Seismic Hazard Assessment for Tabuk City, NW Saudi Arabia

doi:10.4236/gep.2013.13002

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Journal of Geoscience and Environment Protection

2013. Vol.1, No.3, 7-11

Published Online December 2013 in SciRes (http://www.scirp.org/journal/gep) http://dx.doi.org/10.4236/gep.2013.13002

Open Access 7

Seismic Hazard Assessment for Tabuk City, NW

Saudi Arabia

Ziyad I. Al-Besher

King Abdul-Aziz City for Science and Technology, Riyadh, Saudi Arabia

Email: [email protected]

Received August 2013

Tabuk city is located within the Red Sea and Gulf of Aqaba active tectonic environment where it has ex-

perienced considerable earthquakes in the historical and instrumental period. Recently, Tabuk city is ex-

pected to become one of the future economic communities in Saudi Arabia. Accordingly, assessment of

seismic hazard of Tabuk city plays an important role to minimize earthquake damage and to anticipate the

future safe development for the strategic projects. For this purposes, earthquake data were collected from

local and regional data centers to construct earthquake catalogue. The earthquake source zones that affect

Tabuk city, maximum magnitude and closest distance have been identified. The stochastic approach has

been applied through this study for seismic hazard assessment in terms of peak ground acceleration and

the response spectra. The results illustrated that, the maximum peak ground acceleration resulted from

Tabuk source zone with moment magnitude (Mw) of 7.5. The calculated peak ground acceleration of 218

cm/sec2 at distance of 10 Km for Tabuk City at the bedrock. The response spectra of Pseudo-Spectral

Acceleration (PSA) have been calculated at 5% of the critical damping with a value of 470 cm/sec2 at 10

Km distance. The results of the present study are highly recommended to improve Saudi Building Code

(SBC) for earthquake resistant design in Tabuk city.

Keywords: Tabuk Cit y; Seismicity; Seismic Hazard Assessment; Response Spectra

Introduction

Tabuk City lies in the northwestern part of Kingdom of Sau-

di Arabia between latitudes of 27.50˚ - 28.50˚N, and longitudes

of 36.00˚ - 37.00˚E (Figure 1). This city includes many resi-

dential buildings in the rural villages and urban zones. In addi-

tion the area is characterized by fast grow in the infrastructures

and great developmental projects. The last major quake re-

ported in the region was in 1995, which caused some damage in

Tabuk region. A sequence of earthquakes struck Tabuk city in

June 2004, approximately 60 km southeast of Tabuk City. The

largest event (ML = 5.2) occurred on June 22. Although the

region is sparsely populated, the event was widely felt in Tabuk

city and its surroundings (Al-Damegh et al., 2009). The 2004

Tabuk earthquake sequence generated a lot of concern for the

earthquake hazard in this area because the region has been gen-

erally considered aseismic.

Because there are no previously studies to assess the seismic

hazard of Tabuk city, despite its location in an active tectonic

environment, the assessment of seismic hazard represents an

important and necessary issue.

Tectonic Setting

The Tabuk is an area located in the northwestern part of the

Arabian Plate. This Plate is surrounded by three tectonic re-

gimes; divergent, convergent, and transform. The divergent

boundaries in the west and south represent the recent spreading

along the Red Sea and the Gulf of Aden respectively. The ac-

tive convergent margin lies to the northeast in the Turkish-

Iranian Plateau, where continental collision has given rise to

this Plateau. A major continental strike-slip fault zone bounds

the plate in the northwest.

This is the Dead Sea Transform fault which runs from the

Gulf of Aqaba through the Dead Sea and Syria into southern

Turkey. A second transform boundary exists to the southeast

(offshore of Oman) in the intra-oceanic Owen Fracture Zone.

This boundary with the Indian Plate is the oldest and least ac-

tive tectonic margin of the Arabian Plate.

The tectonic pattern of the region is inherited from the base-

ment tectonics of ancient Nubian-Arabian shield, which re-

ceived its main structural imprint during the late Precambrian

orogenies. The most important among the tectonic features is

the northwest-southeast striking faults (parallel to the Red Sea),

the Gulf of Suez rift, the Najd faults system, the Aqaba fault

line, and northeast-southwest and east-west trending pattern.

The geology and tectonics of western Saudi Arabia is dominat-

ed by the Arabian Shield, the Red Sea, and the Gulf of Aden in

the west-southwest.

Since the start of rifting, the Arabian plate moved northeast-

ward from Egypt and Sudan, northward from Somalia, and

rotated counterclockwise about a point in the vicinity of the

Gulf of Suez. Such movement is accommodated by compres-

sion and strike-slip faulting along the Bitlis and Zagros belts,

where the Arabian plate is undergoing a subduction beneath the

Eurasian plate, and by strike-slip displacement along the Dead

Sea transform fault. At present, the northern part of the Arabian

plate moves northwestward with respect to the Eurasian plate at

a rate of 20 ± 3 mm/yr.

Seismicity of t he Study Area

According to Ambraseys et al. (1994), Tabuk was affected

Z. I. AL-BESHER

Open Access

Figure 1.

Location map of the study area.

by some historical strong earthquakes (Figure 2) as follows;

March 18 1068; January 4 1588; December 26 1906 and Feb-

ruary 26 1909. The collected catalogue includes different mag-

nitude scales but surface wave and body wave magnitudes are

the common. For the homogeneity of the catalogue, all these

scales were converted into Moment magnitude (the most relia-

ble) based on Harvard CMT catalogue. The Spatial distribution

of the instrumental seismicity (1906 till 2012) is plotted in

Figure 3.

Declustering attempts have been made to separate the time

independent part of seismicity from time-dependent or clus-

tered seismicity. Furthermore, the aftershocks and the fore-

shocks have been removed from earthquake catalogue using

Gardner and Knopoff (1974) approach.

Depending on the integration between geological, geophysi-

cal and seismological data the earthquake source zones have

been identified. Seismicity within these sources is assumed to

be uniform in terms of distribution and type of earthquakes. It

is therefore, assumed that seismic activity from source can be

characterized by a single earthquake generating process. It is

also assumed that earthquakes have equal probability of occur-

ring at any point within the seismic zone. Accordingly, the

main affected zones for Tabuk city are shown in Figure 4 as

follows; 1) Tabuk Zone; 2) Northern Red Sea Zone and 3) Gulf

of Aqaba Zone.

The maximum expected earthquake for each source zone has

been calculated (Table 1). This earthquake used sometimes in

place of maximum credible (Reiter, 1991). Another kind is the

maximum historic earthquake, which often defines the lower

bounds of the maximum credible events.

Ground Motion Simulation

For the assessment of sei smic hazard in terms of acceleration

and response spectra, a stochastic technique proposed by Boore

(2003) is used. The prediction of ground-motion or response

amplitude as a function of earthquake magnitude and distance

is of fundamental importance for the assessment of seismic

hazard. The attenuation relations are usually developed empiri-

cally by regression analysis of the observed ground-motion

parameters, most likely the peak horizontal acceleration. Lack

of acceleration database necessitated the development of pre-

Figure 2.

Spatial distribution of the historical earthquakes (827-1906).

Figure 3.

Spatial distribution of the earthquakes.

Figure 4.

Seismotectonic source zones affecting Tabuk area.

diction equations using stochastic methods in conjunction with

a theoretical source model.

The purpose of this study is to simulate the peak ground ac-

celeration (PGA) expected at Tabuk City and the response

spectra at 1%, 3%, 5% and 10% damped pseudo-acceleration.

Z. I. AL-BESHER

Open Access

Table 1.

Seismicity parameters for all of the identified seismic sources.

Zone Mmax (obs) Mmax

Tabuk 7.0 7.0*

Northern Red Sea 6.9 7.56

Gulf of Aqaba 7.3 7.8

where M(obs): Maximum observed magnitude; Mmax: Maximum expected Magni-

tude; *M(obs) plus .5 magnitu de un i ts.

A version 2.0 FORTRAN program from boore (2003) has been

used for simulating earthquake ground-motion in the study area.

The application of the stochastic simulation method requires the

spectral shape as a function of the earthquake size. Boore (2003)

breaks the total spectrum of the horizontal motion at a site (Y

(Mo, R, f)) into contributions from earthquake source (E), path

(P), site (G) and the instrument or type motion (I), so that:

Y (Mo, R, f) = E (Mo, f) P(R, f) G (f) I (f) (1)

By separating the spectrum of ground-motion into source,

path and Site components, the models based on the stochastic

method can be easily modified to account for specific situations

or for improved information about particular aspects of the

model.

The Source (E (Mo, f))

The shape and amplitude of the source spectrum part in

“Equation (1)” must be specified as a function of the earth-

quake size. The most commonly used model for the earthquake

source spectrum is the ω-square model. In this model, scaling

of the spectra from one magnitude to another is determined by

specifying the dependence of the corner frequency (ƒo) on

seismic moment. Although, the ω-square model is widely used,

in practice a variety of other models through stochastic method

are also in use. Aki (1967) recognized that assuming a similari-

ty in the earthquake source implies that:

00constant

(2)

where the constant can be related to the stress drop (∆σ). Fol-

lowing Brune (1970; 1971), the corner frequency is given by

the following equation:

( )

1/3

4.9 10|

βσ

=×∆

(3)

where, ƒo is in Hz,

s (the shear-wave velocity in the vicinity of

the source) in km/s, ∆σ in bars, and Mo in dyne . c m .

The source spectra for all the models are given by the fol-

lowing equation:

0 00

((),)E MCM SMf=

(4)

where C is a constant given by:

θφss o

C(R).V. F(4prbR)=

(5)

where Rθϕ is the radiation pattern, usually averaged over a

suitable range of azimuths and take-off angles, V represents a

reduction factor that accounts for the partitioning of energy into

two horizontal components, F is the amplification due to free

surface, ρs and βs are the density and shear-wave velocity in the

vicinity of the source, and Ro is a reference distance, usually set

equal to 1 km. In equation 4.4, the parameter Mo is the seismic

moment. In general, the moment magnitude is used rather than

seismic moment as a more familiar measure of the earthquake

size. The S (Mo,f) in equation 5.4 is the displacement source

spectrum.

Using the “Equation (5)”, an average radiation pattern for

S-wave is taken as Rθϕ = 0.55 (Boore & Boatwright, 1984) for

all zones. These parameters are related to the source vicinity.

Hussein et al. (1998) determined the stress drop (∆σ) in bar for

the maximum earthquake occurred on November 11, 1995 in

Aqaba zone. In other zones the standard values of (∆σ = 30 bar)

are used. This value is taken because a significant stress drop

for large earthquakes is about 30 bar.

The Path (P(R, f), Duration)

The effects of the path are represented by simple functions

that account for geometrical spreading, attenuation (combining

intrinsic and scattering attenuation) and the general increase of

duration with distance due to the propagation and scattering.

The path P(R, f) in “Equation (1)” is given by the following:

( )

expP(R,f /) ()

ZRfRQ f

−=





(6)

where CQ is the seismic velocity used in the determination of

Q(f), and the geometrical spreading Z(R) is given by a piece-

wise continuous series of straight lines, as follow:

( )()

( )

1 12

RRR

ZRZRRR R

ZRR R

≤





= ≤≤









≤







(7)

where R is usually taken as the closest distance to the rupture

surface rather than the hypocentral distance.

The ground-motion duration (Tgm), according to Atkinson

and Boore (1995), is determined as the sum of the source dura-

tion (To), which is related to the inverse of a corner frequency,

and a path dependent duration that accounts for dispersion bR.

Atkinson (1993) computed duration for each record of 1500

events that matches the observed relationship between the peak

ground velocity (PGV) and the Fourier spectrum of velocity

using the random process theory equations. The computed

slope (b) was found to be 0.16 for (10 ≤ R < 70 km), −0.03 for

(70 ≤ R < 130 km), 0.04 for (130 < R < 1000 km) and 0.0 for <

10 km. A negative slope in the transition zone from direct wave

to granite layer (Lg) phase (70 to 130 km) is due to the addi-

tional energy that is injected in the time window of the signal as

the “Moho bounce” rays arrive.

For this study, the three segments geometrical spreading op-

erator (Atkinson & Boore, 1995) is used. The R−1 geometrical

spreading is assumed for a distance less than 70 km, R0.0 for a

distance between 70 km and 130 km and R−0.5 for greater than

130 km distances. Due to the small hypocentral distance of

Tabuk earthquake to the city, the effect of attenuation is very

small.

The Site (G(f ) )

The modifications of the ground-motion due to local site ge-

ology are known as site effects. The site effects G(f) in “Equa-

Z. I. AL-BESHER

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tion (1)” are classified into amplification A(f) and diminution D

(f) as follow:

( )( )( )

FfAfDf=

(8)

The attenuation or diminution operator D (f) in “Equation (8)”

accounts for the path independent loss of high frequency in the

ground-motions, at which a very rapid decay of spectral ampli-

tudes happen for f ≥ fmax. Simple multiplicative filter can ac-

count for the attenuation of the high frequency motions. Two

filters are in common use: the first is fmax or the high cut filter

(Hanks, 1982; Boore, 1983) as given by:

( )

max

Df f

−







=+ 







(9)

The second is κo filter (Anderson & Hough, 1984), as given

by:

( )()

expDf f

=− ĸ

(10)

where κo is the spectral decay parameter. Both these filters can

be combined in any application as described by the following

equation:

( )()

max

exp1 f

Dff f

−







=−+









(11)

Through this study, the local site effects haven’t been taken

into account during the calculation of Peak Ground Accelera-

tion (PGA), Peak Ground Velocity (PGV) and Peak Ground Dis-

placement (PGD).

Figure 5 represents a time simulated history for PGA, PGV

and PGD at 10-km distance Tabuk City resulted from Tabuk

zone at the bedrock. Figure 6 shows the distribution of PGA

through Tabuk City. It is noticed that the simulated values of

the three outputs increase eastward where the main earthquake

is present.

Response Spectra

The response spectrum is the most important characterization

of seismic ground-motion in earthquake engineering, which

forms the basis for most designs. A Single Degree of Freedom

(SDOF) system is a mechanical system with mass m, which

provides inertia and stiffness K that provides a restoring force,

whose deformation can be fully described by single coordinate

(Bommer, 2000). The natural period (T) of vibration for such a

SDOF system is given by the following equation:

= (12)

If a series of SDOF systems with a given level of structural

damping are all subjected to an acceleration time history acting

at their base, each mass will respond differently according to its

natural period and as a relationship between the period and the

frequency content of the ground-motion. The maximum abso-

lute value of the response for each SDOF oscillator can be cal-

culated and plotted against the corresponding value of period

(T). The resulting plot, called a response spectrum, shows the

maximum response that a SDOF system experience when sub-

jected to the ground-motion represented by that particular ac-

020 40 60 80100

Time(sec)

-300

-200

-100

100

200

Acceleration(cm/s

)

020 40 6080 100

Time(sec)

-40

-20

Velocity(cm/sec)

020 40 6080 100

Time(sec)

-20

Displacement(cm)

Figure 5.

Time series of the si mulated PGA at dis tance 1 0-k m from Tabuk source

zone.

Figure 6.

Distribution of PGA in Tabuk city.

celerogram. The response spectrum reflects the characteristics of the

earthquake that generate motion and nature of the recording site.

For low levels of damping (less than 20% of critical), the vel-

ocity spectra (SV) and the acceleration spectra (SA) could be

estimated from the displacement spectra (SD):

PSVSD T

= ⋅

(13)

.2PSA SDPSVT

ππ



= ⋅



=



(14)

Z. I. AL-BESHER

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The spectra obtained by this way are known as pseudo-spec-

tra and is the cause of nomenclature PSA and PSV. An advan-

tage offered by pseudo spectra is that the calculation of SD is

the simplest and least time-consuming of the three spectra;

hence “Equations (13) and (14)” are a convenient way to esti-

mate the spectral velocity and acceleration. The response spec-

tra are calculated for three selected damping values of 0.03,

0.05 and 0.1 of the critical damping, which are chosen to be

relevant to various structural characteristics. Figure 7 repre-

sents the response spectra of Pseudo-spectral Acceleration at

10-km distance from Tabuk source zone.

Figure 7.

Response spectra for pseudo-spectral acceler ation at 10-km di s-

tance from Tabuk source zone.

Conclusion

The current study trials simulate high frequency ground-mo-

tion produced by the damaging earthquakes at the northwestern

part of Saudi Arabia. This is an area where no recording system

is installed for measuring such motion. The stochastic simula-

tion method was applied to estimate the maximum ground-

motion number of the selected points through Tabuk City. The

calculated ground motions are represented by PGA. In addition,

the Pseudo-Spectral Acceleration (PSA) was calculated as well

at these sites for simulation. An effective source in the area is

the Tabuk source zone, which is located to the northeast of

Tabuk City. The maximum moment magnitude of Mw = 7.5 ha s

been obtained from this zone, explains higher values of PGA

and PSA through the Tabuk City.

It is concluded that, the maximum Peak Ground Acceleration

was found to be 218 cm/sec2 (gal) at the bedrock while, the

response spectrum, which reflects the characteristics of earth-

quake and the nature of the recording site, is calculated at vari-

ous damping ratios (0.03, 0.05 and 0.1) of the critical damping.

The estimated values from present study have been compared

with those of the Global Seismic Hazard Assessment Program

(GSHAP, Grϋnthal et al., 1999) and Al-Haddad et al. (1994).

The estimated values of PGA in in maps of GSHAP and Al-

Haddad et al. (1994) are in good agreement with the results of

the present study.

The estimated PGA could contribute significantly to the de-

termination of the national seismic codes. These results could

be a tool for engineers, decision-makers and planners to miti-

gate the earthquake effects and allow them to plan earthquake

resistant design through Tabuk City.

Acknowledgements

The author is extremely grateful to the Research Center, col-

lege of Science, King Saud University for supporting this pro-

ject.

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