Prepared in cooperation with the 

Ministry of Petroleum and Mineral Resources, 

Deputy Ministry of Mineral Resources, 

Kingdom of Saudi Arabia 
AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY 

Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the current-year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey publications released prior to the current year are listed in the most recent annual "Price and Availability List" Publications 

that are listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and 

Availability List" are no longer available. 

Prices of reports released to the open files are given in the listing "U.S. Geological Survey Open-File Reports," updated monthly, which is for sale in microfiche from the U.S. Geological Survey, Books and Open-File Reports Section, Federal Center, Box 

25425, Denver, CO 80225. Reports released through the NTIS may be obtaine.d by writing to the National Technical Information 

Service, U.S. Department of Commerce, Springfield, VA 22161; please include NTIS report number with inquiry. 

Order U.S. Geological Survey publications by mail or over the counter from the offices given below. 

BY MAIL 

Books 

Professional Papers, Bulletins, Water-Supply Papers, Techniques 

of Water-Resources Investigations, Circulars, publications of general interest (such as leaflets, pamphlets, booklets), single copies of Earthquakes 

& Volcanoes, Preliminary Determination of Epicenters, and some miscellaneous reports, including some of the foregoing series that have gone 

out of print at the Superintendent of Documents, are obtainable by mail 

from 

U.S. Geological Survey, Books and Open-File Reports 

Federal Center, Box 25425 

Denver, CO 80225 

Subscriptions to periodicals (Earthquakes & Volcanoes and 

Preliminary Determination of Epicenters) can be obtained ONLY from 

the 

Superintendent of Documents 

Government Printing Offlce 

Washington, D.C. 20402 

(Check or money order must be payable to Superintendent of Documents.) 

Maps 

For maps, address mail orders to 

U.S. Geological Survey, Map Distribution 

Federal Center, Box 25286 

Denver, CO 80225 

Residents of Alaska may order maps from 

Alaska Distribution Section, U.S. Geological Survey, 

New Federal Building - Box 12 

101 Twelfth Ave., Fairbanks, AK mot 

OVER THE COUNTER 

Books 

Books of the U.S. Geological Survey are available over the 

counter at the following Geological Survey Public Inquiries Offices, all 

of which are authorized agents of the Superintendent of Documents: 

• WASHINGTON, D.C.--Main Interior Bldg., 2600 corridor, 

18th and C Sts., NW. 

• DENVER, Colorado--Federal Bldg., Rm. 169, 1961 Stout St. 

• LOS ANGELES, California--Federal Bldg., Rm. 7638, 300 N. 

Los Angeles St. 

• MENLO PARK, California--Bldg. 3 (Stop 533), Rm. 3128, 

345 Middlefield Rd. 

• RESTON, Virginia--503 National Center, Rm. 1C402, 12201 

Sunrise Valley Dr. 

• SALT LAKE CITY, Utah--Federal Bldg., Rm. 8105, 125 

South State St. 

• SAN FRANCISCO, California--Customhouse, Rm. 504, 555 

Battery St. 

• SPOKANE, Washington--U.S. Courthouse, Rm. 678, West 

920 Riverside Ave .. 

• ANCHORAGE, Alaska--Rm. 101,4230 University Dr. 

• ANCHORAGE, Alaska--Federal Bldg, Rm. E-146, 701 C St. 

Maps 

Maps may be purchased over the counter at the U.S. Geological Swvey offices where books are sold (all addresses in above list) and 

at the following Geological Survey offices: 

• ROLLA, Mlssourl--1400 Independence Rd. 

• DENVER, Colorado--Map Distribution, Bldg. 810, Federal 

Center 

• FAIRBANKS, Alaska--New Federal Bldg., 101 Twelfth Ave. 
Probabilistic Estimates of the 

Seismic Ground-Motion Hazard in 

Western Saudi Arabia 

By PAUL C. THENHAUS, S.T. ALGERMISSEN, 

DAVID M. PERKINS, STANLEY L. HANSON, 

and WILLIAM H. DIMENT 

Prepared in cooperation with the 

Ministry of Petroleum and Mineral Resources, 

Deputy Ministry of Mineral Resources, 

Kingdom of Saudi Arabia 

U.S. GEOLOGICAL SURVEY BULLETIN 1868 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, industry, or firm names in this publication is for 

descriptive purposes only and does not imply endorsement by the U.S. 

Government. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1989 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Ubrary of Congress Cataloging-in-Publication Data 

Probabilistic estimates of the seismic ground-motion hazard in western Saudi 

Arabia. 

(U.S. Geological Survey bulletin; 1868) 

Supt. of Docs. no.: I 19.3:1868 

1. Seismology-Saudi Arabia. 2. Geodynamics. I. Thenhaus, Paul 

C. II. Series. 

QE75.89 no. 1868 557.3 s [551.2'2'09538] ~434 

[QE537 .2.S33] 
CONTENTS 

Abstract 1 

Introduction 1 

Overview of probabilistic model 2 

Hazard model 2 

Seismic-source-zone model 2 

Earthquake-occurrence model 3 

Ground-motion-attenuation model 3 

Probability distribution of ground motion 4 

Tectonic settings of seismic source zones 5 

Dead Sea rift 5 

Gulf of Aqaba (Elat) 5 

Dead Sea Basin 7 

Lebanon Mountains segment 8 

Fault-source model for Dead Sea rift 8 

Gulf of Suez 8 

Red Sea rift 10 

Fault-source model for Red Sea rift 13 

Gulf of Aden 13 

Fault-source model for Gulf of Aden 15 

Afar depression 15 

Western Arabian Peninsula 16 

Earthquake-recurrence model 18 

Seismicity parameters 18 

Seismic-rate allocation sequence and methods 20 

Maximum magnitudes 20 

Modeling earthquake events 21 

Ground-motion attenuation 21 

Preferred attenuation model 26 

Ground-motion hazard maps 26 

Discussion and recommendations 29 

Acknowledgments 36 

References cited 36 

Appendix-Seismic activity rates of source zones and source faults 42 

FIGURES 

1. Diagram and graphs showing elements of the probabilistic hazard 

model 2 

2-5. Maps showing: 

2. Tectonic, geologic, and geographical features and sites 6 

3. Seismic source zones 7 

4. Fault-source model 9 

5. Seismicity of the region 12 

6. Block diagrams showing process of rigid-plate rifting 14 

7. Chart showing allocation scheme of seismic activity rate 19 

Contents Ill 
8. Cross section showing interpreted velocity structure along the 

southwestern Saudi Arabian geophysical transect 22 

9. Index map showing the location of three ground-motion profiles 24 

10-15. Graphs showing: 

10. Profiles of peak horizontal acceleration applying various 

assumptions to two attenuation models 25 

11. Profiles of peak horizontal velocity applying various assumptions to 

two attenuation models 25 

12. Acceleration attenuation curves illustrating high and low extremes of 

attenuation relations 27 

13. Velocity attenuation curves illustrating high and low extremes of 

attenuation relations 28 

14. Acceleration attenuation curves for the preferred hazard 

model 29 

15. Velocity attenuation curves for the preferred hazard model 30 

16-19. Maps showing: 

16. Peak horizontal acceleration with a 90 percent probability of not 

being exceeded in 100 yr, given by the preferred hazard model 31 

17. Peak horizontal velocity with a 90 percent probability of not being 

exceeded in 100 yr, given by the preferred hazard model 32 

18. Peak horizontal acceleration with a 90 percent probability of not 

being exceeded in 100 yr, given by the upper-bound hazard 

model 33 

19. Peak horizontal velocity with a 90 percent probability of not being 

exceeded in 100 yr, given by the upper-bound hazard model 34 

20. Graph showing profiles of peak horizontal acceleration with and without 

attenuation variability 35 

21. Graph showing profiles of peak horizontal velocity with and without 

attenuation variability 35 

TABLES 

1. Estimates of b-values using two maximum-likelihood techniques 18 

2. Comparison of peak ground-motion values at Haql, Yanbu, Jiddah, and Jizan 

obtained from the preferred and upper-bound regional hazard models 28 

IV Contents 
Probabilistic Estimates of the 

Seismic Ground-Motion Hazard in 

Western Saudi Arabia 

By Paul C. Thenhaus, S.T. Algermissen, David M. Perkins, 

Stanley L. Hanson, and William H. Diment 

Abstract 

Estimates of seismic horizontal ground acceleration 

and velocity having a 90 percent probability of nonexceedance in 1 00 years in western Saudi Arabia indicate the 

highest relative levels of ground motion are expected in 

regions neighboring the Gulf of Aqaba and North Yemen. 

Estimated ground motions within the shield area are 

relatively low, whereas the central and northern coastal plain 

regions are characterized by intermediate-level groundmotion values that are generally governed by far-field effects 

of earthquakes in the central Red Sea rift. 

The seismic hazard estimates are derived from regional 

seismic source zones that are based on interpretations and 

assumptions relating potential seismic activity to the Precambrian through Tertiary structural framework of the region. 

Because statistical variability in ground-motion attenuation 

relations is known to be a dominant source of uncertainty in 

estimated probabilistic ground-motion values, and because 

strong-motion records do not exist for the Arabia peninsula, 

hazard maps have been developed using both a preferred 

low-Q attenuation function and a conservative high-Q 

attenuation function. Values resulting from the high-Q 

attenuation function are considered upper-bound estimates 

to the hazard model. 

Progress toward a more complete understanding of the 

seismic hazards of western Saudi Arabia is expected from 

broad-scope, interdisciplinary geological, seismological, and 

related geophysical investigations aimed at improving our 

understanding of all parameters that have gone into the 

probabilistic ground-motion estimates. Identification of areas 

in which geological hazards such as landsliding and 

liquefaction may occur for ground shaking levels suggested 

in this report will do much to quantify these hazards in a 

preliminary manner. 

Manuscript approved for publication, July 22, 1988. 

INTRODUCTION 

The use of probabilistic ground-motion maps to 

represent seismic hazard has evolved from experience 

with a number of other types of map representations 

during the 30-yr history of regional seismic hazard 

mapping in the United States. Historical seismicity maps 

are factual and can serve to warn that earthquakes occur 

more widely than people usually recognize. Epicenter 

maps, however, only provide information about the 

geographical location of earthquakes and, in some cases, 

information about the depth of earthquakes. Epicenter 

maps provide no direct information concerning: (1) 

expected levels of ground motion, and (2) the 

generalization of historical seismicity to likely areas of 

future seismicity. Historic maximum intensity maps 

provide information on levels of damaging ground 

motion, but they do not provide information on the 

relative frequency of occurrence of specific levels of 

ground motion in various geographical areas. Intensity 

maps may, therefore, overstate the hazard in areas where 

a large earthquake occurred historically but where the 

frequency of such occurrences is low. Probabilistic 

seismic ground-motion maps properly focus on 

hazardous ground motions, allow the use of 

interdisciplinary geological, seismological, and geophysical information to assist in the generalization of the 

historic record to likely areas of future earthquakes, and 

properly represent the hazard with respect to the 

expected frequency of earthquake and ground-motion 

occurrence. Such maps of the contiguous United States 

(Algermissen and Perkins, 1976; Algermissen and 

others, 1982) have found widespread use in building code 

applications, regional land-use planning, emergency 

preparedness, and insurance analyses. In addition, such 

Introduction 
A 

• Source 2 (POINT) 1.0 

.s: .s: 

~: f.: E ~ ~ 0.8 

/\1 /\1 

~~0.6 VI 

'II{ 

ci 

-

~ 

~ 0.4 

~ 

~ 1-" 

(FAULT) 

E ~ 

1.1... 0'-------- ACCELERATION ACCELERATION 

MAGNITUDEFigure 1. Elements of the probabilistic hazard model. A, Types of seismic source zones and grid of points at which the 

hazard is to be computed. 8, Statistical analysis of seismicity data. C, Ground-motion attenuation curves for various 

magnitude levels. D, Cumulative conditional probability distribution of acceleration at a site on the grid of points. E, The 

extreme probability Fmax,t(a} for various accelerations and exposure times (T ). Bold arrows key graphs to their respective 

elements in the source zones and on the computation grid. 

maps can serve to prioritize regions for further research 

and point out those studies most useful in furthering the 

understanding of the seismic hazard problem. 

OVERVIEW OF PROBABILISTIC MODEL 

Hazard Model 

In this study, earthquakes are exponentially 

distributed with regard to magnitude and randomly 

distributed with regard to time. The exponential 

magnitude distribution is an assumption based on 

empirical observation. The distribution of earthquakes in 

time is assumed to be Poissonian. The assumption of a 

Poisson process for earthquakes in time is consistent with 

historical earthquake occurrence insofar as it affects the 

regional probabilistic hazard calculation. On a regional 

basis, large shocks closely approximate a Poisson 

process, whereas small shocks may depart significantly 

from a Poisson process. The ground motions associated 

with small earthquakes are of only marginal interest in 

engineering applications, and consequently the Poisson 

assumption serves as a useful and simple model (Cornell, 

1968). 

The production of a probabilistic ground-motion 

hazard map involves the development of four constituent 

models. These are: (1) a seismic-source-zone model that 

defines the spatial distribution of earthquakes within the 

region of concern (fig. lA), (2) an earthquakeoccurrence model that describes the recurrence of events 

in time within the seismic source zones (fig. lB), (3) a 

ground-motion-attenuation model that describes 

mathematically the manner in which earthquake ground 

motions decrease with distance from an earthquake 

source for various magnitude levels (fig. 1C), and ( 4) a 

probability model for calculating the expected maximum 

amplitude of ground motion within a given period of time 

corresponding to a chosen probability level for a number 

of individual sites in the region (fig. ill). 

Seismic-Source-Zone Model 

Three idealized seismic source areas are shown in 

figure lA. Earthquakes within each source zone can be 

modeled as: (1) point sources in areas (used to represent 

earthquakes for which the fault rupture lengths are small 

compared with the map scale being used); (2) finite 

rupture lengths along two-dimensional (linear) fault 

sources; or (3) a mixed source, for example, point 

sources for small earthquakes and fault (two 

dimensional) sources for larger earthquakes. 

2 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
All earthquakes are assumed to be shallow shocks 

consis tent with the worldwide observations of 

earthquake depths in regions of crustal rifting and 

consistent with the earthquake data base used in the 

development of the attenuation models applied in this 

study. 

Earthquake-Occurrence Model 

For each seismic source, the magnitude distribution was taken to be exponential and of the form given 

by the Richter relation of occurrence frequencies 

(fig. lB): 

logN=a+bM (1) 

where N is the number of earthquakes in a given 

magnitude range centered about magnitude M and a and 

b are constants to be determined. Because seismicity in 

our region of concern is generally low, and research has 

shown that significant errors in the computed b-value 

occur when the total number of earthquakes are less than 

about 40 (Bender, 1982), b-values for regional 

collections of source zones having total numbers of 

earthquakes from 60 to 150 were computed. The bvalues were computed using maximum-likelihood 

techniques described by Bender (1983) and Weichert 

(1980). 

The small number of earthquakes in many of the 

seismic source zones also precludes direct assessment of 

a-values for individual zones. Hence, a regional rate of 

earthquake activity was calculated by summing all the 

activity in all the seismic source zones. This regional rate 

of activity was then apportioned among the individual 

source zones so that the amount of activity applied to any 

one zone is in rough proportion to the number of 

earthquake events occurring in that zone. The procedure, 

therefore, attempts to maintain the relative seismic flux 

among source zones consistent with the historic record of 

events. 

The future spatial occurrence of earthquakes is 

assumed to be uniform throughout each source zone. 

That is, if each seismic source area is divided into n small 

divisions (such as shown in fig. 1A) and if the number of 

earthquakes likely to occur in any magnitude range is N, 

then the number of earthquakes likely to occur in this 

magnitude range in each small division or block of a 

source area is 

N 

n 

(2) 

If seismicity is distributed along a fault of length L, the 

distribution of earthquakes is somewhat more 

complicated. We have used the relationship between 

fault rupture length (L) and magnitude (M) suggested by 

Mark (1977): 

log (L) = 1.915 + 0.389 M (3) 

where L is the average surface rupture length in meters 

and M is as already defined. If there are N M -M 11 1 

earthquakes in the magnitude interval M2-M1 that have 

an average length of rupture (determined from equation 

3) of Lave and we are modeling a fault of length X, the 

earthquakes are distributed at the rate of 

(4) 

earthquakes per unit of length along the fault. If one end 

of a fault is located at X1 and the other end at X2, the 

earthquake rupture centers are assumed to occur 

·e Lave Lave UflltOrmly between xl + 2 and X2-2 along the fault. 

Bender (1984b) provides details regarding the finitefault-rupture model used in this study. 

Ground-Motion-Attenuation Model 

The ground-motion-attenuation function is a 

critical component of the probabilistic ground-motion 

mapping procedure (fig. 1C). Research has shown that 

much of the variability in estimates of peak groundmotion values is due to uncertainties inherent in groundmotion-attenuation models (Bender, 1984a; McGuire 

and Shedlock, 1981). Although there are a number of 

published attenuation models available for various 

tectonic settings and applications (see for example, Campbell, 1985), choice of a single, correct model for western 

Saudi Arabia is hindered by lack of strong-motion 

recordings and lack of reliable isoseismal maps due to a 

relatively low rate of earthquake activity and a sparse 

population distribution. Hence, there is no direct 

sampling of the propagation path of earthquake waves 

within this region. This lack of data leaves a fundamental 

question as to whether the region is characterized by 

high, low, or moderate attenuation. Our approach is to 

investigate the ground-motion hazard implications of 

both a conservative low-attenuation model (high Q) and 

a preferred high-attenuation model (low 0), thereby 

providing upper-bound and preferred ground-motion 

estimates within the constraints of seismic source zones 

and seismic activity rates of this study. 

Overview of Probabilistic Model 3 
Probability Distribution of Ground Motion 

Once the seismic activity rate per unit area or per 

unit length of fault is determined for the seismic source 

zones of the region, and a ground-motion-attenuation 

function is decided upon, a distribution of ground motion 

is calculated (fig. ill) for a number of individual sites 

located on an appropriate grid pattern (fig. 1A) 

throughout the region. 

From the distribution of ground motion at each site 

(fig. 1D), it is possible to determine directly the expected 

number of times a particular amplitude of ground 

motion is likely to occur in a given period of years at a 

given site, and thereby, the maximum amplitude of 

ground motion in a given number of years corresponding 

to any level of probability (fig. lE). The relationship 

between return period, Ry (a), exposure time, T, and 

probability of exceedance during that exposure time, 

1-Fmax,t (a), is explained by the following development. 

First, the distribution of the expected number of 

occurrences of ground motion at each location is 

calculated. The peak ground motion, for example, the 

peak acceleration corresponding to some extreme 

probability, is then calculated from the distribution of the 

expected number of occurrences in the following 

manner. Let the peak acceleration be a, then 

F (a) =P (A sa I M>Mmin] (5) 

is the probability that an observed acceleration A is less 

than or equal to the value a, given that an earthquake 

with magnitude M, greater than some minimum 

magnitude of interest, has occurred. The calculation at a 

given grid point or along a fault is performed for every 

acceleration a of interest using: 

F(a) expected number of occurrences with A sa and M> Mmia 

total expected number of occurrences (M>Mmia) 

A typical F (a) is shown in figure W. 

F (a) is the probability that the acceleration value a 

will not be exceeded, given that an earthquake larger 

than the minimum magnitude occurs in the source zone. 

Therefore (1.0-F (a)) is the probability that a will be 

exceeded. If q, is the yearly rate at which earthquakes 

larger than the minimum magnitude occur in the zone, 

then the expression 

q, (1.0-F (a)) (6) 

gives the annual rate at which earthquake accelerations 

greater than a occur. In T years, 

rq, (1.0-F (a)) (7) 

gives the expected number of occurrences of 

accelerations greater than a for a period of T years. If 

earthquake occurrences are rare, and the annual rate of 

occurrence is the same from year to year (that is, for 

Poisson occurrences), then probability theory tells us that 

exp (-(expected number)) (8) 

is the probability that no such occurrence happens. Thus, 

the probability that acceleration value a is not exceeded 

in T years due to an earthquake in the source zone 

considered is given by the expression 

exp (-Tfj> (1.0-F(a))) 

which can be rewritten as, 

F ( ) _ e-t [1-F(a)] max,t a - (9) 

where T now equals a particular exposure time, t. Using 

equation 9, a table of (a) and F (a) is constructed. For a 

particular exposure time T=t, Fmax,t(a) is calculated, and 

the value of a for a given extreme probability, say 

Fmax,t(a) =0.90, is found by interpolation. 

It is convenient here to define the term return 

period in years as: 

1 

Ry (a) 1-F (a) (10) 

where R (a) is the average number of events that must 

occur to get an acceleration exceeding a. The return 

period in years is given approximately by 

R1 (a) R(a) 

(11) expected number of events per year (M~Mmia) 

We obtain from equations 10 and 11: 

t fj>t (1-F(a)) = Ry (a) (12) 

thus, from equations 9 and 12: 

F (a) -e-t!Ry (a) max,t - (13) 

4 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
and t 

In (Fmax,t (a)) = ---

Ry (a) 

(14) 

For an extreme probability of Fmax,t (a) = 0.90 and an 

exposure time oft = 100 yr: 

or 

100 

In (0.90) =-Ry (a) 

100 

Ry (a)= 0.1054 = 949 yr. 

Thus, the average return period for the ground motions 

we have mapped is about 950 yr. 

It may be useful to point out that using equation 13 

and setting the exposure time equal to the average return 

period Ry (a), that is 

t =Ry (a), 

we have Fmax,t (a) =e-1 = 0.37 . (15) 

Thus the acceleration with a return period of Ry (a) =t 

years has a probability of 

1-Fmax,t (a)= 1-0.37 =0.63 or 63 percent 

of being exceeded in t years. The point is that 

accelerations (or any other parameter) with a particular 

return period have a 63 percent probability of being 

exceeded during an exposure time equal to that return 

period. 

Frequently, it is convenient to express the 

maximum ground motion in terms of the annual 

probability of exceedance. Let rr (a) be the probability of 

exceedance of ground motion a in T years; then 

F ( ) 1 ( ) -T/R (a) max, T a = -1" T a = e Y (16) 

and 

rr( a) = 1-e-T!Ry (a) (17) 

for T = 1 yr, equation 17 becomes 

1 

rr( a)= 1-e -R, (a) . 

when Ry (a) is sufficiently large (say, greater than 10 yr), 

1 

rr(a) = Ry (a) . 

TECTONIC SETTINGS OF SEISMIC 

SOURCE ZONES 

Dead Sea Rift 

The Dead Sea rift is a 1,000-km-long sinistral-slip 

boundary between the Arabian and Sinai plates (fig. 2). 

Although commonly referred to as a rift, the structures 

defining the plate boundary result from transform 

movement along a shear zone that connects the Red Sea 

spreading ridge with the Zagros collisional zone of 

Turkey. Inception of shear movement coincided with 

initial rifting of the Red Sea in pre-Miocene time, and 

offset Precambrian through Cretaceous rock units 

indicate about 105 km of cumulative left-slip movement. 

The average slip rate is estimated to be 0.7-1.0 cm/yr 

over the last 4-5 m.y. Late Quaternary fault displacements have been mapped along the length of the 

transform boundary (Garfunkel, 1981); however, 

structural style and trend and contemporary seismic 

activity vary considerably. We have, therefore, segmented 

the transform plate boundary into three separate source 

zones to reflect these differences (fig. 3). A growing body 

of knowledge indicates that discrete segments of large 

strike-slip faults possess varying seismological characteristics generally governed by large-scale crustal 

asperities and varying strike of the fault trace within a 

uniform, regional stress regime (Sykes and Seeber, 1982; 

Sieh, 1981; Nishenko and McCann, 1981; Sykes and 

Nishenko, 1984). The three source zone segments, from 

south to north, are (1) the Gulf of Aqaba-Arva Valley, 

(2) the Dead Sea rift, and (3) the Lebanon Mountains 

segments. Finer distinctions than used in this study could 

perhaps be made in the Dead Sea rift and the Jordan 

fault segments because of subaerial exposure of most of 

the fault system and a larger number of earthquakes 

recorded in this region. However, because this study 

concerns western Saudi Arabia, finer distinctions in 

seismic sources in the central and northern fault 

segments would have little influence on ground motions 

that affect northwestern Saudi Arabia. 

Gulf of Aqaba (Eiat) 

The Gulf of Aqaba (fig. 2) is the widest and 

deepest segment of the transform plate boundary and is 

characterized by the structural development of enechelon rhomb-grabens (pull-apart basins). Three such 

grabens, each filled by a few kilometers of sediment, have 

Tectonic Settings of Seismic Source Zones 5 
Q Tertiary-Holocene volcanics 

------.-- Normal fault-Bar and ball on 

downthrown side 

Strike-slip fault-Queried where 

movement or fault zone is 

hypothetical 

Thrust fault- Sawteeth on upper plate 

Faults-Undifferentiated as to mode 

of displacement. Dashed where 

inferred 

Ocean spreading-ridge transform 

zone-Undifferentiated as to 

certainty of zone's existence. Solid 

line between transforms indicates 

spreading ridge axes where processes 

of ocean-floor spreading are occurring 

with high degree of certainty 

Shear zones-Inferred from Bouguer 

gravity in the Gulf of Suez region 

~,, Geologic/Tectonic province limits 

Figure 2. Index map of selected tectonic, geologic, and geographical features and sites referenced in the text. Sources 

of map compilation by region: (1) Western Arabia peninsula: Ministry of Defence, United Kingdom (1980), Brown (1972), 

Greenwood and others (1980), Coleman (1973), Merghelani and others (1981); (2) Dead Sea rift: Garfunkel (1981), Eyal 

and others (1981), Garfunkel and others (1981); (3) Gulf of Suez: Garfunkel and Bartov (1977), Riad (1977); (4) Red Sea: 

Drake and Girdler (1964), Hall (1980), Coleman (1973); (5) Gulf of Aden: Laughton (1966), Laughton and others (1970), 

Cochran (1981, 1982); and (6) Afar: Kronberg and others (1975), Barberi and others (1975). 

been inferred to underlie the Gulf of Aqaba ( Garfunkel, 

1981). Rhomb-graben development is a common 

structural style along transform plate boundaries where 

the strike of the transform deviates from the regional slip 

direction; such grabens have been long recognized in the 

United States as important structural elements along the 

transform plate boundary of southern California (Crowell, 1974). Local extension across the transform zone 

results in small-magnitude crustal separation and a 

"leaky" transform boundary. Garfunkel (1981) estimated 

35-40 km of young slip is adequate to open the rhombgrabens of the Gulf of Aqaba. Separation of the Precambrian crust across the transform boundary is estimated to 

be about 1 km in the central and southern segments. 

Older, Miocene and post-Miocene slip was distributed 

throughout a broader region of shear neighboring the 

gulf. Sinistrally offset Miocene dikes intruded in the 

Precambrian crystalline rocks of eastern Sinai suggest 

24 km of cumulative post-Miocene offset on a shear 

system of anastomosing strike-slip faults in a 30-km-wide 

6 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
25° 

200 400 

KILOMETERS 

200 

MILES 

63° 

Figure 3. Seismic source zones of the Arabian shield, Red Sea rift, and contiguous tectonic systems used in this study. 

Stippled area highlights outcrop boundary of Precambrian crystalline rocks in western Saudi Arabia. 

zone neighboring the gulf (Eyal and others, 1981 ). 

Similar displacement is estimated for the eastern coast of 

the Gulf of Aqaba (Eyal and others, 1981). Thus, some 

60 km of the total 110-km offset along the transform 

system is taken up by faults within the gulf itself. North of 

the Gulf of Aqaba, faults of the Dead Sea transform 

boundary are subaerially exposed in the Arva Valley. 

Active en-echelon left-slip faults are evident within this 

topographic saddle, whereas normal-slip faults delimit 

some of the margins (Garfunkel, 1981). In the northern 

Arva Valley, the Arva Valley fault bounds the eastern 

margin of the topographic depression and has a distinct 

topographic expression; late Quaternary movement 

along this fault occurred over a length exceeding 100 km 

(Garfunkel and others, 1981; Garfunkel, 1981). 

Dead Sea Basin 

The trend of the transform boundary shifts from a 

predominantly north-northeast trend through the Gulf of 

Aqaba-Arva Valley area to a predominantly north-south 

trend along the Dead Sea basin (fig. 2). Topographically, 

the Dead Sea occupies the lowest portion of the subaerTectortic Settings of Seismic Source Zones 7 
iarially exposed transform boundary. The Arva fault 

bounds the east side of the Dead Sea basin, but structural 

expression of the fault diminishes to the north. The 

Jordan (Jericho) fault, a major sinistral fault en echelon 

to the Arva, bounds the Dead Sea basin to the west. The 

Dead Sea occupies a rhomb-graben between the two 

sinistral slip faults. Trenching studies across the Jordan 

(Jericho) fault indicate two large earthquakes in the past 

2,000 yr; one event occurred between 200 B.C. and A.D. 

200 and the other between A.D. 700 and A.D. 900 

(Reches and Hoexter, 1981). Other, smaller events could 

be inferred. Precise dating of the events is precluded 

because of poor soil development and the predominance 

of coarse clastic deposits in the region. Left-lateral 

distortion and dislocation of walls of an 8th-century 

palace located somewhat west of the main trace of the 

Jordan fault (Reches and Hoexter, 1981) indicate that 

not all the shear strain is accommodated along the main 

trace of the Jordan fault, but some is distributed 

throughout a broader area of the Jordan Valley. Average 

slip rate on the Dead Sea portion of the transform is 

estimated to be 0.7 cm/yr (Reches and Hoexter, 1981), 

which is consistent with the average slip of the overall 

plate boundary of 0.7-1.0 em/yr. The Jordan fault 

continues northward through the Jordan Valley to the 

Sea of Galilee. Mountain fronts on either side of the 

Jordan Valley are fault controlled, and locally at their 

bases, scarps attain several tens of meters in height 

(Garfunkel and others, 1981). In the valley near the 

Jordan fault trace, lacustrine beds are tilted at 30° and, 

locally, are gently folded within a few hundred meters 

from the fault, which indicates a small amount of 

compression across the fault ( Garfunkel and others, 

1981). 

Lebanon Mountains Segment 

North of the Jordan Valley, the transform 

boundary splays into three large fault strands (fig. 2). The 

central strand, the Yammuneh fault, follows a sinuous 

course northward through the Lebanon Mountains. The 

sharp easterly bend in the transform boundary through 

this segment results in a strong constraining bend in the 

left -slip transform system. The resultant transpression is 

postulated as causing large topographic uplifts on either 

side of the fault (Quennell, 1956). The other two large 

splay faults may also be active, but their surface traces die 

out some 50-60 km north of the Jordan Valley (Garfunkel and others, 1981). 

Ben-Menahem (1981) noted a nonuniform 

distribution of seismic slip along the length of the Dead 

Sea transform zone. Although moderate-to-large 

earthquakes have occurred in all of the above three 

segments of the transform zone, historic and 

contemporary microseismic activity is considerably lower 

in the Gulf of Aqaba-Arva Valley segment than in the 

central and northern segments. Lack of historically 

reported large earthquakes (M > 7) in the southern 

segment could be due to the historically sparse 

population of this region; however, low levels of 

microseismic activity recorded during recent monitoring 

tends to support the general distribution of the macroseismic data (Ben-Menahem and Aboodi, 1971). BenMenahem (1981) concluded that a large portion of the 

total slip is accommodated aseismically in the south, 

whereas to the north, as the Zagros collisional zone is 

approached, slip becomes purely seismic. This thesis is 

consistent with the tectonic style of the transform 

boundary in its northern segments. North of the Dead 

Sea, compression across the transform boundary 

increases considerably because of the large constraining 

bend through the Lebanon Mountains that results, 

perhaps, in predominantly stick-slip fault movement. The 

tectonic setting is generally similar to the "big bend" 

region of the San Andreas fault in southern California 

that broke in 1857 in a great earthquake and that has a 

prehistoric record of characteristically breaking in large 

events (Sieh, 1978). Sykes and Seeber (1982) referred to 

these large bends in transform boundaries as tectonic 

knots and concluded that they are areas of considerable 

stress accumulation because of their difficulty in 

accommodating relative plate motion. 

Fault-Source Model for Dead Sea Rift 

The three segments of the Dead Sea rift are 

modeled as mixed-source types of seismic source zones 

(fig. 1A); that is, linear fault-rupture sources are 

modeled within a broader area source (fig. 4 ). 

Earthquakes with Ms ~ 6.5 are modeled as twodimensional sources, whereas earthquakes with Ms :S 6.5 

are modeled as point sources throughout the area 

source. Two fault sources are modeled 20 km apart and 

central to the area source. Double, parallel faults were 

modeled in each of the three segments to better 

represent the en-echelon nature of long Quaternary 

strike-slip faults (that generally bound rhomb-grabens) 

and to account for deformation extending beyond a single 

fault trace. 

Gulf of Suez 

As in the case of the Dead Sea transform (fig. 2), 

additional detail in seismic source zones could perhaps 

be provided for eastern Egypt. However, because 

additional detail in source zones of this region would 

have little affect on resultant ground motions in extreme 

northern Saudi Arabia, we have not attempted detailed 

zones in this region. 

8 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
0 200 400 

KILOMETERS 

~-__._20_0 --=----'--------' 

MILES 

Figure 4. Fault-source model for the Dead Sea-Red Sea-Gulf of Aden rift system used in this study. Stippled area 

highlights outcrop boundary of Precambrian crystalline rocks in western Saudi Arabia. 

Although age of incipient rifting along the Suez 

arm of the Red Sea rift is not precisely known, stratigraphic evidence indicates rifting was well underway by 

late Oligocene to early Miocene time (30-20 m.y. ago.) 

(Garfunkel and Bartov, 1977). An estimated 15-20 km of 

crustal stretching in the southern part of the rift was 

accommodated through faulting and block tilting. 

However, crustal separation was too small to result in 

production of oceanic lithosphere between the Sinai and 

Nubian plates (Garfunkel and Bartov, 1977). Amounts of 

crustal stretching decrease northward along the rift. 

Although the primary episode of deformation 

along the Suez arm of the rift was completed by late 

Miocene time, slow subsidence of the gulf has continued 

through the Quaternary. Quaternary fault scarps are 

generally concentrated along a central trough and along 

the eastern border fault (Garfunkel and Bartov, 1977); 

they retain a zig-zag pattern inherited from middle 

Tertiary structural fabric. Young uplift of the rift margins 

is indicated by fragmentary evidence of uplifted marine 

terraces and reefs (Garfunkel and Bartov, 1977) on the 

flanks of the southern part of the rift. 

North of the gulf, structures of the rift are covered 

by eolian dunes and deltaic deposits of the Nile River. 

However, a continuous physiographic depression persists 

northward along a projection of the central trough; it 

Tectonic Settings of Seismic Source Zones 9 
forms the physiographic furrow in which the Suez Canal 

was excavated. Some of the topographically low areas of 

this depression are below sea level, and Garfunkel and 

Bartov (1977) argued that stratigraphic evidence, 

together with the geomorphic evidence, indicates recent 

subsidence of the canal depression with continental and 

pluvial sedimentation generally having kept pace with 

subsidence. 

The structural framework of Egypt is complex with 

four primary, intersecting fault trends being recognized 

throughout this region. These are (1) the east African 

fault systems that trend north-south, (2) the Erythrean or 

African fault system that trends northwesterly, (3) 

Tethyan faults that trend east-west, and (4) the 

Aualitic faults that trend northeast-southwest (Said, 

1962; Riad, 1977). Riad interpreted a number of 

northwest-trending, parallel to subparallel shears in 

northern Egypt based on a Bouger gravity anomaly map 

of the region (fig. 2). He inferred right-lateral movement 

across the shear zones based on space-photography and 

suggestions of independent stratigraphic studies. The 

northwest strike of the shear zones and dextral sense of 

movement would be conjugate to the sinistral, northeasttrending Dead Sea transform. Opening of the Gulf of 

Suez is suggested (Riad, 1977) to have resulted from 

dextral slip along the shear zones. 

The Gulf of Suez seismic source zone is drawn to 

include those shear zones inferred to be associated with 

the gulf (fig. 3). The eastern boundary of the zone has 

been extended northward to include the topographic 

depression of the Suez Canal. The zone collects a 

number of historically reported earthquakes in northern 

Egypt that have been assigned Modified Mercalli 

intensities (MMI) as high as X (Poirier and Taber, 1980). 

It is assumed that seismic potential is uniform 

throughout the zone although this, in fact, is not entirely 

clear. Structural deformation is most intense in the Gulf 

of Suez in the southern part of the source zone; however, 

historically reported events are concentrated in the 

northern half of the zone. This may be a result of longer 

settlement of the Nile delta region as compared to the 

desert regions neighboring the Gulf of Suez. 

Red Sea Rift 

Drake and Girdler (1964) recognized three physiographic provinces of the Red Sea (fig. 2). These are the 

coastal shelves, a broad main trough, and a deep axial 

trough. The three provinces are present to varying 

degrees along the length of the Red Sea. North of lat 24° 

N., an axial trough is not clearly developed and the shelf 

areas are quite narrow. Between lat 19° N. and 24° N., an 

axial trough is characterized by a discontinuous series of 

"deeps" in excess of 1,000 fathoms, and a main trough is 

well developed. South of lat 19° N., shelf areas are broad 

and the axial trough has a continuous, narrow expression 

to about lat 15° N. where the main trough shoals into the 

Straits of Bab-al-Mandab. 

Structural deformation is intense in the axial zone 

and characterized by numerous normal faults (Ross and 

Schlee, 1973, 1977; Phillips and Ross, 1970). Marginal to 

the axial trough, deformation of the main trough is less 

intense but also characterized by abundant normal faults 

and folds. Many shallow folds are related to salt flowage 

and diapirism (Ross and Schlee, 1973, 1977). The 

transition from axial trough to main trough (roughly 

following the 500-fathom bathymetric contour) is 

marked in places by narrow, intensely faulted zones or 

large downfaulted blocks (Ross and Schlee, 1973, 1977). 

Shelf areas are generally characterized by normal faults 

(more prevalent in the southern Red Sea) and folds 

related to salt flowage and diapiric intrusion. 

The Red Sea was recognized early in the 

development of global plate tectonic theory as ~art of the 

worldwide rift system (Sykes, 1967). However, Important 

aspects concerning the rift's fundame?tal crust?l 

structure and tectonic development remam uncertam 

and are topics of much debate in recent geological 

literature. Central to the debate is the nature of the crust 

beneath the broad shelf areas of the southern Red Sea. 

One school of thought holds that the shelf areas in the 

southern Red Sea are underlain by Miocene age oceanic 

crust or, at least, mantle-derived crust. A second school 

of thought argues that the shelf areas are character~e~ 

by rifted and extended continental crust. Although It IS 

generally accepted that ocean-floor spr~adi~g ( creati?n 

of new oceanic crust) has been occurrmg m the axtal 

trough of the southern Red Sea (south of ~bout lat 19.~ 0 

N.) since Pliocene time, a variety of geologtcal, geophysical, and plate kinematic arguments have been proposed 

both suggesting and denying the possibility of early 

Miocene ocean-floor spreading. 

Magnetic signatures obtained across the shelves 

are subdued due to a late Miocene evaporite sequence, 

which is about 3 km thick, that precludes direct 

observation of deeper basement structure through 

standard seismic-reflection techniques. Broad, linear 

magnetic anomalies on the shelves. sh~w an abrupt 

transition to high-frequency anomalies m the coastal 

plain of southwestern Saudi Arabia .. The abrupt 

transition in signature has commonly been mterpreted as 

the contact between oceanic and continental crust (Gettings, 1977; Hall, 1980; Hall and others, 1977; Girdler and 

Styles, 1974 ). Furthermore, these investigators su~ested 

that the low-amplitude broad wavelength anomalies of 

the shelves are roughly symmetric about the axial trough 

and generally correlate with Miocene age. an~malies of 

the Heirtzler and others (1968) magnetic time scale. 

Alternatively, Cochran (1983), following Fraziers' (1970) 

1 o Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
structural interpretation of an area off northern Afar, 

suggested that the offshore total-intensity magnetic 

anomalies are due to salt diapirs along large basement 

faults and, hence, cannot be correlated with oceanfloor-spreading anomalies. Cochran (1983) further noted 

a correlation of total-intensity magnetic anomalies with 

free-air gravity anomalies. Cochran stated that this 

correlation would not be expected if the magnetic 

anomalies were spreading anomalies, but that this 

correlation would be allowable if the magnetic signatures 

derive from faulted basement and salt structures. 

Recent plate kinematic reconstructions (Le Pichon 

and Francheteau, 1978; Cochran 1981, 1983), while 

applying different fundamental constraints on the 

amount of rotation of the Arabian and Nubian plates 

about the Euler pole of McKenzie and others (1970) and 

using different interpretations of the total number of 

rigid plates in the region, require an amount of total plate 

separation in the southern Red Sea more than twice the 

width of Pliocene-present ocean floor in the axial trough. 

The required extra space could be accommodated 

through pre-Pliocene ocean-floor spreading, extended 

and thinned continental crust, or any combination of 

unknown percentages of thinned continental crust and 

mafic intrusion. Abundant geologic evidence exists of 

significant amounts of mafic intrusion into the 

continental crust bordering the Red Sea (Schmidt and 

others, 1983; Blank, 1977). An unresolved issue, 

however, is the lateral extent of extended continental 

crust. In models that incorporate Miocene age oceanfloor spreading, this zone is relatively narrow and crops 

out immediately east of the coastal plain (Schmidt and 

others, 1983). Alternatively, in Cochran's (1983) 

interpretation, extended continental crust extends far 

seaward, beneath the shelves and main trough of the 

southern Red Sea. Furthermore, the floors of the central 

and northern Red Sea are suggested to be extended 

continental crust as Cochran (1983) argued that 

organized ocean-floor spreading has not yet commenced 

north of about lat 21° N. The region between lat 2P N. 

and 25° N. is suggested (Cochran, 1983) to be currently 

in transition from diffuse continental extension to an 

ocean-floor spreading mode of plate separation. 

Bohannon (1986) has suggested a tectonic model 

of early Red Sea rift development that incorporates lower 

to middle crustal expansion through large-scale mafic 

intrusion with concomitant brittle extension in the upper 

crust. The magnitude of brittle extension is suggested to 

be about 40 percent, which is a relatively small value in 

comparison to estimates of over 100 percent extension 

along some passive continental margins (Le Pichon and 

Sibuet, 1981 ). Incorporating geologic interpretations 

from field mapping with an interpretation of a seismicrefraction profile (Mooney and others, 1985) that 

extends across the southern Arabian shield to the 

Farasan Islands, Bohannon (1986) envisioned brittle 

extension accommodated by widely spaced up-to-basin 

listric normal faults above an east -dipping low-angle 

detachment that crops out near the eastern coastal plain 

and roots beneath the Arabian Escarpment. The brittlely 

extended crust is exposed in a 75-km-wide belt east of the 

coastal plain and exhibits characteristic arcuate traces of 

normal faults. Although Bohannon (1986) suggested it is 

possible that extended continental crust exists beneath 

the coastal plain, he argued that west of the coastline the 

crust is of oceanic character. 

The regional distribution of seismicity throughout 

the Red Sea has changed little since the early 

investigations of Gutenberg and Richter (1954), Drake 

and Girdler (1964), and Sykes and Landisman (1964). 

Notably, with an additional 20 yr of data and 

establishment of the Worldwide Standard Seismograph 

Network (WWSSN), seismicity remains concentrated 

proximal to the southern Red Sea rift (fig. 5). Relocation 

efforts have refined post-1955 earthquake locations to an 

accuracy of 10-20 km (Sykes and Landisman, 1964; 

Fairhead and Girdler, 1970) for moderate and large 

shocks in the region. Reasonable associations with at 

least region-scale structure can, therefore, be inferred. A 

large aftershock (mb = 5.3) of the North Yemen 

earthquake of December 13, 1982, was recorded by the 

WWSSN and a network of portable seismographs 

deployed in the epicentral region (Langer and Merghelani, 1983). The epicenters determined independently by 

local and teleseismic data are no more than 5 km apart, 

which suggests that contemporary seismic activity in this 

area can be reliably determined from WWSSN data. 

Although seismic activity is concentrated in the 

southern half of the Red Sea and predominantly confined 

to the axial trough, the seismicity is not uniformly 

distributed throughout this area. Some earthquake 

activity is clustered on, or near, inferred transform faults 

of the rift axis (fig. 5). Focal mechanisms for two 

earthquakes located near the southern Red Sea rift axis 

indicate nearly pure strike-slip movement on northeasttrending planes and suggest contemporary seismic 

activity on rift transform faults (Fairhead and Girdler, 

1970). Other significant activity appears to occur along 

other portions of the central rift not having recognized 

transform faults. This activity may be related to either 

intrusive activity along the axial trough, normal fault 

movement associated with the downdropping of blocks 

into the axial trough, or movement along presently 

undetected transforms. 

Figure 3 shows the seismic source zone 

configurations of the Red Sea used to group historic 

seismicity for the determination of earthquake 

recurrence rates. Three primary zones have been 

constructed. A relatively narrow zone based on the 

500-fathom bathymetric contour includes seismicity 

Tectonic Settings of Seismic Source Zones 11 
Figure 5. Preliminary map of seismicity of Saudi Arabia and adjacent regions. Computer-generated symbols are 

earthquakes obtained from the NElS worldwide seismicity catalog. Modified Mercalli intensities are from Poirier and Taher 

(1980) and are plotted as solid circles only for the western Arabian peninsula. Locations of intensity values do not necessarily 

represent epicenter locations; they represent locations of existing reports of shaking effects which may be distant from 

actual epicenters due to very low population density in western Saudi Arabia. Earthquake symbols represent the following 

values of magnitude (M): Triangles = 4sM<5; squares = 5sMs6; hexagons = 6sMs7; diamonds = 7sM<8; 

hourglass = M2:8. Stippled area highlights outcrop boundary of Precambrian crystalline rocks in western Saudi Arabia. 

associated with the highly faulted axial trough. This 

central-rift zone has been subdivided into northern and 

southern halves based on a significant decrease in 

observed seismicity north of lat 21 o N. Distinguishing 

these two segments preserves the influence of the seismically active southern axial trough in the seismic-hazard 

maps. In the northern half, although an axial trough is 

not well defined north of lat 24° N., the few epicenters 

that are present are contained by the 500-fathom 

contour. The broad zone that spans the width of the Red 

Sea is intended to separate primary, rift-related crust and 

structure of the main trough and shelves from 

predominantly Precambrian crust and structure of the 

Arabian and Nubian shield areas. Because of the 

previously discussed conflicting interpretations of 

offshore crustal structure, the eastern boundary of this 

zone can be interpreted in several different ways. Under 

hypotheses that advocate a primary plate separation 

mode of ocean-floor spreading, the zone boundary 

approximates the full extent of inferred Miocene age 

oceanic crust. Under the hypothesis of deep-to-middle 

crustal expansion through extensive diking, the zone 

boundary separates crust of mantle derivation to the west 

from continental crustal structure to the east. Under 

hypotheses of rifted continental crust extending beneath 

the shelf areas to the margins of the axial trough, the 

zone boundary separates the region of significant brittle 

crustal extension from less deformed regions on shore. 

12 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
Under this last type of hypothesis the definition of the 

zone boundary is a matter of degree of deformation, and 

the boundary becomes arbitrarily located. 

Fault-Source Model for Red Sea Rift 

The fault-source earthquake model for the Red 

Sea ri~t (fig. 4) is developed from fundamental plate 

tectomc concepts that serve to explain tectonic processes 

of active oceanic spreading ridges. The block diagrams of 

figure 6 schematically illustrate these concepts. First, 

although the locations of evolving spreading ridges and 

transforms may be predisposed by ancient structures of 

the Precambrian continental crust (Sykes, 1978), once 

ocean-floor spreading is established, structures of the 

continental and oceanic domains are geologically distinct. 

Second, once organized ocean-floor spreading is 

established, seismically active transform faults exist 

between offset spreading ridges and have earthquakes 

that exhibit strike-slip focal mechanisms. Wilson (1965) 

first introduced this concept. Recent microearthquake 

studies using ocean-bottom seismographs (OBS) have 

shown this distribution of seismicity to be generally 

characteristic of oceanic transforms (Francis and others, 

1978; Forsyth and Rowlett, 1979; Project Rose Scientists, 

1981; Rowlett, 1981). Other information gained from 

OBS monitoring indicates that seismicity is distributed 

across the interior plate corners of some ridge-transform 

intersections. At the regional scale of this investigation, 

however, this detail is not significant to the fault source 

model. A third point, not well illustrated by figure 6, is 

the characterization of ridge-crest earthquakes by 

normal focal mechanisms oriented parallel to the ridge 

strike (Sykes, 1967; Trehu and others, 1981). A 

generalization used in the fault-source model is that the 

larger spreading-ridge earthquakes, at any given 

spreading rate, occur off the spreading-ridge axis by 

perhaps 10 km or more (Solomon and Burr, 1979; Burr 

and Solomon, 1978). Although observed spreading-ridge 

earthquakes can neither confirm nor deny this 

generalization, Solomon and Burr (1979) suggested this 

to be a reasonable assumption based on considerations of 

observed high heat flow and very thin crust directly within 

axial troughs. 

These concepts and generalizations have been 

derived from studies of mature ocean-spreading systems 

worldwide, and it is not entirely clear that they can be 

applied in a strict sense to the incipient ocean-spreading 

system of the Red Sea. Conflicting interpretations of the 

composition and structure of the crust neighboring the 

axial trough of the southern Red Sea and the unresolved 

mode of plate separation in the northern Red Sea (brittle 

extension versus ocean-floor spreading) raise uncertainties in developing a fault-source earthquake model 

based on worldwide examples of mature ocean-spreading 

ridges. Because of the youth of the Red Sea rift, an axial 

rift valley is incompletely developed and offsets of the 

axial-trough centerline (assumed centerline of rift 

spreading) across inferred transforms are extremely 

small, being less than 15 km (Coleman, 1977) as 

compared to hundreds of kilometers for some mature 

spreading systems (Stewart, 1983). The specific geometry 

of the Red Sea ridge-transform fault system cannot be 

defined with confidence. 

Our approach, therefore, has been to style fault 

sources in accord with an ocean-spreading-ridge model 

but to relax some constraints of the rigid-plate transform 

theory illustrated in figure 6. 

Earthquakes are modeled as linear fault ruptures 

on parallel, spreading-ridge faults modeled 10 km either 

side of the axial-trough centerline as represented by 

Coleman (1977; fig. 4). Furthermore, in the southern 

Red Sea axial-trough zone, transform faults as inferred 

by Hall (1980) have been modeled perpendicular to the 

spreading-ridge axis; these faults include those identified 

with reasonable certainty as well as those that are 

speculative. These transforms, which have been modeled 

beyond the terminal ends of the offset axial-trough 

centerline, extend across the entire width of the axial 

trough as defined by the 500-fathom bathymetric 

contour. No transform faults were modeled in the 

northern axial-trough zone because the paucity of 

historically recorded seismicity in this region precludes 

the determination of seismic activity rates for such faults. 

A single exception is an inferred north-northeast-trending fault that follows a steep bathymetric break 

at the Gulf of Suez-Red Sea junction. An mb = 6 

earthquake occurred at this location on March 31, 1969 

(Fairhead and Girdler, 1970; Ben-Menahem and 

Aboodi, 1971), and the zone, which has remained 

intermittently active, produced four earthquakes of 

mb=4-4.5 through 1984. The zone's unique structural 

setting and continuing history of seismic activity 

distinguishes it from other inferred structures of the 

northern Red Sea. We have, therefore, represented this 

seismically active zone as a distinct seismic source. 

Gulf of Aden 

The Red Sea rift and the continental rifts of east 

Africa are joined to the worldwide rift system through the 

Sheba Ridge spreading center that extends the length of 

the Gulf of Aden (fig. 2). To the east, at about long 58° 

E., Sheba Ridge is terminated by right-lateral transform 

slip along the Owens Fracture Zone. To the west, Sheba 

Ridge shoals into the Afar region of east Africa where its 

structural expression as a distinct ocean-spreading ridge 

is lost in a broad region of distributed extension (fig. 2). 

Geomorphic expression of Sheba Ridge narrows from a 

Tectonic Settings of Seismic Source Zones 13 
i xxxx i 

I I 

B I I 

I I 

:...._A~+--8 --+l 1 I I 

D 

j-FT-1 XXXXI-FT-i 

I I I I 

I I I I 

I I I 

I 

I 

I 

I 

I 

I 

I I I 1+-A__...~~s~~ I I I 

Figure 6. Schematic block diagrams showing formation of oceanic crust through rigid-plate rifting. A, 

Magnetic anomaly time, T = 0. Precambrian crystalline crust with preexisting structural fabrics and faults. 8, 

Magnetic anomaly time, T = 1. Initial intrusion of oceanic crustal material along preexisting zones of weakness 

separates the Precambrian crust into blocks A and B. Dashed line indicates axial centerline of the new rift. 

Spreading is assumed to be symmetric to, and at a constant rate along, the rift axes. Creation of new crust 

across the right-hand offset of the spreading ridge is accommodated by left-lateral displacement along a 

transform fault. Zone of seismicity associated with transform movement extends between the spreading ridge 

axes and is shown by the line of ·x·s· raised out of the plane of the diagram. C, Magnetic anomaly time, T = 3. 

Continued symmetrical spreading along the rift axes, in time, results in accretion of young oceanic crust to 

blocks A and B and fossil transforms (FT) beyond the offset of the ridge axes. Note that the length of active 

transform faulting has not changed. Note also that fossil transforms have not formed through shear movement, 

but rather, through simple accretion juxtaposing crusts of different ages and composition. 0, Magnetic 

anomaly time, T = 7. Continued symmetrical rifting results in a broad area of oceanic crust. Note that the active 

zone of transform movement has not changed from diagram 8. Further note the growth of fossil transforms 

from diagram C. 

300-km-wide mountain range near the Owens Fracture 

Zone to a narrow valley occupying the Gulf of Tad jura, 

although expression of the axial valley does not greatly 

change (Laughton, 1966; Laughton and others, 1970). 

Numerous northeast -southwest -trending transform 

faults offset Sheba Ridge along its length (fig. 2); the 

most spectacular fault is the Alula-Fartak trench (Laughton, 1966) at long 52° E. where Sheba Ridge is offset a 

distance of 160 km. The transform is marked by a linear 

northeast-trending trench 240 km long, 16 km across, 

and 2,800 fathom deep (Laughton, 1966). Earthquakes 

associated with the trench show strike-slip mechanisms 

characteristic of oceanic transforms (Sykes and Landisman, 1964). Transforms and fracture zones of Sheba 

Ridge are typically characterized by linear northeastsouthwest-trending zones of mountainous topography as 

14 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
much as 16 km wide in the western gulf but considerably 

wider in the eastern gulf. Bouguer gravity anomalies, 

magnetic anomalies, and crustal velocities from seismicrefraction studies indicate Sheba Ridge is underlain by 

oceanic crust as much as 10 m.y. old (Girdler, 1970; 

Laughton and others, 1970; Cochran, 1981). Cochran 

(1982) suggested that there is a 80-km-wide zone of 

diffuse continental extension and mafic dike intrusion 

that extends to the Arabia coast beyond the 10-m.y. 

magnetic anomaly. Structure within this zone is 

suggested to have resulted from an early (1G--15 m.y. 

ago) crustal stretching and attenuation episode that 

preceded ocean-floor spreading (Cochran, 1982). 

Most of the seismicity of the Gulf of Aden shows a 

close spatial association with the axial valley of Sheba 

Ridge and associated transform faults (Fairhead and 

Girdler, 1970; McKenzie and others, 1970; Sykes, 1970; 

Sykes and Landisman, 1964; fig. 5). Some seismicity, 

however, occurs in the region of mountainous 

topography off the spreading axis and may be related to 

block adjustments in this region or a small amount of 

distributed ocean-floor spreading (Sykes and Landisman, 

1964; Sykes, 1967). The largest earthquakes to occur in 

the gulf are associated with the Alula-Fartak transform 

zone. Magnitudes (mb) for events occurring in 1959 and 

1967 were 6.7 and 5.8, respectively (Sykes and Landisman, 1964; Fairhead and Girdler, 1970). 

Two seismic source zones have been constructed in 

the Gulf of Aden (fig. 3). A central rift zone collects 

seismicity occurring in close proximity to the axial valley. 

Near long 46° E., a second, broader zone envelops the 

central rift zone and collects seismicity occurring within 

the broad area of mountainous terrain neighboring the 

central rift. The source zones terminate at the Owens 

Fracture Zone to the east. To the west, the central rift 

zone extends through the Gulf of Tadjurah into the Afar 

beyond Lake Asal to include the Ghoubbet-Asal rift and 

associated seismic activity (Abdallah and others, 1979). 

Westward, beyond Lake Asal, seismicity becomes diffuse 

and structural expression of a primary rift zone is 

obscured by significant distributed extension throughout 

the Afar. 

Fault-Source Model for Gulf of Aden 

As in the Red Sea fault-source model, spreadingridge earthquakes have been modeled as linear ruptures 

on faults placed parallel to, and 10 km either side of, the 

spreading axis of the Gulf of Aden (fig. 4). West of long 

48° E., transform offsets are numerous but very short, 

generally less than about 20 km (Cochran, 1981). 

Because such short rupture lengths would have no effect 

on ground-motion values onshore, they have not been 

modeled in this study. Between long 48° E. and 54° E. 

relatively long transform offsets of Sheba Ridge, 

including the Alula-Fartak trench, are present (Cochran, 

1981). These longer offsets have been included in the 

fault model as northeast-southwest-trending faults that 

have linear earthquake ruptures (fig. 4). 

Afar Depression 

The Afar depression (fig. 2) of northeastern Ethiopia is the subaerially exposed structural junction of the 

Red Sea, Gulf of Aden, and East Africa rift systems. 

Because this unique tectonic setting provides direct 

observation of tectonic processes of a spreading-ridge 

triple junction, many diverse geologic investigations have 

addressed the structural and tectonic evolution of this 

region (see for example, Pilger and Rosier, 1975). To 

review the many significant contributions to the tectonics 

and structure of this region is beyond the scope of this 

investigation. Because our area of interest is the Arabian 

Peninsula, we are concerned here with only far-field 

effects of earthquakes originating within the Afar, and 

much detail that could perhaps be modeled in seismic 

sources specific to the Afar region would have no effect 

on earthquake ground motions at distances with which 

we are concerned. The seismic source zones are, 

therefore, derived from only regional aspects of the 

tectonic setting. 

The central Afar is a highly extended region 

characterized by a thin crust of mafic composition, 

voluminous Quaternary tholeitic volcanism, and intense 

normal faulting along north-northwest and northwest 

trends (Barberi and Varet, 1977; Mohr, 1975; Morton 

and Black, 1975; fig. 2). The region is conterminous to 

the inland extension of Sheba Ridge to Lake Asal that 

has been proposed by Abdallah and others (1979) based 

on contemporary extensional tectonism observed in the 

Asai-Ghoubbet graben and an associated well-defined 

seismicity trend. Northwest of Lake Asal, in the central 

Afar, seismicity is diffuse and two focal mechanisms in 

the region indicate east-west-trending planes of strikeslip displacement (McKenzie and others, 1970). In 

microplate kinematic models proposed by Barberi and 

Varet (1977) and Le Pichon and Francheteau (1978), 

zones of northeast-trending shear accommodate 

transform motion between offset, northwest-trending 

zones of extension. 

The metamorphic continental crust of the Danakil 

block (fig. 2) marks the northeastern boundary of the 

Afar. In many tectonic models of this region, the Danakil 

block is inferred to be a splinter of continental crust 

separated from continental crust of the Nubian and 

Tectonic Settings of Seismic Source Zones 15 
Arabian plates by oceanic crust of the Afar depression to 

the southwest and that of the Red Sea to the northeast 

(Le Pichon and Francheteau, 1978; Barberi and Varet, 

1977; Hall, 1980). Offset of the primary locus of 

spreading from the southern Red Sea to the Afar is 

inferred to occur by means of a transform zone through 

the Dahlac Islands, north of the Danakil block, to the 

Gulf of Zula (fig. 2). In contrast to the aseismic 

appearance of the shelf areas of the southern Red Sea, an 

active seismic zone extends across the western shelf in the 

vicinity of the proposed transform (fig. 5). We have 

modeled this inferred transform zone as a fault -source 

for earthquakes jn this region. 

The western and southern boundaries of the Afar 

depression are marked by relatively narrow, bedrock 

escarpment terranes which are transitional zones 

between the structurally low, strongly faulted and 

extended central Afar region and the structurally high, 

slightly deformed regions of the Ethiopia and Somalian 

plateaus (fig. 2). Large-displacement normal faults and, 

in places, up-to-basin block rotations along (listric?) 

normal faults characterize the general structure of the 

escarpment terranes (Zanettin and Justin-Visentin, 

1975; Juch, 1975; Gouin and Mohr, 1964). The 

escarpments converge to the southwest and follow the 

margins of the Ethiopian rift into the East African rift 

system. 

A strong north-south seismicity trend has long 

been recognized along the length of the Ethiopian 

escarpment (Gouin and Mohr, 1964). Gouin (1970, 

1979) found the Ethiopian escarpment to be the most 

seismically active zone in Ethiopia. Only a few epicenters 

occur in close proximity to the southeast escarpment, 

although the Aysha Horst, which bounds the extreme 

southeast Afar, appears to be associated with significant 

activity (fig. 5). 

Our seismic source zones for the Afar region are 

quite general because we are only concerned with low 

levels of earthquake ground motions at large distances 

from earthquakes in this tectonically active region. The 

Ethiopia and southeast escarpments (including the 

Aysha Horst) comprise seismic source zones bounding 

the Afar region (fig. 3). The source zones are terminated 

at a rather arbitrary location to the southwest where they 

converge and funnel into the Ethiopian rift. At some 

point towards the southwest, presently undefined, it 

would appear that contemporary tectonism within the 

escarpments is no longer influenced by the distributed, 

extensional processes of the Afar but is more closely 

related to the tectonics of the Ethiopian rift. 

The central Afar has been separated from areas to 

the north and south based on fault trends, intensity of 

tectonism, and a dispersed pattern of seismicity. 

Dominant northeast and north-northeast fault 

trends of the southwestern Afar, which are associated 

with the intersection of the Ethiopian rift (fig. 2), have 

been excluded from the central Afar seismic source. No 

significant seismicity has been historically documented in 

this region. To the north, sparse seismicity of the Danakil 

block and Danakil depression have been excluded from 

the central Afar source zone. 

Western Arabian Peninsula 

Precambrian crystalline basement is widely 

exposed throughout western Saudi Arabia (fig. 2), 

although large areas are covered by voluminous Tertiary 

basalts. Three Precambrian tectonic provinces have been 

recognized in the structural framework of the shield 

(Greenwood and others, 1980). The Najd fault system is 

the most prominent structural feature of the Arabian 

shield and comprises the youngest of three Precambrian 

tectonic provinces (Greenwood and others, 1980). Shear 

zones of the Najd system separate the Hijaz-Asir 

province to the southwest from the Shammar province to 

the northeast (fig. 2). At least four major northwesttrending left -lateral wrench zones of the Najd cross the 

shield from the northeast coast of the Red Sea to the 

southeast shield area where the fault system is concealed 

beneath Mesozoic sedimentary rocks. Cumulative Precambrian left-slip on the Najd system is estimated to be 

about 240 km (Brown, 1972). Greenwood and others 

(1980) suggested that the Najd tectonic province is also 

structurally depressed because of the smaller area of 

exposed high-grade metamorphic rocks and larger area 

of unmetamorphosed rocks as compared to the 

neighboring Shammar and Hijaz-Asir provinces. 

The Hijaz-Asir Precambrian tectonic province lies 

between the Najd fault system and the coastal plain (fig. 

2). In the southeastern part of this province, braided 

north-trending fault zones predominate with complex 

patterns of left- and right-lateral, normal and reverse 

faulting (Greenwood and others, 1980). In the central 

part of the province, faults have a distinctive northeastsouthwest trend. The Ad Damm fault (fig. 2) is a 

prominent structure of this region and extends about 130 

km to the northeast from the coastal plain to beyond Taif. 

Pallister (1984) has inferred right-lateral Precambrian 

movement on this fault. Significantly, Merghelani and 

others (1981) have noted a high level of contemporary 

microseismicity in the vicinity of the fault. 

The Shammar tectonic province is exposed over a 

relatively small area of the extreme northeastern shield 

area and is characterized by north- and northeasttrending lineaments and granitic intrusions (Greenwood 

and others, 1980). 

16 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
Contemporary seismic activity in the shield area 

appears low as no significant earthquakes have been 

recorded in this area since the establishment of the 

WWSSN in the mid-1960's. However, historical accounts 

researched by Poirier and Taber (1980) indicate that 

strong earthquake shaking has been experienced at 

Khaybar, Tayma, and Yanbu in the year 1068 and 

Medina in 1122 and 1256 (figs. 2, 5). Taif experienced 

earthquake shaking in the year 1270 and Mecca in 1481. 

Detailed accounts for these events are not given by 

Poirier and Taher (1980); however, they did note that 

ground cracking and fissuring accompanied widespread 

destruction in Arabia in the 1068 event. They assigned a 

MMI of IX to these reports of shaking. The 1256 Medina 

event (MMI =IV) is likely related to volcanic activity. 

Smoke was seen from Medina, and a lava flow 

approached the city. At present, lacking any further 

historical information and any field geologic 

investigations relating to the reported ground fissuring, 

the locations of the earthquakes that caused these 

reported shaking phenomena cannot be determined 

confidently. It may be reasonable to assume, however, 

that if the locations of ground disruption for the 1068 

event (Khaybar and Tayma and presumably between 

these locations) approximate the epicentral region, this 

event was of tectonic origin as opposed to the apparent 

volcanic origin of the 1256 (MMI =IV) Medina event. 

In contrast to the paucity of information related to 

historical earthquakes in western Saudi Arabia, Ambraseys and Melville (1983) have outlined a generally 

continuous record of moderate-sized earthquakes that 

occurred in Yemen over the past 12 centuries. Most 

recently, the December 13, 1982 (Ms = 5.7) earthquake 

near Dhamar, North Yemen, caused considerable 

destruction and loss of life (Arya and others, 1985). The 

earthquake occurred in an area of active extensional 

tectonism characterized by horst-and-graben structure 

and Holocene basaltic volcanism (Geukens, 1966). 

The dearth of information concerning the historic 

and contemporary seismicity of the Arabian shield 

presently precludes any systematic seismotectonic 

analysis of the region. To construct seismic source zones 

for this region, therefore, we have imposed a hypothesis 

that relates any significant future earthquake activity in 

the shield to seismic reactivation of Precambrian faults. 

Thus, the source zones conform to Precambrian tectonic 

provinces and regional Precambrian structural trends. 

The approach is justified, in part, by studies of intraplate 

earthquakes in the United States (for example, Hamilton, 1981) and worldwide (Sykes, 1978) which concluded 

that there is fairly clear evidence for larger intraplate 

earthquakes to be caused by reactivation of ancient, 

Precambrian faults or fault systems. Some support for 

applying this approach to source zones on the Arabian 

Shield is lent by microseismicity studies of Merghelani 

(1979, 1981) and Merghelani and others (1981). These 

studies suggest that microseismic epicenters are 

concentrated near structural lineaments of probable Precambrian origin and near Precambrian faults such as the 

northeast-trending Ad Damm fault near Taif. 

Speculatively, there may be an association between 

shaking felt at Taif in 1270 and Mecca in 1481 (Poirier 

and Taber, 1980) and the Ad Damm fault that is in close 

proximity to both of these locations. Incidental support of 

the approach to source zones comes from recognition of 

the profound role played by Precambrian faults and fault 

systems in the development and disposition of the east 

Africa rift system (McConnell, 1967, 1972; Harpum, 

1970). However, the fact that the Red Sea rift cuts across 

major, northeast-trending Precambrian trends could 

perhaps be used as an argument against significant 

Tertiary structural control by Precambrian structure. At 

best, the Red Sea rift strikes parallel to a subordinate, 

relatively poorly developed north-northwest-trending 

system of short Precambrian faults and structural 

lineaments. 

The N ajd fault system comprises the northeasternmost zone of the study area (figs. 2, 3). To the northwest, 

the N ajd source zone is terminated by zones of the Dead 

Sea rift and Red Sea rift. To the southeast, the Najd 

source zone extends beneath Mesozoic sedimentary 

cover rock to include the terminous of the fault zone as 

depicted by Brown (1972). Included within the Najd zone 

are the historically reported shaking events near Medina, 

Kaybar, and Tayma (Poirier and Taher, 1980). 

Southwest of the Najd fault system, the northeasttrending faults of the central Hijaz-Asir Precambrian 

tectonic province have been separated from the braided 

north- and north-northeast-trending faults of this 

province. The Ad Damm fault is a prominant feature of 

this northeast-trending zone. 

In the southwestern portion of the Arabian 

Peninsula, the seismicity of the Yemen area is collected 

by a zone that is defined primarily by the regional 

exposure of the Yemen Trap Series volcanics. Although 

not explicitly keyed to structure, the zone as drawn is 

thought to include the primary region of significant 

contemporary extensional tectonism in the southwestern 

part of the Arabian Peninsula. The northern boundary is 

of utmost importance to the city of Jizan, Saudi Arabia; 

unfortunately, however, this boundary is poorly defined 

at present. Jizan has been included in the zone of 

relatively high seismic activity of North Yemen because 

of the January 11, 1941, earthquake (Ms =5.8; Ambraseys and Melville, 1983) that occurred about 75 km east 

of Jizan near Riyah. The earthquake caused widespread 

damage in North Yemen; the earthquake was apparently 

felt without causing damage in Jizan. 

Tectonic Settings of Seismic Source Zones 17 
Table 1. Estimates of b-values using two maximum-likelihood techniques 

Tectonic category Maximum-likelihood methods 

Fit to annual rates Weichert (1980) method 

Oceanic 

Transform .......................... . 

Nontransform ...................... . 

Continental ........................... . 

Combined tectonic categories ......... . 

EARTHQUAKE-RECURRENCE MODEL 

The worldwide earthquake catalog of the National 

Earthquake Information Service (NElS), U.S. 

Geological Survey, served as the primary source for 

earthquake information in the region with entries crosschecked and additions made from other published 

seismicity studies in the region (Poirier and Taber, 1980; 

Ambraseys and Melville, 1983; Fairhead and Girdler, 

1970; Sykes, 1970; Sykes and Landisman, 1964). 

Significantly more documentary work is required to 

ensure a complete historical catalog. 

Fairhead and Girdler (1970) concluded that 

earthquakes of mb =:: 5.0 (Ms;;::4.8) and mb =:: 4.8 (Ms~4.5 

were well detected in the Red Sea and Gulf of Aden, 

respectively, since the early to mid-1960's. Our analysis 

corraborates their work and indicates 20 yr of complete 

reporting for events mb;::: 4.5; earthquakes smaller than 

mb = 4.5 are not of interest in this study because they are 

not generally associated with damage to structures. Joint 

epicenter relocations by Fairhead and Girdler (1970) 

indicated that, for earthquakes occurring in the northern 

and central Red Sea between 1962 and 1968 and having 

magnitudes mb =:: 4.1, epicentral locations in the catalog 

were in error by no more than 0.20° in either latitude or 

longitude. In the southern Red Sea-northern Ethiopian 

region, one relocated epicenter shifted 0.6° in latitude. 

The remaining relocations, however, were less than 0.35°. 

In the eastern Afar-Gulf of Aden region, one relocated 

epicenter shifted 0.46° in longitude, but remaining 

relocations were less than 0.35°. Our conclusion is that 

epicenter locations as reported in NElS catalog 

(formerly the U.S. Coast and Geodetic Survey catalog) 

through 1984 are adequate for a regional investigation 

where seismic sources correspond to regional geologic 

provinces. Further investigations o~ indi~idual epice?te~s 

would be required for a study deahng With local, seismically active faults. 

-0.92 -0.89 

-1.01 -1.04 

-0.89 -1.02 

-1.00 -1.11 

Measures of earthquake size appear in the catalog 

as body-wave magnitude (mb), surface-wave magnitude 

(Ms ), local magnitude (ML ), and epicentral intensity (Io ). 

For statistical analysis of earthquake recurrence 

frequencies, the various measures of earthquake size 

were converted to Ms magnitudes. A least -squares 

regression fit to the catalog data gives the equation: 

Similar fits to magnitude-intensity data of the catalog 

yield: 

Ms = 0.6/o + 0.85 . 

Fits of ML on / 0 gave results identical to Ms on Io above, 

although the few earthquakes having ML measures are 

less than ML =5.5. Catalog ML magnitudes were 

assumed to be equivalent to Ms magnitudes. 

This approach assumes similar source pro~erti~s 

for all earthquakes, although such an assumption ts 

probably not strictly correct considering the different 

tectonic settings represented in the earthquake catalog. 

Ground-motion amplitudes and predominant periods of 

ground motion for various magnitude measures are likely 

to vary among the tectonic environments. However, 

significantly more seismological research throughout the 

region is required to substantiate these variations and to 

allow a proper treatment of magnitudes and attenuation 

throughout the region. 

Seismicity Parameters 

Determination of constants a and b in the Richter 

relation of occurrence frequencies, log N =a+ bM, 

18 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
Maximum 

like!" hood 

Regional 

seismic 

activity rate, 

Final 

Minimum Maximum seismic 

chi-squared likelihood rates 

I 

I 

I 

I 

I 

I 

I 

I 

I 

Oceanic: non-transform 

Gulf of Aden 

Figure 7. Allocation scheme of distributing regional seismic 

activity rate into constituent seismic source zones and faults. 

Steps labeled A through F correspond to descriptions of 

procedures in text. 

followed from a series of statistical tests of the 

earthquake data base. The seismic source zones were 

classified into oceanic rifting and continental divisions to 

investigate the possible existence of statistically 

significant differences in b-values for various tectonic 

settings represented in the earthquake catalog. Oceanicrifting zones were subdivided into areas of transform 

faults and areas not having known or inferred transforms. 

In the Red Sea and Gulf of Aden, earthquakes located 

generally within 30 km of a known or inferred transform 

fault or transform zone were assumed to be associated 

with transform faulting. Time of complete earthquake 

reporting was estimated for both oceanic and continental 

earthquake catalogs according to the methods described 

by Stepp (1973) and Weichert (1980). Estimates of 

b-values of maximum-likelihood fits to the entire catalog 

data base and to subcatalogs are summarized in table 1. 

Bender (1983) has published curves showing 50 

percent and 90 percent confidence regions forb-value fits 

to simulated seismicity data. As expected, when the 

numbers of earthquakes in a sample become smaller, the 

b-values have progressively larger confidence limits. For 

40 earthquakes, about the number used in each tectonic 

category above, the 75th percentile and 25th percentile 

b-value estimates lie at about 1.15 times band 0.87 times 

b, respectively (derived from fig. 5 of Bender, 1983). On 

this basis, we concluded that the data are insufficient to 

prove that the different tectonic categories have b-values 

different from that of the fit to the combined data set. 

Accordingly, we adopted one of the two maximumlikelihood fits to the combined data. We chose -1.00 

from the maximum-likelihood fit to annual rates instead 

of the Weichert (1980) method because, although both 

techniques gave comparable values for predicted rates 

for modest-size earthquakes, the fit to annual rates 

results in rates about 50 percent higher for earthquakes 

in the magnitude 6.55-7.15 category. We decided to 

accept the more conservative value. However, because 

the hazard estimates are not critically sensitive to 

earthquake occurrence rates at high magnitudes, the 

choice of one over the other would not result in a 

significant difference in ground-motion values. 

There are too few earthquakes to fit a-values for 

individual source zones or source faults; hence, it is 

necessary to allocate the seismicity from a regional 

seismic activity rate into the constituent source zones and 

faults. Two optimal ways for statistically performing this 

allocation exist: maximum-likelihood and minimum-chisquared procedures. For the maximum-likelihood 

technique of allocation, the rates are allocated according 

to the total number of earthquakes observed in a zone. 

This count is taken for earthquakes above some 

magnitude for which the seismicity of the regional source 

zones are determined to be complete. 

Whereas the maximum-likelihood technique is 

insensitive to the magnitudes of the earthquakes 

observed in a zone, the minimum-chi-squared technique 

is not and gives somewhat higher allocations to zones in 

which higher magnitude events have occurred. Extensive 

testing with simulated data suggests that the minimumchi-squared technique may be superior to the maximumlikelihood technique when independence of earthquake 

occurrences within a zone is not assured and the 

occurrence of a high-magnitude event appears to inhibit 

the occurrence of subsequent lower magnitude events for 

the duration of the catalog. Furthermore, because of 

lower sensitivity to the occurrence of lower magnitude 

events, the minimum-chi-squared technique may be the 

preferred technique when there is a considerable 

difference in the completeness times of the lower 

magnitude categories. The maximum-likelihood technique, because it only depends on the number of 

earthquakes observed and not on their magnitude, overweights the contribution of small events. 

For oceanic transform faults (sections D and E 

below) we assumed a priori that the rate per unit length 

among all the transform faults should be equal and chose 

the allocation technique that gave a reasonable approximation to th~t assumption. The calculated seismic rates 

for all sources are tabulated in the Appendix. 

Earthquake-Recurrence Model 19 
Seismic-Rate Allocation 

Sequence and Methods 

Figure 7 schematically illustrates the various steps 

of the seismic rate allocation procedure. These steps, A 

through F, are outlined below. 

Allocation Method 

A Allocation of regional seismic Maximum likelihood. 

activity rate into three 

tectonic categories of zones: 

oceanic nontransform zones, 

oceanic transform zones, and 

continental zones. 

Comment: Seismicity rates resulting from this allocation for 

magnitude-6.5 earthquakes are about 50 percent 

higher than rates from direct fits to the seismicity of 

the individual tectonic categories. 

B. Seismicity rate of oceanic, Maximum likelihood. 

nontransform zones into 

northern and southern Red 

Sea zones and zones around 

Sheba Ridge in the Gulf of 

Aden. 

C Seismicity rate of oceanic, 

transform zones into overall 

rates for both the southern 

Red Sea and Gulf of Aden 

transforms. 

Minimum chi-squared 

(unequal completeness for 

magnitudes below 5.5). 

Comment: Direct allocation into individual transform fault 

zones at this stage would have reduced the Red Sea 

transform rates by a factor of 2 overall and would 

have produced rates inconsistent with our 

assumption of equal-rate-per-unit-fault-length. 

D. Overall seismicity rate for 

Gulf of Aden transforms into 

individual transform faults. 

E. Overall seismicity rate for 

southern Red Sea transforms 

into individual transform 

faults. 

F. Overall seismicity rate for the 

continental tectonic category 

into individual source zones. 

Maximum likelihood 

(best conformance equalrate-per-unit-fault-length 

assumption). 

Maximum likelihood (best 

conformance with our equalrate-per-unit-fault-length 

assumption). 

Minimum chi-square 

(unequal completeness) 

Comment: Eight zones are assigned approximately the same 

rates under either maximum-likelihood or 

minimum-chi-squared allocation; four zones had 

allocated rates varying between 0.5 and 3 times the 

rate allocation from the maximum-likelihood 

method. 

Maximum Magnitudes 

For each seismic source zone, an estimate of the 

maximum possible size of potential earthquakes is 

required to truncate the exponential frequency 

distribution. Due primarily to the lack of insight into 

distinctions in potential earthquake-source dimensions 

among many of the seismic source zones, maximum 

magnitudes were assigned to broad geologic categories of 

source zones. The highest maximum magnitude 

(Ms = 8.0) was assigned to the three segments of the 

Dead Sea transform on the basis of the historical 

occurrence of an estimated Ms = 8.0 on the northern 

segment (Ben-Menahem, 1981). Long fault lengths and 

historical occurrences of earthquakes having estimated 

magnitudes ;;::: 7.0 in the central and southern segments 

(Ben-Menahem, 1981) were the bases of assigning a 

maximum magnitude Ms = 8.0 to these segments. 

The maximum magnitude assigned to seismic 

source zones of the Red Sea and Gulf of Aden is 

Ms = 6.5. Spreading ridge faults (located 10 km off the 

ridge axes) similarly are assigned Ms = 6.5. This estimate 

is about one magnitude unit higher than the highest 

instrumentally determined magnitudes for earthquakes 

that could not be confidently associated with known or 

inferred transform faults. The corresponding mb 

magnitude is about 6.25 which fits weii with Solomon and 

Burr's (1979, fig. 1) nominal upper bound for spreadingridge-axis earthquakes on ridges with half-spreading 

rates of 0.6 cm/yr (the estimated half-rate of the Red Sea 

spreading ridge). 

Oceanic transforms, on the other hand, were 

assigned a maximum magnitude of Ms = 7.2. This 

estimate is based on the 1959 occurrence of an mb = 6.7 

(Ms =7.1) earthquake on the Allula-Fartak transform in 

the Gulf of Aden (Sykes and Landisman, 1964). Because 

the remaining oceanic transforms in the Red Sea and 

Gulf of Aden are considerably shorter than the AllulaFartak zone, this size event might be considered an 

overestimate of the maximum potential earthquake for 

these transforms. However, complicating any simple 

relation between transform length and earthquake size 

are observations by Burr and Solomon (1978) and 

Solomon and Burr (1979) that maximum moments, 

average fault width, and maximum moment sum increase 

with decreasing spreading velocities. From the empirical 

relations of Solomon and Burr between full spreading 

rate and moment for transform fault earthquakes, 

Ms = 7.2 may underestimate the maximum-magnitude 

earthquake by about 0.25 magnitude unit. The spreading 

rate of the Red Sea rift would plot as a low extreme to 

their data base derived from worldwide examples of 

mature spreading ridges. It is not clear whether their 

empirical relations are valid for the poorly developed 

transform zones of the Red Sea spreading ridge. 

The remaining zones of western Saudi Arabia, the 

Afar region of Ethiopia, and northeastern Egypt were 

also assigned maximum magnitudes of Ms = 7.2. This 

value is about one magnitude unit higher than historically 

reported intensities (converted to Ms) or instrumentally 

recorded events in any of these regions. 

20 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
The ground-motion estimates of this study are not 

critically sensitive to the choice of maximum magnitudes. 

Because of the low rate of seismic activity in the region, 

the mapped ground-motion values are well below the 

near-field ground motions produced by the chosen 

maximum-magnitude earthquakes. Hence, the mapped 

ground motions are dominated by the more frequent 

occurrence of low- to moderate-magnitude events. At 

longer return periods than addressed here, where ground 

motions approach those produced by the maximummagnitude earthquakes, choice of upper-bound 

earthquakes becomes a critical factor in determining 

ground-motion levels (Bender, 1984b). 

MODELING EARTHQUAKE EVENTS 

Earthquakes are assumed to be equally likely 

throughout a zone and are modeled as point sources or 

as line sources of a length appropriate to 

magnitude/fault-rupture-length relations (Mark and 

Bonilla, 1977; Mark, 1977). Area seismic sources in this 

study are characterized by a low rate of earthquake 

activity; hence, all earthquakes in these sources are 

modeled as point sources. Acceleration values in zones of 

low seismicity are correspondingly low at the return 

period addressed here, even when maximum magnitudes 

are high. Because these low accelerations affect large 

areas, the ratio of the isoseismal area to the size of the 

linear earthquake source is large, and a point -source 

approximation can be justified (Bender, 1982, 1984b ). 

However, in seismic source zones having relatively 

moderate to high rates of earthquake activity, modeling 

the larger earthquakes as linear ruptures is warranted 

and results in near-source ground-motion levels 

substantially higher than for point-source models 

(Bender, 1982, 1984b ). This is the case for the rift axes 

and transform faults of the study. 

A distinction was maintained in modeling 

earthquakes on the continental transform faults (Dead 

Sea Rift system) and the oceanic transforms (southern 

Red Sea and Gulf of Aden). Whereas the seismicity on 

continental transforms was modeled as a mixed-source 

type, that is, moderate to large earthquakes were 

modeled on faults and small earthquakes were modeled 

as point sources in an area around the faults, the oceanic 

transforms were modeled exclusively as fault sources. 

This modeling distinction was maintained, in part, 

because of tectonic dissimilarities between the two types 

of transforms, but also because of extreme differences in 

size of the transform zones relative to the map scale 

being used here. Active fault deformation associated with 

large transform plate boundaries through heterogeneous 

continental crust can seldom be confined to a single, 

narrowly confined fault strand. The preceding 

discussions of the Dead Sea transform illustrate this 

point for that region. On the other hand, small offsets of 

young oceanic spreading ridges define only a very 

restricted area where earthquakes related to rigid-plate 

transform faulting would be expected. On the regional 

scale of mapping being performed here, that area 

approaches a short linear source. Hence, representation 

of any area source becomes unimportant to the groundmotion hazard estimate. 

GROUND-MOTION ATTENUATION 

A number of aspects of the gross crustal structure 

in western Saudi Arabia and the eastern Red Sea lead us 

to believe that attenuation of ground motions from 

earthquakes in this region may present a difficult 

problem. As there are no seismograph or strong-motion 

records from moderate-sized events, there is no direct 

data on strong-ground-motion attenuation characteristics in western Saudi Arabia. Hence, an attenuation 

model developed outside of the region must be applied, 

but this can only be justified if it can be demonstrated 

that the tectonic settings are generally similar. This 

becomes a difficult proposition in view of the uncertainty 

concerning the tectonic history, crustal composition, and 

structure of the Red Sea rift. 

Studies of attenuation of shear waves in the oceanic 

crust from a small number of earthquakes indicate very 

poor propagation (very low 0) for source-to-site paths 

crossing, or originating near, oceanic spreading ridges, 

due to the presence of high heat flow and partial melt 

immediately beneath the spreading ridge (Molnar and 

Oliver, 1969; Solomon, 1973). However, for some 

earthquakes on transform faults, source-to-site paths 

that do not cross spreading-ridge axes are characterized 

by efficient transmission of shear waves (generally high 

0) (Molnar and Oliver, 1969; Solomon, 1973). 

Observations from different transform faults may not be 

directly correlative, however, because of the diversity of 

crustal structural and magmatic activity among oceanic 

transforms (Stewart, 1983). Efficient transmission of the 

Sn (short-period shear waves) phase has been recognized 

across passive continental margins despite considerable 

lateral variations in crustal structure, composition, and 

thickness (Molnar and Oliver, 1969). 

The coastal region of Saudi Arabia, where the 

largest population centers are located, is the most 

important region in this study. At question are the 

attenuation characteristics along source-to-site paths 

from the eastern Red Sea to the coastal area where the 

propagation path does not cross the spreading-ridge axis 

and from the Arabian shield area westward where the 

propagation path crosses abrupt lateral changes in 

crustal thickness and structure. 

Ground-Motion Attenuation 21 
SW ~ED SEA SHELF &=--_.,t4

._ ___ _ 

COASTAL PLAIN ARABIAN SHIELD 

ARABIAN ------------~.__PLATFORM --- NE 

--HIJAZ-ASIR TECTONIC PROVINCE ---..o+- NAJD TECT. _L SHAMMAR TECT._.. PROVINCE --:r- PROVINCE 

-l 

0 

10 

E 6.5 20 ~ 6.7 

30 

J: 40 

..... 

Q,. 50 

w 8.0 8.0 

60 Q 8.3 8.3 

70 8.3 

8.5 8.5 

BOO ~---~----~----~--~~--~----L----~L------L----~-----L----~-----~~ zoo 400 600 BOO 1000 llOO 

DISTANCE, km 

INDEX MAP 

Figure 8. Interpreted velocity structure along the southwestern Saudi Arabian geophysical transect and seismic deeprefraction line (from Gettings and others, 1983). 

The Red Sea is generally characterized by high heat 

flow. Measured heat flow values in the axial trough of the 

southern Red Sea range as high as 5 HFU (1 

HFU = 1 x 10-6 cal/cm2 sec), which is somewhat more 

than three times the worldwide mean, and apparently 

decrease rapidly beneath the shelf to about 2 HFU at the 

coast (Gettings and others, 1983). High heat flow typifies 

regions of the world that are known to have high 

attenuation (low Q). Lithospheric strength, however, has 

been suggested as correlating with low attenuation (high 

Q) (Oliver and Isacks, 1967; Molnar and Oliver, 1969). 

Interpretations of a seismic deep-refraction line across 

the southern Arabia shield, coastal plain, and shelf 

indicate a shallow mantle and a high-velocity crustal 

wedge beneath the shelf that thickens rapidly to 20 km 

beneath the coastal plain where the wedge is in abrupt 

vertical contact with nearly normal shield-type crustal 

structure (Mooney and others, 1985; Gettings and 

others, 1983) (fig. 8). The high-velocity wedge is inferred 

to be mafic crust emplaced through either ocean-floor 

spreading or extensive diking in the early history of Red 

Sea rifting 20--25 m.y. ago. It is not clear at present what 

effect this high-velocity, presumably high strength, 

oceanic crustal wedge would have on propagation paths 

22 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
of earthquake waves originating in the eastern Red Sea 

or along transform faults within this region of moderate 

to high heat flow. 

Propagation paths that trend east to west, from the 

shield area to the coastal plain, would seemingly best be 

described by an attenuation function incorporating low 

earthquake-wave attenuation (high 0), which is typical of 

shield areas worldwide. However, some areas of the 

shield, north of the refraction line traverse, are 

characterized by generally north-south trending zones of 

extensive Tertiary and Holocene basaltic volcanism. High 

heat flow and partial melts at all crustal levels in these 

volcanic zones could substantially alter attenuation 

characteristics for propagation paths that cross them. 

The abrupt lateral and vertical change in crustal 

structure beneath thick unconsolidated sediments of the 

coastal plain raises further questions of possible focusing 

effects and (or) amplification of earthquake waves in the 

coastal plain region. 

In light of the uncertainties outlined above, we 

have investigated the consequences of modeling both 

low-0 and high-0 attenuation functions for all the 

seismic source zones. We consider the ground motions 

derived from the high-0 attenuation function as 

generally upper bound values for the seismic source 

zones and seismic activity parameters of this study. There 

appears to be a good possibility that future research 

could justify the use of at least two attenuation functions: 

possibly a low-0 attenuation for zones of the Red Sea rift 

and a high-0 attenuation for the zones of the shield area. 

At present, however, pertinent data are too sparse to 

justify adding this complexity to the model. 

Figures 10 and 11 are profiles of ground-motion 

values calculated along lines extending from the Red Sea 

axial trough into the western Arabia shield (fig. 9). The 

figures summarize the range of ground motions 

calculated at Y anbu, Jiddah, and Jizan and apply various 

assumptions in two attenuation models that pair peak 

horizontal ground acceleration and peak horizontal 

ground velocity. Whereas peak acceleration values 

usually occur at ground-motion frequencies of about 5 

Hz, peak velocity values generally occur at frequencies of 

about 1-2 Hz. Therefore, velocity is generally a better 

measure of ground motion for applications concerned 

with long-period ground motions, such as c:~arthquake 

effects on tall structures or high-rise constructiion located 

some distance from an earthquake source (see Steinbrugge, 1984, for discussion of some examples). 

Acceleration, on the other hand, provides a better 

measure of ground motion at the higher frequencies 

experienced closer to the earthquake source and is 

generally better suited to considerations involved in 

low-rise construction. 

The attenuation models used in this investigation 

are those of Nuttli and Herrmann (1984) and K W. 

Campbell (written commun., 1984; 1987) that are 

derived using arithmetic mean peak values for the two 

horizontal components of strong ground motion. Nuttli 

and Herrmann's (1984) attenuation model is based on a 

semitheoretical approach that incorporates a small 

amount of strong-motion data from small earthquakes in 

the upper Mississippi embayment and Atlantic coastal 

plain of the Eastern United States and theoretical 

relations between mb and far-field accelerations and 

velocities in a high-0 region. The relations are valid for 

4.5 $.mb $.7.4. Because the NElS catalog data was 

previously converted to Ms magnitudes for seismic rate 

determinations, a table of acceleration values as a 

function of mb and epicentral distance was constructed 

using Nuttli and Herrmann's attenuation model. Ms 

magnitudes corresponding to Nuttli and Herrmann's mb 

magnitudes were then determined using Ms = 2.0 mb-5.6 

(Nuttli, 1983), which holds for 4.5 $.mb $. 7.0 (the range 

of magnitudes of significance in this study). Groundmotion values associated with catalog Ms values at 

various distances were then determined by interpolation 

in the table. 

Nuttli and Herrmann's (1984) attenuation 

relations are developed for thick unconsolidated or stiff 

soil sites typical of the Mississippi embayment and 

coastal plain regions of the Eastern United States; a 

surficial geologic setting grossly similar to the coastal 

region of western Saudi Arabia. 0 at 1 Hz is taken as 

1,500 in their relations. For mb ~ 4.5, their relations for 

average mean peak horizontal acceleration (Ah, in 

cm/sec2

) and velocity (Vh, in em/sec) are of the form: 

where A and C are constants setting the level of the 

ground-motion curves for median values determined 

from available data for the Mississippi Valley, R is 

epicentral distance, h is minimum focal depth dependent 

on magnitude and given by logh(minJ =-1.730+ 0.456mb 

for mb ~ 4.5 (Nuttli, 1983), and B and Dare equivalent to 

coefficients of anelastic attenuation in the Mississippi 

Valley region. The specific relations, given by Nuttli and 

Herrmann (1984) are: 

logAh =0.57 +0.50mb-0.83 log (R2 +h2)¥l-0.00069R, 

and 

log Vh = -3.60 + 1.00ffib-0.83 log (R2 + h2)¥l -0.00033R. 

Ground-Motion Attenuation 23 
200 400 

KILOMETERS 

200 

MILES 

Figure 9. Index map showing the location of three ground-motion profiles depicted in figures 10 and 11. 

In our application of these attenuation relations, 

focal depths (h) for earthquakes of all magnitudes have 

been held constant at 5 km. We did this for two reasons. 

First, Nuttli and Herrmann's (1984) attenuation 

relations are developed for an intraplate seismic zone 

that might have nucleation depths for large earthquakes 

down to a 40-km depth, which is the approximate base of 

the continental crust in the Central United States. Such 

depths for earthquake nucleation in the Red Sea are 

unreasonable. Second, because of the lack of strongmotion data and large uncertainty in characterizing the 

nature of ground-motion attenuation along the Red Sea 

coast, we wanted an upper-bound estimate of ground 

motion, given the seismic source zones and seismic 

activity rates of this study. The above treatment of the 

Nuttli and Herrmann (1984) attenuation relationships 

provides such an estimate. 

The second set of attenuation models for mean 

peak horizontal acceleration and velocity were obtained 

from K.W. Campbell (written commun., 1984; 1987) and 

follow the form 

In Y=a+bM-d·In[R +c1exp(c~] +Ie;Ki + € , 

where Y is the peak ground-motion parameter, M is the 

magnitude (ML for M < 6.0 and determined by 

24 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
100 Acceleration profile of Yanbu 

g) 

z 

0 

i= 

c( 

a: 10-1 w 

_, 

w 

0 

0 

c( 

0 

0 

_, 10-2 .J 

0 200 300 400 

A DISTANCE (km) A' 

100 Acceleration profile of Jiddah 

g) 

z 

0 

j: 

c( 

a: 10-1 w 

_, 

w 

0 

0 

c( 

(!' 

0 

_, 

10-2 _j 

0 100 200 300 400 

8 8' 

100 Acceleration profile of Jizan 

g) 

-z 

0 

j: 

c( 

a: 

w 

_, 10-1 

w 

0 

0 

c( 

0 

0 

_, 

10-2 

0 100 200 300 400 

c DISTANCE (km) C' 

Figure 10. Profiles of peak horizontal acceleration along 

transects shown in figure 9 applying various assumptions to 

two attenuation models. Vertical arrows indicate the locations 

of the cities of Yanbu, Jiddah, and Jizan along their 

respective profiles. MNH = modified Nuttli and Herrmann 

(1984) attenuation relation (focal depths have b1een held 

constant at 5 km). HQ2 and LQ2 = K.W. Campbell's (written 

commun., 1984) attenuation relation incorporatin!~ high-Q 

and low-Q values, respectively, assuming a 2-km depth to 

the top of the seismogenic zone (assumed top of crystalline 

basement). HQ10 and LQ10 = K.W. Campbell's (written 

commun., 1984) attenuation relation incorporatin!~ high-Q 

and low-Q values, respectively, assuming a 1 0-km depth to 

the top of the seismogenic zone. 

Velocity profile of Yanbu 

102 

u 

G) 

In 

' E 

2 

> 

1- 101 

0 

0 

_, 

w 

> 

(!' 

0 

_, 

100 

0 100 200 300 400 

A DISTANCE (km) A' 

Velocity profile of Jiddah 

102 

u 

G) 

In 

' E 

2 

> 

1- 101 

0 

0 

_, 

w HQ2 > 

(!' 

0 

_, LQ2 

100 

0 100 200 300 400 

8 DISTANCE (km) 8' 

Velocity profile of Jizan 

102 

u 

G) 

In 

' E 

2 

> 

1-

101 0 

0 

_, 

w 

> 

(!' 

0 

_, 

100 

0 100 200 300 400 

c DISTANCE (km) C' 

Figure 11. Profiles of peak horizontal velocity along 

transects shown in figure 9 applying various 

assumptions to two attenuation models. Labels are as 

in figure 10. 

ML = 2.241 + 0.621Ms from Western United States data, 

K.W. Campbell, written commun., 1984; Ms for M"2 6.0), 

R is the shortest distance between the site and the 

seismogenic zone of rupture, K; is a set of source and site 

variables, and € is a random error term (Campbell, 1981, 

1982, 1983, 1984). The coefficents a through e are 

parameters determined from the strong-motion data 

Ground-Motion Attenuation 25 
base. .K; variables are site parameters, H, which is 

thickness of surficial sediments, and F, which is fault type. 

For this study, depth of surficial sediments was taken to 

be 2 km along the coastal plain and all fault parameters 

are taken as normal faulting. The specific relations used 

in this study for peak horizontal acceleration (PHA, in 

cm/sec2

) and velocity (PHV, in em/sec) were (K.W. 

Campbell, written commun., 1984; 1987): 

In PHA = -3.303 + 0.85M-1.25In[R + 0.0872e 0·

878 M] + 

0.34F-0.0059R, and 

In PHV = -1.584 + 1.18M-1.24ln[R + 0.00907e0·

951M] + 

0.49F + 0.534tanh(0.415H)-0.0059R. 

The relations are derived from worldwide strongmotion data and incorporate California-type anelastic 

attenuation in which 0 corresponds to 180 at 1 Hz 

(Joyner and Boore, 1981). Four profiles of ground 

motions (figs. 10, 11) were generated using Campbell's 

relations incorporating high- and low-0 values and 2-

and 10-km depths to top of crystalline basement. Top of 

crystalline basement is taken to be the top of the seismogenic zone of rupture from which source-to-site 

distances are measured. This measure of distance is 

appropriate for moderate to large earthquakes where 

fault rupture may extend to the surface, because it is 

unlikely that the small stresses associated with rupture 

within the sedimentary section would contribute 

significant energy to surface ground motions (Campbell, 

1984). 

Figures 12 and 13 illustrate the low and high 

extremes in the attenuation relations used in this study. 

With focal depth constant at 5 km and high 0, Nuttli and 

Herrmann (1984) relations result in extremely high 

ground motions for moderate to large earthquakes. We 

feel that ground-motion values derived from these curves 

are upper-bound ground-motion estimates given the 

consituent source zone and fault models of this study. 

The lower curves at any given magnitude (figs. 12, 

13) were estimated from K.W. Campbell's (written commun., 1984; 1987) attenuation relations incorporating 

low 0 (high attenuation) and a 10-km depth to the top of 

the seismogenic zone (in this application, the top of 

crystalline basement). Curves for Campbell's attenuation 

relations assuming high 0 and a 10-km depth, and both 

high and low 0 at a 2-km depth, fall between these two 

sets of curves in figures 12 and 13. 

Preferred Attenuation Model 

Our preferred interpretation of ground-motion 

attenuation in coastal Saudi Arabia, based on presently 

available data, is given by Campbell's low-0, 2-km-depth 

attenuation models for acceleration and velocity. 

Attenuation curves for these models are shown in figures 

14 and 15. A low-0 model is preferred because of high 

shallow-crustal temperatures in and along the Red Sea 

rift. As previously mentioned, heat flow measurements 

across the southern Red Sea coast range from as high as 

5 HFU in the axial trough to 2 HFU at the coast 

(Gettings and others, 1983). In comparison, heat flow 

values for the Western United States, a region known to 

have high attenuation (low 0 ), is generally characterized 

by values ~ 1.5 HFU (Diment and others, 1975). In 

contrast, the Central and Eastern United States, known 

to be regions of low attenuation (high 0 ), are 

characterized regionally by values s 1.5 HFU (Diment 

and others, 1975). 

GROUND-MOTION HAZARD MAPS 

Figures 16 and 17 show the regional distribution of 

peak acceleration and velocity having a 0.90 (90 percent) 

probability of not being exceeded in 100 yr for the 

preferred attenuation model. The highest predicted 

ground-motion values occur in extreme northwestern 

Saudi Arabia neighboring the Gulf of Aqaba. 

Accelerations as high as 0.30 g and velocities as high as 

21 em/sec are predicted and are governed by the faultsource model, the moderate rate of earthquake activity, 

and the potential for large earthquakes in the Gulf of 

Aqaba The town of Haql, located on the northeastern 

shore of the gulf is included in this region of 

comparatively high ground motion. Ground-motion 

values in coastal regions neighboring the Red Sea range 

from 0.05 g and 2 em/sec at Yanbu, and 0.07 g and 4 

em/sec at Jiddah, to 0.13 g and 7 em/sec at Jizan. The 

ground-motion values at Yanbu and Jiddah are governed 

by the seismic sources of the central Red Sea. The 

seismic source zones corresponding to the Precambrian 

shield tectonic provinces (figs. 2, 3) have a low rate of 

earthquake activity and result in relatively low ground 

motions of around 0.04 g and 2 em/sec over broad inland 

areas. The higher ground motions toward the coast at 

Yanbu and Jiddah are a result of distant ground motions 

from more frequent, larger earthquakes in the central 

Red Sea fault zones. The ground motions predicted at 

26 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
---- NUTTLI AND HERI~MANN (1984): CONSTANT 5-km DEPTH FOR ALL MAGNITUDES 

------CAMPBELL (WRITTEN COMMUN.,1984): LOW Q, 10•km DEPTH TO TOP 

OF SEISMOGENIC ZONE 

z 0 

010 

1-

<C 

a: 

w 

-I 

w 

0 

0 -1 

<C 10 

" 0 

-I 

-2 

10 

0 1 2 3 4 

10 10 10 10 10 

LOG DISTANCE (km) 

Figure 12. Acceleration attenuation curves illustrating the high and low extremes of attenuation relations investigated in the 

study. Magnitudes are M. for Nuttli and Herrmann (1984) curves and M. for M'2:.6, ML forM< 6 for Campbell curves (written 

commun., 1984). See text for treatments of magnitude measures in the attenuation functions. Distance is epicentral distance 

for Nuttli and Herrmann curves and closest distance' to the seismogenic zone of rupture for Campbell curves. 

Jizan, however, are not a result of seismic activity 

modeled in the central Red Sea but rather result from 

including Jizan in the zone of generally high seismicity in 

North Yemen. Whether it is appropriate to include Jizan 

in this seismic source zone cannot be confidently assessed 

until the seismotectonics of the southwest Arabian 

Peninsula are better understood. 

Figures 18 and 19 show probabilistic ground 

motions using the high-Q Nuttli and Herrmann (1984) 

relations. Regardless of what the actual ground- motion 

attenuation in western Saudi Arabia is (which could only 

be resolved through additional work beyond the S<:ope of 

this report), our extreme treatment of the NuUli and 

Herrmann (1984) ground-motion relations ensures 

conservative regional ground-motion estimates for the 

seismic source zones and the earthquake recurrence 

model. Table 2 compares values at Haql, Y anbu, Jiddah, 

and Jizan for the preferred and upper-bound models. 

The differences between the preferred and upper-bound 

values of ground motion at any given location are largethe upper-bound estimates being from 40-50 percent 

larger at Haql to 150 percent larger at Yanbu. The larger 

differences at lower ground motions are attributable to 

the slower decrease of ground motion with distance in 

the Nuttli and Herrmann (1984) attenuation relations as 

compared to K.W. Campbell's (written commun., 1984) 

relations. Although the differences in ground motion 

between the preferred and upper-bound models are 

large, the regional ground-motion values themselves are, 

in general, moderate to low as a result of the low rates of 

seismic activity in the Arabia shield area or, as along the 

coast, large distances from the modeled fault sources in 

the Red Sea rift. 

Research has shown that statistical variability in 

ground-motion attenuation relations is a dominant 

source of uncertainty in estimated ground-motion values 

(McGuire and Shedlock, 1981; Bender, 1984a). The 

standard deviation ( o) in loge acceleration appears to lie 

within a range of 0.35 s us 0.65 and is generally assumed 

to hold for all magnitudes and distances of the 

Ground-Motion Hazard Maps 27 
,...._, 

CJ 

Q) 

tn 

' E 

CJ 

......, 

> 

1-

--0 

0 

...1 

w 

> 

(!) 

0 

...1 

103 

2 

10 

1 

10 

------ NUTTLI AND HERRMANN (1984): CONSTANT 5-km DEPTH FOR ALL MAGNITUDES 

CAMPBELL (WRITTEN COMMUN.,1984): LOW Q, 10•km DEPTH TO TOP 

OF SEISMOGENIC ZONE 

10°~----------~--------~_.~~----~--~----------~ 0 1 2 3 4 

10 10 10 10 10 

LOG DISTANCE (km) 

Figure 13. Velocity attenuation curves illustrating the high and low extremes of attenuation relations investigated in this 

study. Magnitudes and distance measures are as in figure 12. 

Table 2. Comparison of peak ground-motion values at Haql, Yanbu, Jiddah, and Jizan obtained from the preferred and 

upper-bound regional hazard models 

[Values in parentheses result from assuming a standard deviation of 0.5 in the attenuation relations) 

Hazard model 

Location Preferred Upper-bound Percent increase of upper-bound model 

Acceleration Velocity Acceleration Velocity Acceleration Velocity 

(percent g) (em/sec) (percent g) (em/sec) 

Haql .......................... 30 (39) 21 (26) 43 (60) 31 (39) 43 (54) 48 (50) 

Yanbu ........................ 4 (5) 2 (2) 10 (13) 5 (6) 150 (160) 150 (200) 

Jiddah ........................ 6 (7) 3 (4) 14 (19) 7 (8) 133 (171) 133 (100) 

Jizan ......................... 12 (15) 5 (6) 20 (26) 8 (10) 67 (73) 60 (67) 

28 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
z 

0 

-

1-

<C 

a: 

w -1 

..J 10 

w 

0 

0 

<C 

CJ 

0 

..J -2 

10 

0 1 2 3 

10 10 10 10 

LOG DISTANCE (km) 

Figure 14. Acceleration attenuation curves for the preferred hazard model: K.W. Campbell (written commun., 1984), 

low Q, 2-km depth to the top of the seismogenic zone. Magnitudes are M. for M?!:. 6 and ML for M < 6. Distance is the 

shortest distance to the zone of seismogenic rupture. 

relation (Bender, 1984a). To illustrate the effect of this 

uncertainty on the estimated ground motions of this 

study, figures 20 and 21 compare ground-motion levels 

along the profiles of figure 9 for the preferred and 

upper-bound models with and without ground-motion 

attenuation variability. A standard deviation in loge 

(acceleration or velocity) of0.5 is assumed. Results show 

that incorporating attenuation variability increases 

ground motions as much as 60 percent for the highest 

levels of ground motion in the central Red Sea on the 

Jiddah profile. Increases in ground-motion values for 

coastal and interior regions are considerably less. 

DISCUSSION AND RECOMMENDATIONS 

The approach to estimating the seismic hazard in 

the western Arabia Peninsula presented in this report 

was developed with full consideration of the limited data 

bases and scientific knowledge regarding the seismotectonics of the region. Indeed, the preferred and the 

upper-bound hazard maps depict the consequences of 

models that depend more on scientific judgment than on 

well-established scientific fact. However, this work has 

served to highlight those questions that must be 

addressed to improve future hazard estimates. The 

hazard maps are submitted as an initial attempt to define 

the seismic hazard as reasonably as possible using 

existing data and information. However, an equally 

important aspect of this study is to identify areas of 

investigation of a somewhat broader scope that would 

serve to refine the overall understanding of the seismic 

hazard problem in western Saudi Arabia. In addition to 

outstanding questions discussed in the preceding sections 

of this report, three fruitful areas of further study can be 

identified: (1) historical and contemporary earthquake 

studies, (2) studies in contemporary tectonics and paleaseismic field investigations, and (3) investigations into 

secondary ground-failure hazards. 

Refinement of the existing historical-earthquake 

data base is likely to result from searches of primary 

Arabic sources. The search performed for this study was 

Discussion and Recommendations 29 
> 1 !:: 10 

(.) 

0 

...I 

w 

> 

" g 10° '---------~--.._ ..... ..-._ ..... ___ .. 3 

10° 101 10

2 10 

LOG DISTANCE (km) 

Figure 15. Velocity attenuation curves for the preferred hazard model: K.W. Campbell (written commun., 1984), low 

Q, 2-km depth to the top of the seismogenic zone. Magnitude and distance measures are as in figure 14. 

restricted to the English literature which, for noninstrumental earthquakes of record in the region, is composed 

of secondary sources. Thorough searches of primary 

sources have considerably improved historicalearthquake catalogs for areas of the Western United 

States (Oaks and Algermissen, 1984), in one case 

producing 50 percent more historical earthquakes than 

documented in existing catalogs of the southern Basin 

and Range region (Dubois and others, 1982). In the 

Eastern United States, a region of relatively low seismic 

activity, it was the documented occurrence of large 

historical earthquakes in 1811-1812 (New Madrid, Mo.) 

and 1886 (Charleston, S.C.) that indicated a seismic 

hazard problem for this region thus providing impetus 

for scientific study. A correlative situation in western 

Saudi Arabia appears in the identification of strong 

earthquake shaking in the towns of Khaybar and Tayma 

in the year 1068 (estimated MMI =IX, Poirier and 

Taber, 1980). In terms of contemporary seismicity, the 

northwest Arabia peninsula appears virtually aseismic 

due in large part, perhaps, to a lack of local seismic 

monitoring. The historical accounts of strong shaking at 

Khaybar and Tayma are the first indications that a 

potentially serious seismic hazard may exist in the area. 

Geologic field investigations of the reported ground 

disruptions between Khaybar and Tayma could serve to 

confirm the historical reports and, if substantiated, could 

perhaps provide a preliminary epicentral area for the 

event. Geophysical studies and detailed microseismic 

monitoring could then provide detailed velocity structure 

and subsurface structure of the area along with accurate 

microseismic locations. The physical dimensions and 

geometry of the seismic source could thus be estimated 

and the potential local seismic hazard evaluated. This 

simplified investigation scenario follows the form of 

highly successful earthquake investigations in the Central 

and Eastern United States (McKeown and Pakiser, 1982; 

Bollinger and Wheeler, 1982). 

Sufficient circumstantial evidence associating 

seismicity with the Ad Damm fault exists to warrant 

further earthquake-specific investigations for this fault. If 

this fault is found to be seismically active, it could very 

well dominate the earthquake ground-motion hazard at 

Taif and Mecca because of its close proximity to both of 

these locations. Ground-motion hazard at Jiddah would 

result from both the generally far field ground motions 

30 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
28° 30° 32° 34° 36° 38° 40° 42° 44° 46° 48° 50° 52° 54° 56° 58° 60° 62° 

Figure 16. Peak horizontal acceleration with a 90 percent probability of not being exceeded in 100 yr, given by the 

preferred hazard model. Contours are in percent gravity. 

produced by the recurrence of earthquakes on the Ad 

Damm fault and those produced by earthquake sources 

of the central Red Sea Rift. At this point, all that can be 

confidently stated is that if the Ad Damm fault is 

seismically active, it would be an important element to 

include in future hazard assessments of coastal Saudi 

Arabia. 

Although the presently known earthquake history 

of western Saudi Arabia resembles that of the Eastern 

United States, in that the rate of significant earthquake 

occurrences is apparently low, the recency of tectonic 

deformation of western Saudi Arabia corresponds to that 

of the Western United States. That is, TertiaryQuaternary tectonics dominate the regional geologic 

structure of both areas. Because geomorphic/geologic 

studies of faults and fault scarps have found widespread 

success in identifying active faults in the Western United 

States tectonic setting, there is reason to believe that such 

investigations would similarly find success in western 

Saudi Arabia. Methods of investigation may not be 

directly transferable because of different regional 

climatic histories. Nevertheless, studies of landform 

degradation and climatic history could lead to paleoseismic, fault -scarp investigation methods specific to the 

regions conterminous to the Red Sea. Relative- and 

absolute-age data necessary for distinguishing ages of 

Discussion and Recommendations 31 
28° 30° 32° 34° 36° 38° 40° 42° 44° 46° 48° 50° 

10° 

go 

Figure 17. Peak horizontal velocity with a 90 percent probability of not being exceeded in 100 yr, given by the preferred 

hazard model. Contours are in centimeters per second. 

faulting are already identifiable for the central and 

northern coastal regions in the form of raised terraces 

and coral reefs and interstratified Quaternary basalts 

(Whitney, 1982). In one case, offset of a raised coral reef 

located about 45 km south of Jiddah has been suggested 

as due to late Pleistocene-Holocene age fault 

displacement (Pallister, 1982). However, previous fault 

studies have been only incidental to the primary goal of 

basic geologic mapping. Standard geologic mapping in 

the Western United States has proved to be far too 

insensitive to Quaternary tectonics and faulting to be 

useful in addressing geologically recent fault movements. 

Geologic maps are generally compiled without the 

required detail and without the proper emphasis on 

recent tectonics that is needed for careful evaluation of 

young fault movement. It is important in seismic fault 

studies of coastal Saudi Arabia to distinguish tectonic, 

paleoseismic fault rupture from faulting due to salt 

diapirism, flowage, and solution. Tectonic faults extend 

to considerable crustal depths and are capable of storing 

sufficient stress to rupture in significant earthquakes. In 

contrast, diapiric and solutional faulting is restricted to 

the stratigraphic section and is generally not associated 

with earthquakes that produce damaging ground 

32 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
Figure 18. Peak horizontal acceleration with a 90 percent probability of not being exceeded in 100 yr, given by the 

upper-bound hazard model. Contours are in percent gravity. 

motions. Nonetheless, such faulting can result in surface 

displacement and thus pose a ground-displacement 

hazard to structures located on or near such faults. 

The recommendation for site-specific investigation 

may appear incongruous with the regional approach of 

this study. However, it is well known that hazard 

estimates can change dramatically for specific areas when 

causal seismogenic faults are identified and earthquake 

occurrence frequencies and maximum magnitudes are 

determined specifically for such faults. The consequence 

is that the hazard can be tied to specific faults, or classes 

of faults, rather than averaged over broad areas, thus 

resulting in a more accurate portrayal of a more local 

seismic hazard. 

Secondary ground-failure hazards resulting from 

earthquakes, such as surface rupture, liquefaction, and 

landsliding, have not been topics of study in the current 

analysis. However, a thorough understanding of the 

earthquake hazard would include a knowledge of those 

areas susceptible to induced ground failure. Sediments 

most susceptible to liquefaction are Holocene unconsolidated, cohesionless sands of delta, river-channel, floodplain or eolian deposits, and uncompacted artificial fill, 

all located in areas having high water tables (Youd and 

Discussion and Recommendations 33 
Figure 19. Peak horizontal velocity with a 90 percent probability of not being exceeded in 100 yr, given by the 

upper-bound hazard model. Contours are in centimeters per second. 

Perkins, 1978; Youd, 1984). Such conditions may exist at 

a number of locations along the Red Sea coastal plain. 

For example, ground-water levels near the wadi systems 

in the Jizan coastal plain are generally 10m or shallower, 

with saturated alluvium of fluvial and eolian origin 

extending to depths greater than 100 m (Muller and 

others, 1984). In Jiddah, a high water table and 

development on unengineered fill and reclaimed land 

pose a potentially serious ground-failure hazard in the 

event of strong ground shaking. 

Near-future progress towards a more complete 

understanding of the seismic hazard in the western 

Arabia peninsula is dependent upon a broad scope of 

interrelated geologic, seismologic, and geophysical 

investigations directed at specific "targets of 

opportunity." Historically reported ground disruptions 

associated with strong earthquake shaking, such as those 

reported at Tayma and Khaybar, provide such a target 

for scientific validation of the historical reports and 

scientific insights into possible earthquake sources of the 

region. With the experience and results of such an 

investigation, future studies could be structured in other 

areas of the western peninsula in full consideration of 

scientific methods of investigation that are likely to be 

applied successfully in the region. 

34 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
c, 

-

z 

0 

i= 

< 

a: 

w 

irl 10-1 

0 

0 

< 

(!) 

0 

..J 

c, 

-

z 

0 

i= 

< a: 

w 

..J 

w 

0 

0 

< 

(!) 

0 

..J 

z 

0 

~ a: 

w 

...J 

10° 

10-1 

10- 2 

~ 10- 1 

0 

< 

(!) 

0 

..J 

0 

8 

100 

100 

Acceleration profile of Yanbu 

200 300 

DISTANCE (km) 

400 

A' 

Acceleration profile of Jiddah 

200 300 400 

DISTANCE (km) 8' 

Acceleration profile of Jizan 

100 200 300 400 

C' DISTANCE (km) 

Figure 20. Profiles of peak horizontal acceleration along 

transects shown in figure 9 showing K.W. Campbell's 

(written commun., 1984) attenuation model (LQ2 in figure 

10) and Nuttli and Herrmann's (N.H.) (1984) attenuation 

model (MNH in figure 10) with and without attenuation 

variability. Attenuation variability modeled as standard 

deviation 0.5 in log. acceleration. 

- 10

2 

() 

a.> 

en 

...... 

E 

() 

-

>-

1- 101 

0 

0 

...J 

w 

> 

(!) 

0 

...J 

0 102 

a.> 

en 

...... 

E 

() 

>-

1- 1 

0 10 

0 

..J 

w 

> 

(!) 

0 

...J 

1cf 

>-

!:: 

0 101 

0 

...J 

w 

> 

(!) 

0 

...J 

100 

0 100 

8 

Velocity profile of Yanbu 

200 300 

DISTANCE (km) 

Velocity profile of Jiddah 

200 300 

DISTANCE (km) 

400 

A' 

400 

8' 

Velocity profile of Jizan 

1cf._----~----~----~-----­ o 100 

c 

200 300 

DISTANCE (km) 

400 

C' 

Figure 21. Profiles of peak horizontal velocity along 

transects of figure 9. Labels are as in figure 20. Attenuation 

variability modeled as standard deviation 0.5 in log. 

velocity. 

Discussion and Recommendations 35 
ACKNOWLEDGMENTS 

Funding for this work was provided by the Ministry 

of Petroleum and Mineral Resources, Deputy Ministry 

for Mineral Resources, Kingdom of Saudi Arabia. We 

thank J.W. Dewey and K.W. Campbell for their reviews 

and helpful suggestions. 

REFERENCES CITED 

Abdallah, A., Courtillott, V., Kasser, M., LePain, A., Lepine, J., 

Robineau, B., Ruegg, J., Tapponnier, P., and Tarantota, 

A., 1979, Relevence of Afar seismicity and volcanism to 

the mechanics of accreting plate boundaries: Nature, v. 

282, p. 17-23. 

Algermissen, S.T., and Perkins, D.M., 1976, A probabilistic 

estimate of maximum acceleration in rock in the 

contiguous United States: U.S. Geological Survey OpenFile Report 76-416, 45 p. 

Algermissen, S.T., Perkins, D.M., Thenhaus, P.C., Hanson, 

S.L., and Bender, B.L., 1982, Probabilistic estimates of 

maximum acceleration and velocity in the contiguous 

United States: U.S. Geological Survey Open-File Report 

82-1033, 99 p. 

Ambraseys, N.N., and Melville, C.P., 1983, Seismicity of 

Yemen: Nature, v. 3, no. 591, p. 321-323. 

Arya, A.S., Srivastava, L.S., and Gupta, S.P., 1985, Survey of 

damages during the Dhamar earthquake of 13 December 

1982 in the Yemen Arab Republic: Seismological Society 

of America Bulletin, v. 75, p. 597-610. 

Barberi, F., Ferrara, G., Santacloce, R., and Varet, J., 1975, 

Structural evolution of the Afar triple junction, in Piler, 

A., and Rosier, A., eds., Afar depression of Ethiopia: 

Inter-Union Commission on Geodynamics, Scientific 

Report 14, Verlagsrbuchhanolung, Stuttgart, p. 38-54. 

Barberi, F., and Varet, J., 1977, Volcanism of Afar-Smallscale plate tectonics implications: Geological Society of 

America Bulletin, v. 88, no. 9, p. 1251-1266. 

Bender, Bernice, 1982, Sensitivity analysis for seismic risk using 

a fault rupture model: U.S. Geological Survey Open-File 

Report 82-294, 75 p. 

___ 1983, Maximum likelihood estimation of b-values for 

magnitude grouped data: Seismological Society of America Bulletin, v. 73, p. 831-851. 

___ 1984a, Incorporating acceleration variability into 

seismic hazard analysis: Seismological Society of America Bulletin, v. 74, p. 1451-1462. 

___ 1984b, Seismic hazard estimation using a finite fault 

rupture model: Seismological Society of America 

Bulletin, v. 74, p. 1899-1923. 

Ben-Menahem, Ari, 1981, Variation of slip and creep along the 

Levant Rift over the past 4500 years, in Freund, R., ed., 

The Dead Sea Rift-International Symposium on the 

Dead Sea Rift, Jerusalem, Sept. 10-20, 1979: Tectonophysics, v. 80, no. 1-4 (special issue), p. 183-197. 

Ben-Menahem, Ari, and Aboodi, Ezra, 1971, Tectonic patterns 

in the northern Red Sea region: Journal of Geophysical 

Research, v. 76, no. 11, p. 2674-2689. 

___ 1981, Micro- and macroseismicity of the Dead Sea Rift 

and off-coast eastern Mediterranean, in Freund, R., ed., 

The Dead Sea Rift-International Symposium on the 

Dead Sea Rift, Jerusalem, Sept. 10-20, 1979: Tectonophysics, v. 80, no. 1-4 (special issue), p. 199-233. 

Blank, H.R., Jr., 1977, Aeromagnetic and geologic study of 

tertiary dikes and related structures on the Arabian 

margin of the Red Sea, in Red Sea research 1970-1975: 

Saudi Arabian Directorate General of Mineral 

Resources Bulletin 22, p. G1-G18. 

Bohannon, R.G., 1986, Tectonic configuration of the Western 

Arabian Continental Margin, Southern Red Sea: 

Tectonics, v. 5, p. 477-499. 

Bollinger, G.A., and Wheeler, R.L., 1982, The Giles County, 

Virginia, seismogenic zone-Seismological results and 

geological interpretations: U.S. Geological Survey OpenFile Report 82-585, 136 p. 

Brown, G.F., 1972, Tectonic map of the Arabian Peninsula: 

Saudi Arabian Directorate General of Mineral 

Resources Arabian Peninsula Map AP-2, scale 

1:4,000,000. 

Burr, N.C., and Solomon, S.C., 1978, The relationship of source 

parameters of oceanic transform earthquakes to plate 

velocity and transform length: Journal of Geophysical 

Research, v. 83, p. 1193-1205. 

Campbell, K. W., 1981, Near-source attenuation of peak 

horizontal acceleration: Seismological Society of America Bulletin, v. 71, p. 2039-2070. 

___ 1982, Near-source scaling characteristics of peak 

horizontal acceleration for moderate-to-large 

earthquakes, in Boatwright, John, ed. and organizer, 

Proceedings of Conference XVI, The dynamic 

characteristics of faulting as inferred from recordings of 

strong ground motion: U.S. Geological Survey Open-File 

Report 82-591, v. 1, p. 120-184. 

___ 1983, The effects of site characteristics on near-source 

recordings of strong ground motion, in Hays, W.W., ed., 

Proceedings of Conference XXII, A workshop on site 

specific effects of soil and rock on ground motion and the 

implications for earthquake-resistant design: U.S. 

Geological Survey Open-File Report 83-845, p. 

280-309. 

___ 1984, Near-source attenuation of strong ground motion 

for moderate to large earthquakes-An update and 

suggested application to the Wasatch fault zone of northcentral Utah, in Hays, W.W., and Gori, P.L., eds., 

Proceedings of Conference XXVI, A workshop on 

evaluation of regional and urban earthquake hazards and 

risk in Utah: U.S. Geological Survey Open-File Report 

84-763,p. 438-499. 

___ 1985, Strong motion attenuation relations-A ten year 

perspective: Earthquake Spectra, v. 1, p. 759-804. 

___ 1987, Predicting strong ground motion in Utah, in Gori, 

P.L., and Hays, W.W., eds., Assessment of regional 

earthquake hazard and risk along the Wasatch front, 

Utah: U.S. Geological Survey Open-File Report 87-585, 

p. L1-L90. 

36 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
Cochran, J.R., 1981, The Gulf of Aden-Structure and 

evolution of a young ocean basin and continental margin: 

Journal of Geophysical Research, v. 86, no. B1, p. 

263-287. 

--- 1982, The magnetic quiet zone in the eastern Gulf of 

Aden-Implications for the early development of the 

continental margin: Geophysical Journal of the Royal 

Astronomical Society, v. 68, no. 2, p. 171-201. 

__ 1983, A model for the development of the Red Sea: 

American Association of Petroleum Geologists Bulletin, 

v. 67, no. 1., p. 41-69. 

Coleman, R.G., 1973, Geologic map of the Red Sea, in Red Sea 

Research 1970-1975: Saudi Arabian Directorate 

General of Mineral Resources Bulletin 22, pl. 1. 

___ 1977, Geologic background of the Red Sea, in Red Sea 

Research, 1970-1975: Saudi Arabian Directorate 

General of Mineral Resources Bulletin 22, p. C1-C9. 

Cornell, C.A., 1968, Engineering seismic risk analysis: 

Seismological Society of America Bulletin, v. 58, p. 

1583-1606. 

Crowell, J.C., 1974, Origin of late Cenozoic basins in southern 

California, in Dickinson, W.R., ed., Tectonics and 

sedimentation: Society of Economic Paleontologists and 

Mineralogists Special Paper 22, p. 190-204. 

Diment, W.H., Urban, T.C., Sass, J.H., Marshall, B.U., Munroe, RJ., and Lachenbruch, A.H., 1975, Temperatures 

and heat contents based on conductive transport of heat, 

in White, O.E., and Williams, D.L., eds., Assessment of 

geothermal resources of the United States-1975: U.S. 

Geological Survey Circular 726, p. 84-103. 

Drake, C.L., and Girdler, R.W., 1964, A geophysical study of 

the Red Sea: Geophysical Journal, v. 8, p. 473-495. 

Dubois, S.M., Sbar, M.L., and Nowak, T.A., 1982, Historical 

seismicity in Arizona: U.S. Nuclear Regulatory 

Commission Report NUREG/CR-2577, 199 p. 

Eyal, M., Eyal, Y., Bartov, Y., and Steinitz, G., 1981, The 

tectonic development of the western margin of the Gulf 

of Elat (Aqaba) Rift, in Freund, R., ed., The Dead Sea 

Rift-International Symposium on the Dead Sea Rift, 

Jerusalem, Sept. 10-20, 1979: Tectonophysics, v. 80, 

no. 1-4 (special issue), p. 39-66. 

Fairhead, J.D., and Girdler, R.W., 1970, The seismicity of the 

Red Sea, Gulf of Aden and Afar triangle: Philosophical 

Transactions of the Royal Society of London, v. 267, 

p. 49-74. 

Forsyth, D., and Rowlett, H., 1979, Microearthquakes and 

recent faulting at the intersection of the Vema fracture 

zone and the mid-Atlantic ridge [ abs.]: EOS, 

Transactions of the American Geophysical Union, v. 60, 

p. 376. 

Francis, T.J.G., Porter, LT., and Lilwall, R.C., 1978, 

Microearthquakes near the eastern end of St. Paul's 

fracture zone: Geophysical Journal of the Royal 

Astronomical Society, v. 53, p. 201-217. Frazier, S.B., 

1970, Adjacent structures of Ethiopia-That portion of 

the Red Sea coast including Dahlah Kebir Island and the 

Gulf of Zula: Philosophical Transactions of the Royal 

Society of London, v. 267, p. 131-141. 

Garfunkel, Zvi, 1981, Internal structure of the Dead Sea leaky 

transform (rift) in relation to plate kinematics, in Freund, 

R., ed., The Dead Sea Rift-International Symposium on 

the Dead Sea Rift, Jerusalem, Sept. 10-20, 1979: Tectonophysics, v. 80, no. 1-4 (special issue), p. 81-108. 

Garfunkel, Z., and Bartov, Y., 1977, The tectonics of the Suez 

rift: Israel Geological Survey Bulletin 71, 44 p. 

Garfunkel, Z., Zak, 1., and Freund, R., 1981, Active faulting in 

the Dead Sea rift, in Freund, R., ed., The Dead Sea 

Rift-International Symposium on the Dead Sea Rift, 

Jerusalem, Sept. 10-20, 1979: Tectonophysics, v. 80, 

no. 1-4 (special issue), p. 1-26. 

Gettings, M.E., 1977, Delineation of the continental margin in 

the southern Red Sea region from new gravity evidence, 

in Red Sea research 1970-1975: Saudi Arabian 

Directorate General of Mineral Resources Bulletin 22, 

p. K1-K11. 

Gettings, M.E., Blank, H.R., Mooney, W.D., and Healy, J.H., 

1983, Crustal structure of southwestern Saudi Arabia: 

Saudi Arabian Deputy Ministry for Mineral Resources 

Open-File Report USGS-OF-03-59, 51 p. 

Geukens, F., 1966, Geology of the Arabian Peninsula-Yemen 

[translated from French by S.D. Bowers]: U.S. 

Geological Survey Professional Paper 560-B, 23 p. 

Girdler, R.W., 1970, An aeromagnetic survey of the junction of 

the Red Sea, Gulf of Aden and Ethiopean rifts-A 

preliminary report: Philosophical Transactions of the 

Royal Society of London, v. 267, p. 359-368. 

Girdler, R.W., and Styles, P., 1974, Two stage Red Sea floor 

spreading: Nature, v. 247, p. 7-11. 

Gouin, P., 1970, Seismic and gravity data from Afar in relation 

to surrounding areas: Philosophical Transactions of the 

Royal Society of London, v. 267, p. 339-358. 

__ 1979, Earthquake history of Ethiopia and the hom of 

Africa: Ottawa, Canada, International Development 

Research Center, report IDRC-118e, 259 p. 

Gouin, P., and Mohr, P.A., 1964, Gravity traverses in Ethiopia: 

Bulletin of the Geophysical Observatory, v. 3, no. 2, 

p. 185-239. 

Greenwood, W.R., Anderson, R.E., Fleck, R.J., and Roberts, 

R.J., 1980, Precambrian geologic history and plate 

tectonic evolution of the Arabian shield: Saudi Arabian 

Directorate General of Mineral Resources Bulletin 24, 

35 p. 

Gutenberg, B., and Richter, C. F., 1954, Seismicity of the earth 

[2d ed.]: Princeton, N.J., Princeton University Press, 

310 p. 

Hall, S.A., 1980, A total intensity magnetic anomaly map of the 

Red Sea and its interpretation: U.S. Geological Survey 

Open-File Report 80-131, 271 p. 

Hall, S.A., Andreasen, G.E., and Girdler, R.W., 1977, Totalintensity magnetic anomaly map of the Red Sea and 

adjacent coastal areas-A description and preliminary 

interpretation, in Red Sea research 1970-1975: Saudi 

Arabian Directorate General of Mineral Resources 

Bulletin 22, p. F1-F15. 

Hamilton, R.M., 1981, Geologic origin of eastern U.S. 

seismicity, in Beavers, J .E., ed., Earthquakes and 

earthquake engineering-The Eastern United States, 

v. 1: Ann Arbor, Mich., Ann Arbor Science Inc., p. 3-24. 

References Cited 37 
Harpum, J .R., 1970, Summary of the geology of Tanzania, 

[chap.] V; Structure and geotectonics of the Precambrian: Dodoma, Tanzania, Mineral Resources Division, 

58p. 

Heirtzler, J.R., Dickson, G .0., Herron, E.M., Pitman, W.C., III, 

and Le Pichon, X., 1968, Marine magnetic anomalies, 

geomagnetic reversals, and motions of the ocean floor 

and continents: Journal of Geophysical Research, v. 73, 

p. 2119-2136. 

Joyner, W.B., and Boore, D.M., 1981, Peak horizontal 

acceleration and velocity from strong-motion records 

including records from the 1979 Imperial Valley, California, earthquake: Seismological Society of America 

Bulletin, v. 71, p. 2011-2038. 

Juch, D., 1975, Geology of the southeastern escarpment of 

Ethiopia between 30° and 42° longitude east, in Pilger, 

A., and Rosier, A., eds., Afar depression of Ethiopia: 

Inter-Union Commission on Geodynamics, Scientific 

Report no. 14, Verlagsbuchhandlung, Stuttgart, 

p. 310-315. 

Kronberg, P., Schonfeld, M., Gunther, R., and Tsombos, P., 

1975, Tectonic relations of the Afar region and adjacent 

areas, in Pilger, A., and Rosier, A., eds., Afar depression 

of Ethiopia: Inter-Union Commission on Geodynamics, 

Scientific report 14, Verlagsbuchhandlung, Stuttgart, 

p. 19-27, 2 pis., scale 1:1,000,000. 

Langer C.J., and Merghelani, H.M., 1983, Aftershocks of the 

Yemen earthquake of December 13, 1982-A detailed 

study from locally recorded data [ abs.]: Earthquake 

Notes, v. 54, p. 20. 

Laughton, A.S., 1966, The Gulf of Aden: Philosophical 

Transactions of the Royal Society of London, v. 259, ser. 

A, p. 150-171. 

Laughton, A.S., Whitmarsh, R.B., and Jones, M.T., 1970, The 

evolution of the Gulf of Aden: Philosophical 

Transactions of the Royal Society of London, v. 267, 

p. 227-266. 

Le Pichon, X., and Francheteau, J ., 1978, A plate-tectonic 

analysis of the Red Sea-Gulf of Aden area, in Oren, 

0 .H., ed., Structure and tectonics of the eastern Mediterranean: Tectonophysics, v. 46, p. 369-406. 

Le Pichon, X., and Sibuet, J.C., 1981, Passive margins-A 

model of formation: Journal of Geophysical Research, 

v. 86,p.3708-3720. 

Mark, R.K., 1977, Application of linear statistical models of 

earthquake magnitude versus fault length in estimating 

maximum expectable earthquakes: Geology, v. 5, 

p. 464-466. 

Mark, R.K., and Bonilla, M.G., 1977, Regression analysis of 

earthquake magnitude and surface fault length using the 

1970 data of Bonilla and Buchanan: U.S. Geological 

Survey Open-File Report 77-614, 9 p. 

McConnell, R.B., 1967, The east African rift system: Nature, 

v. 215, p. 578-581. 

___ 1972, Geological development of the rift system of 

eastern Africa: Geological Society of America Bulletin, 

v. 83,p.2549-2572. 

McGuire, R.K., and Shedlock, K.M., 1981, Statistical 

uncertainties in seismic hazard evaluations in the United 

States: Seismological Society of America Bulletin, v. 71, 

no. 4, p. 1287-1308. 

McKenzie, D.P., Davies, D., and Molnar, P., 1970, Plate 

tectonics of the Red Sea and East Africa: Nature, v. 226, 

p. 243-248. 

McKeown, F.A., and Pakiser, L.C., 1982, Investigations of the 

New Madrid, Missouri, earthquake region: U.S. 

Geological Survey Professional Paper 1236, 201 p. 

Merghelani, H.M., 1979, Seismicity of the Tihmat-Asir region, 

Kingdom of Saudi Arabia: U.S. Geological Survey Saudi 

Arabian Project Report 261, 20 p. 

___ 1981, Seismicity of the Yanbu region, Kingdom of Saudi 

Arabia: U.S. Geological Survey Saudi Arabian Project 

Report 371, Technical Record 16, 32 p. 

Merghelani, H.M., Kinkar, A.R., and As-Sawwaf, M.W., 1981, 

Seismicity studies in Saudi Arabia-Microearthquakes in 

the Jiddah area: Saudi Arabian Directorate General of 

Mineral Resources Open-File Report DGMROF-01-08, 64 p. 

Ministry of Defence, Director of Military Survey, United 

Kingdom, 1980, Political map of the Arabian Peninsula: 

U.S. Geological Survey and the Arabian American Oil 

Company, compilers, under sponsorship of the Kingdom 

of Saudi Arabia, Ministry of Petroleum and Mineral 

Resources and the U.S. Department of State, scale 

1:2,250,000. 

Mohr, P.A., 1975, Structural setting and evolution of Afar, in 

Pilger, A., and Rosier, A., eds., Afar depression of 

Ethiopia: Inter-Union Commission on Geodynamics, 

Scientific Report 14, Verlagsbuchhandlung, Stuttgart, 

p. 27-37. 

Molnar, P., and Oliver, J., 1969, Lateral variations of 

attenuation in the upper mantle and discontinuities in the 

lithosphere: Journal of Geophysical Research, v. 74, 

p. 2648-2681. 

Mooney, W.D., Gettings, M.E., Blank, H.R., and Healy, J.H., 

1985, Saudi Arabian seismic deep-refraction profile-A 

travel time interpretation of deep crustal structure: Tectonophysics, v. 111, p. 173-246. 

Morton, W.H., and Black, R., 1975, Crustal attenuation in Afar, 

in Pilger, A., and Rosier, A., eds., Afar depression of 

Ethiopia: Inter-Union Commission on Geodynamics, 

Scientific Report 14, Verlagsbuchhandlung, Stuttgart, 

p. 55-65. 

Muller, E., Fredrich, K., and Klinge, H., 1984, Hydrogeology of 

the Tihamat Asir, in Jado, A.R., and Z.Otl, J.G., 

Quaternary Period in Saudi Arabia, V.Z.: ; New York, 

Springer-Verlag Publishers, p. 174-194. 

Nishenko, S.P., and McCann, W.R., 1981, Seismic potential for 

the world's major plate boundaries, in Simpson, D.W., 

and Richards, P.G., eds, Earthquake prediction, an 

international review, Maurice Ewing series: American 

Geophysical Union, v. 4, p. 20-28. 

Nuttli, O.W., 1983, Average seismic source-parameter relations 

for mid-plate earthquakes: Seismological Society of 

America Bulletin, v. 73, p. 519-535. 

38 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
Nuttli, O.W., and Herrmann, R.B., 1984, Ground motion of 

Mississippi Valley earthquakes: Journal of Technical 

Topics in Civil Engineering, v. 110, p. 54-69. 

Oaks, S.D., and Algermissen, S.T., 1984, Investigation of 

historical seismicity in the Salt Lake City portion of the 

Wasatch front region of Utah using documentary 

sources, in Hays, W.W., and Gori, P.L., eds., Proceedings 

of S:onference XXVI, A workshop on evaluation of 

regional and urban earthquake hazards and risk in Utah: 

U.S. Geological Survey Open-File Report 84-763, 

p. 384-390. 

Oliver, J., and !sacks, B., 1967, Deep earthquake zones, 

anomalous structures in the upper mantle, and the 

lithosphere: Journal of Geophysical Research, v. 72, 

p. 4259-4275. 

Pallister, J.S., 1982, Reconnaissance geologic map of the Shu 

Ayba quadrangle, sheet 20/39, Kingdom of Saudi Arabia: 

Saudi Arabian Deputy Ministry for Mineral Resources 

Open-File Report USGS-OF--02-59, 1 pl. 

___ 1984, Explanatory notes to the geologic map of the AI 

Lith quadrangle, sheet 20 D, Kingdom of Saudi Arabia: 

Saudi Arabian Deputy Ministry for Mineral Resources 

Open-File Report USGS-OF--04-8, 124 p., 1 pl. 

Phillips, J.D., and Ross, D.A., 1970, Continuous seismic reflexion profiles in the Red Sea: Philosophical Transactions of 

the Royal Society of London, v. 267, p. 143-152. 

Pilger, A., and R<>sler, A., 1975, eds., Mar depression of 

Ethiopia: Inter-Union Commission on Geodynamics, 

Scientific Report 14, Verlagsbuchhandlung, Stuttgart, 

416 p. 

Poirier, J.P., and Taber, M.A., 1980, Historical seismicity in the 

Near and Middle East, North Mrica, and Spain from 

Arabic documents (VIIth-XVIIth century): 

Seismological Society of America Bulletin, v. 70, no. 6, 

p. 2185-2201. 

Project Rose Scientists, 1981, Microearthquake activity on the 

Orozco fracture zone-Preliminary results from project 

Rose: Journal of Geophysical Research, v. 86, 

p. 3783-3790. 

Quennell, A.M., 1956, Tectonics of the Dead Sea rift, in 

Association of Mrican Geological Surveys Symposium, 

20th International Geology Congress, Mexico City, 1956: 

p. 385-405. 

Reches, Ze'ev, and Hoexter, D.F., 1981, Holocene seismic and 

tectonic activity in the Dead Sea area, in Freund, R., ed., 

The Dead Sea Rift-International Symposium on the 

Dead Sea Rift, Jerusalem, Sept. 10--20, 1979: Tectonophysics, v. 80, p. 235-254. 

Riad, S., 1977, Shear zones in North Egypt interpreted from 

gravity data: Geophysics, v. 42, no. 6, p. 1207-1214. 

Ross, D.A., and Schlee, J., 1973, Shallow structure and geologic 

development in the southern Red Sea: Geological 

Society of America Bulletin, v. 84, p. 3827-3848. 

___ 1977, Shallow structures and geologic development of 

the southern Red Sea, in Red Sea research 1970--1975: 

Saudi Arabian Directorate General of Mineral 

Resources Bulletin 22, p. E1-E18. 

Rowlett, H., 1981, Seismicity at intersections of spreading 

centers and transform faults: Journal of Geophysical 

Research, v. 86, no. B5, p. 3815-3820. 

Said, R., 1962, The geology of Egypt: New York, Elsevier, 

377 p. 

Schmidt, D.L., Hadley, D.G., and Brown, G.F., 1983, Middle 

Tertiary continental rift and evolution of the Red Sea in 

southwest Saudi Arabia, U.S. Geological Survey OpenFile Report 83-641, 56 p. 

Sieh, K.E., 1978, Prehistoric large earthquakes produced by slip 

on the San Andreas fault at Pallett Creek, California: 

Journal of Geophysical Research, v. 83, p. 3907-3939. 

___ 1981, A review of geological evidence for reccurrence 

times of large earthquakes, in Simpson, D.W., and Richards, P.G., eds., Earthquake prediction, an international 

review, Maurice Ewing series: American Geophysical 

Union, v. 4, p. 181-207. 

Solomon, S.C., 1973, Shear wave attenuation and melting 

beneath the mid-Atlantic ridge: Journal of Geophysical 

Research, v. 78, p. 6044-6059. 

Solomon, S.C., and Burr, N.C., 1979, The relationship of source 

parameters of ridge-crest and transform earthquakes to 

the thermal structure of oceanic lithosphere: Tectonophysics, v. 55, p. 107-126. 

Steinhrugge, K.V., 1984, Effects of shaking on buildings by class 

of construction, in Williams, M.W., Proceedings of the 

Geologic and Hydrologic Hazards Training Program: 

U.S. Geological Survey Open-File Report 84-760, 

p. 153-209. 

Stepp, J.C., 1973, Analysis of completeness of the earthquake 

sample in the Puget Sound area, in Harding, S.T., ed., 

Contributions to seismic zoning: National Oceanic and 

Atmospheric Administration Technical Report 

ERL267-ESL30, p. 16-28. 

Stewart, L.M., 19HJ, Strain release along oceanic transforms: 

New Haven, Conn., Yale University, Ph. D. dissertation, 

272 p. 

Sykes, L.R., 1967, Mechanism of earthquakes and nature of 

faulting on the mid-ocean ridges: Journal of Geophysical 

Research, v. 72, p. 2131-2153. 

___ 1970, Focal mechanisms for earthquakes along the 

world rift system: Seismological Society of America 

Bulletin, v. 60, no. 5, p. 1749-1752. 

___ 1978, Intraplate seismicity reactivation of preexisting 

zones of weakness, alkaline magmatism, and other tectonism postdating continental fragmentation: Reviews of 

Geophysics and Space Physics, no. 4, p. 621-668. 

Sykes, L.R., and Landisman, M., 1964, The seismicity of East 

Africa, the Gulf of Aden and the Arabian and Red Seas: 

Seismological Society of America Bulletin, v. 54, no. 6, 

p. 1927-1940. 

Sykes, L.R., and Nishenko, S.P., 1984, Probabilities of 

occurrence of large plate rupturing earthquakes for the 

San Andreas, San Jacinto, and Imperial faults, California, 1983-2003: Journal of Geophysical Research, v. 89, 

p. 5905-5927. 

Sykes, L.R., and Seeber, L., 1982, Great earthquakes and great 

asperities along the San Andreas fault, southern California [abs.]: EOS, Transactions ofthe American Geophysical Union, v. 63, p. 1030. 

References Cited 39 
Trehu, A.M., Nabelek, J.L., and Solomon, S.C., 1981, Source 

characterization of two Reykjanes Ridge earthquakesSurface waves and moment tensors; P waveforms and 

nonorthogonal nodal planes: Journal of Geophysical 

Research, v. 86, no. B3, p. 1701-1724. 

Weichert, D.H., 1980, Estimation of earthquake recurrence 

parameters for unequal observation periods for different 

magnitudes: Seismological Society of America Bulletin, 

v. 70,p. 1337-1346. 

Whitney, J.W., 1982, Overview of the geomorphology of 

western Saudi Arabia and recommendations for future 

research: Saudi Arabian Deputy Ministry for Mineral 

Resources Open-File Report USGS-OF--02-6, 19 p. 

Wilson, J.T., 1965, A new class of faults and their bearing on 

continental drift: Nature, v. 207, p. 343-347. 

Youd, T.L., 1984, Geologic effects-liquefaction and associated 

ground failure, in Williams, M.W., Proceedings of the 

Geologic and Hydrologic Hazards Training Program: 

U.S. Geological Survey Open-File Report 84-760, 

p. 210-232. 

Youd, T.L., and Perkins, D.M., 1978, Mapping of liquefactioninduced ground failure potential: American Society of 

Civil Engineers, Journal of the Geotechnical Engineering 

Division, v. 104, no. GT4, p. 433-466. 

Zanettin, B., and Justin-Visentin, E., 1975, Tectonical and 

volcanological evolution of the western Afar margin 

(Ethiopia), in Pilger, A., and Rosier, A., eds., Afar 

Depression of Ethiopia: Inter-Union Commission on 

Geodynamics, Scientific Report No. 14, Verlagsbuchhandlung, Stuttgart, p. 300-309. 

40 Probabilistic Seismic Ground-Motion Hazard In Western Saudi Arabia 
SEISMIC ACTIVITY RATES OF 

SOURCE ZONES AND SOURCE FAULTS 
APPENDIX 

SEISMIC ACTIVITY RATES OF SOURCE ZONES 

AND SOURCE FAULTS 

The tabulated seismic activity rates are given at the magnitude 5.0 

level as determined from the allocation scheme illustrated in figure 7 and 

expressed in terms of n!yr/km2 for area sources and n/yr/km for fault 

sources. The b-value for all sources is taken as -1.0. 

Source Description Seismic Activity Rate 

Western Arabia peninsula -7 N~jd zon~ •••••.•••••••••••••••••••••••••••••••••• 1 .03x10 _

8 Hl~az-As~r (NW structlJl"al trend) •.••••••.•••••••• 1 .19x1 0 _

8 HiJaz-Aslr (NE structural trend •••••••••••••••••• 9.50x10 

Yemen •• ••••••••••••••••••••••••.••••••••••••••••• 7. 66x1 0-7 

Dead Sea transform _

6 Lebanon Mountains segment ••••••••.••••••••••••••• 3.78x10 

Dead -5 Sea segment ••••••••••••••••.•••••••••••••••• 1 • 77x1 0 

Gulf 6 of Aqaba segment •••.•••••••••••••••••••••••• 4.88x10-

Gulf of Suez ••••••••••••••••••••••••••••••••••••••• 2.40x1o-6 

Red Sea 

Gulf of Suez-Red Sea junction •••••••••••••••••••• ~.36x10-5 Red Sea shelf areas •••••.••••••••.••••••••••••••• 2.22x1o-7 

6 Northern -5 axial trough •••••••••••••••••••••••••••• 1. ~x1o_

Southern axial trough~ ••••••••••••••••••••••••••• 1.07x10 4 

Southern axial trough transform 

raults, listed in order from _

4 north-to-south (Figure 4) ..•••.•..•...••.•••••... 1.63x1o_4 •••••....•••••.••••••••• 5.87x1o_4 •••••••••••••••.•••••••• 2.92x1o_4 (Dahlac Islands transform) •••••••.••••••••••••••• 4.75x1o_4 ••• ~ •••••••••••••••••••• 4.52x10 4 •••••••••••••••••••••••• 4.95x1o:4 •••••••••••••••••••••••• lf.36x10 

Gulf of Aden 

Mountainous region neighboring _

7 axial trough ••••••••••.•••••••••••••••••••••••• 5.00x10 4 Axial trough ••••••••••••••••••••••••••••••••••••• 2.69x10-

Gulf of Aden transform faults, 

listed in order from west-to- _

4 east (Figure 4) •••••••••••••••••••••••••••••••• 4.93x1o_4 ••••••••••••••••.••••••••••.•.•• 4.93x1o_4 •••••••.•••••••••••••••••••••••• 2.08x10_4 •••••••••••••••••••••••••••••••• 2.08xto_4 

•••••••••••••••••••••••••••••••• 3.19x1o_4 {Alula-Fartak trench) •••••••••••••••••••••••••••• 8.02xto_4 •••••••••••••••••••••••••••••••• 1.29x10 

Central Afar and neighboring regions _

6 Central Afar ••••••••••••••••••••••••••••••••••••• 1 • 07x1 0 _

6 Somalia escarpment ••••••••••••••••••••••••••••••• 1.13x10_6 Ethiopian escarpment ••••••••••••••••••••••••••••• 1.~2xl0_6 Danakil block •••••••••••••••••••••••••••••••••••• 1. 79xl 0 -7 Sudan-Ethiopia boundary zone •••••••••.••••••••••• 3.58x10 

*U.S. GOVERNMENT PRINTING OFFICE: 1989-673-047/86049 


SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS 

Periodicals 

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly). 

Technical Books and Reports 

Professional Papers are mainly comprehensive scientific reports of 

wide and lasting interest and importance to professional scientists and engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

· collections of related papers addressing different aspects of a single scientific topic. 

Bulletins contain significant data and interpretations that are of lasting scientific interest but are generally more limited in scope or 

geographic coverage than Professional Papers. They include the results 

of resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic. 

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrogeology, 

availability of water, quality of water, and use of water. 

Cb'culars present administrative information or important scientific 

information of wide popular interest in a format designed for distribution 

at no cost to the public. Information is usually of short-term interest. 

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at 

depositories indicated in USGS catalogs. 

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

depositories. They are a nonpermanent form of publication that may be 

cited in other publications as sources of information. 

Maps 

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 71/2-or 15-minutequadrangleformats (scales mainly 1:24,000 or 1 :62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only. 

Geophysical Investigations Maps are on topographic or planimetric 

bases at various scales; they show results of surveys using geophysical 

techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology. 

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 7 1/2-minute quadrangle photo geologic maps on planimetric bases 

which show geology as interpreted from aerial photographs. Series also 

includes maps of Mars and the Moon. 

- Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, stratigraphy, and structural relations in certain coal-resource areas. 

Oil and Gas Investigations Charts show stratigraphic information 

for certain oil and gas fields and other areas having petroleum potential. 

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations. 

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic dataofboth regular and irregular areas; principal scale 

is 1:24,000 and regional studies are at 1:250,000 scale or smaller. 

Catalogs 

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, Books 

and Open-File Reports Section, Federal Center, Box 25425, Denver, 

CO 80225. (See latest Price and Availability List.) 

"Publications of the Geological Survey, 1879- 1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche. 

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche. 

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche. 

Supplements for 1982, 1983, 1984, 1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form. 

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only 

"Price and A vallabillty List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet form only. 

Selected copies of a monthly catalog "New Publications of the U.S. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklc:.t form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

National Center, Reston, VA 22092. 

Note.--Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.