found to be seismically active char-

acterized with earthquake swarms, Eilat deep, aragonese deep and Daka deep (Klinger

et al. 1999).

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4.4 Estimation of seismic hazard parameters

4.4.1 Recurrence parameters

The seismicity of a seismogenic zone is quantified in terms of the recurrence relationship:

Log NðMÞ ¼ a� bM

where N is the number of earthquakes of magnitude M or greater per unit time. The a value

is the activity and defines the intercept of the above recurrence relationship (Gutenberg and

Richter 1944) at M equals zero. The number of occurrences per year of a hazardous event

(e.g., the annual frequency that the ground-motion parameter, X, at a site exceeds a

specified value x) is defined as the annual frequency and usually denoted as k (X C x). The

parameter b is the slope, which defines the relative proportion of small and large earth-

quakes (Fig. 4).

Figure (4a) cumulative number of the earthquakes as a function of time of the catalogue

used in this study, figure (4b) an example of a typical frequency magnitude distributions in

this study. The slope of the red line corresponds to the b-value.

All seismic sources were assumed to generate earthquakes according to a doubly

bounded exponential distribution (Cornell and Vanmarcke 1969). This is because the

Gutenberg and Richter (1944) relationship imposes the unrealistic assumption that the

maximum potential earthquake for any seismotectonic setting. The following truncated

exponential recurrence relationship is therefore commonly used in practice:

Nð�MÞ ¼ aexp½�bðM �MminÞ� � exp½ðMmax �MminÞ�

1� exp½�bðMmax �MminÞ�

where a = N(Mmin), Mmin is an arbitrary reference magnitude, Mmax is an upper-bound

magnitude where N (m) = 0 for M [ Mmax, and b = b 9 lin10. In this form, earthquake

frequency approaches zero for some chosen maximum earthquake of a region.

The compiled catalogue has varying levels of completeness and these must be

accounted for when deriving activity parameters for the seismic sources. The parameters of

Table 1 Seismicity parameters for all seismic sources

Zone No. Zone name Mmax k r (k) b r (b) b r (b)

Z-1 Mediterranean Sea 1 6.3 16.98 2.4 2.12 0.09 0.92 0.04

Z-2 Mediterranean Sea 2 5.4 23.21 3.36 2.58 0.12 1.12 0.05

Z-3 Dead Sea 6.7 29.58 2.75 2.73 0.09 1.19 0.04

Z-4 Jordan 4.8 114.05 14.11 3.40 0.14 1.48 0.06

Z-5 East Jordan 4.8 9.46 1.79 2.42 0.13 1.05 0.06

Z-6 Gulf of Suez 6.7 27.82 3.21 2.00 0.11 0.87 0.05

Z-7 Sinai triangle 5.4 17.40 2.56 2.42 0.12 1.05 0.05

Z-8 Red Sea 1 5.6 14.83 2.40 1.94 0.14 0.84 0.05

Z-9 Tabuk region 5.4 16.93 2.93 2.30 0.15 1.00 0.06

Z-10 Red Sea 2 6.0 10.40 1.73 2.03 0.11 0.88 0.05

Z-11 Volcanic zone 6.1 7.78 2.78 2.10 0.16 0.91 0.07

Z-12 Aqaba Gulf 6.8 54.27 6.02 2.20 0.11 0.96 0.05

Mmax is an upper-bound magnitude, Mmin = 3, k is the annual frequency and b = b 9 ln10

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the doubly bounded exponential distribution were obtained in this study using the maxi-

mum likelihood estimation procedure of Weichert (1980).

In many regional hazard studies, an overall b value is often used in order to stabilize the

results by avoiding undue fluctuations of b particularly in zones of low seismicity (Frankel

1995; Frankel et al. 1997; Deif et al. 2009).

4.4.2 Maximum earthquake estimation

Maximum magnitude mmax was calculated within the seismic sources using statistical

procedure of Kijko (2004), using the following equations:

Fig. 4 a Cumulative number of the earthquakes as a function of time of the catalogue used in this study,b an example of a typical frequency magnitude distributions in this study. The slope of the red linecorresponds to the b-value

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mmax ¼ mobsmax þ

Zmmax

mmin

½FmðmÞ�ndm;

where Fm(m) is the cumulative density function (CDF) of magnitude.

From this equation, an estimated value of mmax can be obtained only by iteration. This

equation states that mmax is equal to the largest observed magnitude mobsmax plus an amount

D ¼Rmmax

mminFmðmÞ½ �ndm: This equation is valid for any CDF, Fm(m) and does not require the

fulfillment of any additional conditions. It may also be used when the exact number of

earthquakes, n, is not known. In this case, the number of earthquakes can be replaced by

kT. Such a replacement is equivalent to the assumption that the number of earthquakes

occurring in unit time conforms to a Poisson distribution with a parameter k, with T the

span of the seismic catalogue. It is also important to note that, since the value of the

integral D is never negative, the equation provides a value of mmax, which is never less than

the largest magnitude already observed. The integration D for the Gutenberg–Richter

relationship that bounded from above is given by:

D ¼Zmmax

mmin

1� exp½�bðm� mminÞ1� exp½�bðmobj

maxÞ � mmin�

" #n

dm

This integral is not simple to evaluate; therefore, Kijko (2004) replaced [FM (m)]n by Cramer’s

(1961) approximation exp{-n[1 - [FM(m)]}. The integral is then solved to give mmax.

In any remaining cases, the maximum magnitude was obtained by adding 0.5 units to the

maximum observed magnitude. The paucity of data related to these sources necessitates

applying this approach, although it is not an ideal one. The maximum magnitude values for

each seismic zone are given in Table 1, while the minimum magnitude for each seismic zone

is equal 3. The seismicity parameters of all seismogenic zones are listed in (Table 1).

4.5 Ground-motion prediction models

The ground-motion prediction models have been selected according to the tectonic regime

associated with the earthquakes in each source zone. The models of Youngs et al. (1997)

are used to model the ground motions from subduction zone earthquakes occurring within

the Mediterranean sources, while the model of Boore et al. (1997) has been used for Gulf

of Aqaba–Dead Sea rift system and active shallow crustal zones. These models were

incorporated within a logic-tree framework. Therefore, they must be transformed into

common metrics (Bommer et al. 2005).

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 framework enables the

distance conversion to be implemented very easily. In the current study, distance metrics

are handled in a straight forward manner using the software package CRISIS 2007.

5 Hazard calculations

The hazard calculations were performed using the CRISIS 2007 software (Ordaz et al.

2007). The software is essentially an implementation of the Cornell (1968) PSHA

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framework, with the aleatory variability in the ground motion explicitly included (Bommer

and Abrahamson 2006). All sources were assumed to behave in a Poissonian manner and

only earthquakes of Mw C 3.0 were deemed to be of engineering significance. The alter-

nated seismic source zonations, activity rates, and ground-motion models that have pre-

viously been discussed are all incorporated into the hazard calculations through the use of

logic-tree formulation. Uncertainties, due to errors associated with the delineation of the

source boundary and attenuation model, were mindfully treated using the logic-tree for-

mulation. The numerical calculations were done in points of a grid covering all selected

region, equi-spaced 0.25� in latitude and longitude, in terms of PGA and period spectral

accelerations SA (T) for T = 0.1, 0.2, 0.3, 1.0, and 2.0 s and for 475-year return period.

6 Results

In this study, the probabilistic assessment using the logic-tree framework are treated to

obtain at each point the mean value of the acceleration. The obtained PGA and 5 % damped

horizontal spectral acceleration values at 0.1, 0.2, 0.3, 1.0, and 2.0 s spectral periods were

mapped so that it can be used to generate an approximate uniform hazard spectrum (UHS)

for each point on the hazard maps for the range of periods important for common engineering

structures. The contour hazard maps display the regional distribution of the mean ground

motion in cm/s2 for a rock condition, with 10 % probability of exceedance in 50 years,

which correspond to return periods 475 years (Figs. 5, 6, 7, 8, 9, 10).

38° E 34° E 37° E 36° E 35° E

31° N

30° N

29° N

28° N

27° N

Gul

f of

Aqa

ba

Saudi Arabia

EGYPT

Jordan

RED SEA0 50 100 150

Km

(a)

Fig. 5 Maximum PGA (cm/s2) with a probability of 90 % of not being exceeded in 50 years in the studyarea

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Km

38° E 34° E 37° E 36° E 35° E

31° N

30° N

29° N

28° N

27° N

Gul

f of

Aqa

ba

Saudi Arabia

EGYPT

Jordan

100 150

(b)

RED SEA0 50

Fig. 6 Seismic hazards on rock for spectral acceleration of 0.1 s and 5 % damping for a 475-year returnperiod

38° E 34° E 37° E 36° E 35° E

31° N

30° N

29° N

28° N

27° N

Saudi Arabia

JordanEGYPT

50 100 150

Km Red Sea

Gul

f of

Aqa

ba

0

Fig. 7 Seismic hazards on rockfor spectral acceleration of 0.2 sand 5 % damping for a 475-yearreturn period

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38° E 34° E 37° E 36° E 35° E

31° N

30° N

29° N

28° N

27° N

Saudi Arabia

Jordan

200

200

350

200

500

500

50 100 150

Km

350

650

650

650

500

EGYPT

RED SEA

Gul

f of

Aqa

ba35

0

0

Fig. 8 Seismic hazards on rockfor spectral acceleration of 0.3 sand 5 % damping for a 475-yearreturn period

38° E 34° E 37° E 36° E 35° E

31° N

30° N

29° N

28° N

27° N

Saudi Arabia

Jordan

RED SEA0 50 100 150

Km

200

200

EGYPT

Gul

f of A

qaba

Fig. 9 Seismic hazards on rockfor spectral acceleration of 1.0 sand 5 % damping for a 475-yearreturn period

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7 Discussion and conclusions

The results of the present study are compared with those of the Global Seismic Hazard

Assessment Program (GSHAP) and Al-Amri 1995. The GSHAP map is based on more

generalized seismotectonic model which lack for specific and detailed seismic zonation.

The (GSHAP) map is calculated for a 10 % probability of being exceeded in 50 years on

rock (475-year return period). Our results are similar as the generalized (GSHAP) map

showing a relatively high hazard values around the Gulf of Aqaba area. It compares well

with the hazard values in this study, taking into consideration the uncertainty in reading

values of the more generalized GSHAP map. Also, this study is similar as the results of Al-

Amri 1995, with estimates of the 475-year PGA around Gulf of Aqaba ranging from 100 to

325 gal.

As main results derived from the analysis of the different maps, its worthy to note that,

for return period, maximum PGA values are predicted around the Gulf of Aqaba–Dead Sea

rift system. Isolines are quite parallel around the coast and decreases toward the continent.

All seismic hazard maps presented in this study show the highest values of peak ground

acceleration along the Gulf of Aqaba-Arava Valley and decrease eastward and westward.

For instant, the peak ground acceleration value in the Gulf of Aqaba-Arava Valley fault for

50 years lifetime is 325 gals, while the lowest values in the eastern region and it is about

75 gals. In this study, we represent a new evaluation in terms of PGA and SA (T) for the

first time in this area. The results should be considered in the revision of the respective

seismic codes, land-use planning, and earthquake-resistant structure design.

Acknowledgments The authors extend their appreciation to the Deanship of Scientific Research at KingSaud University for funding the work through the research group project No RGPVPP-122.

38° E 34° E 37° E 36° E 35° E

31° N

30° N

29° N

28° N

27° N

Gul

f of

Aqa

ba

Saudi Arabia

Jordan

EGYPT

RED SEA0 50 100 150

Km

Fig. 10 Seismic hazards on rockfor spectral acceleration of 2.0 sand 5 % damping for a 475-yearreturn period

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