THE QUANTITATIVE SEISMICITY MAPS OF THE ARABIAN PENINSULA AND ADJOINING AREAS

Three quantitative seismicity maps, the a-value map, the b-value map and the estimated maximum magnitude map, have been prepared for the Arabian Peninsula and surroundings. For preparation of these maps, the frequency-magnitude relation has been used where the constants a and b were estimated using maximum likelihood method. These maps are superimposed on the modified seismotectonic map of Arabian peninsula. These quantitative seismicity maps reveal that the Arabian peninsula consists of four high seismic activity zones: Zagros high, Dead sea rift system high, southern Red sea high and Gulf of Aden and Arab sea high. The b-values determined by this study are compared with those determined by other workers for various regions in the Arabian peninsula. They show a good agreement .The maximum magnitude map reveals a Mmax highs comparable with the a and b highs .


INTRODUCTION
The seismicity of the region expressed in quantitative terms is always more meaningful than just presenting maps showing the location of epicenters of earthquakes because the former enable one to compare the relative seismicity level of different regions of the earth and their tectonic relations.The seismicity parameters were introduced by Gutenberg and Richter (1954) in their pioneer work in 1954.
Several investigation (e.g., Ritsema, 1954.Ullman and Maaz, Ullman 1966; Karnik, 1971) have tried to express seismicity of a regin in terms of energy release or strain release.Other investigators (e.g.Miyamura, 1962;Kaila and Narain, 1971;Kaila etal., 1972;Kaila and Roa, 1975) have tried to express seismicity in term of parameters a and b of the frequency magnitude relation.Hattori (1974) determined the spatial distribution of the b-values at every degree of latitude and longitude of different seismic regions of the world.Karnik (1969)

PENINSULA
The majority of earthquake in the Arabian Peninsula are concentrated along three major belts (Fig. 1) the first is Zagros fold belt that extends about Owing to data limitation in some source compartment, this method is selected since a minimum number of events can be used (Welkner, 1967).However, for small number of events, the upper and lower limits of confidence for the b is value becomes larger even for small probability as shown by Aki (1965).In equation ( 2), the value of b unbounded without restriction so that the condition imposed in Utsu (1965) equation for total energy of a given group of earthquakes is taken.The minimum magnitude in a given set of data is 4.0.
The maximum magnitude (Mm) values were also determined from Pisarenko etal (1996).

MAXIMUM MAGNITUDE MAP
The third seismicity map (Fig. 4) shows the estimated maximum magnitude value.The range of maximum magnitude from 6 to 7 is seen to be located in the Aden Gulf.The range of maximum magnitude about Gulf of Aqabah and Sueze Gulf is from 5.5 to 6.
The maximum magnitude for the southen Red sea is 6.5, the same as in land invicinity of point (13 o N, 43 o E).The range of maximum magnitude from 6.5 to 7.0 is seen in the Zagros folded Belt.

DISCUSSION AND CONCLUSIONS
Our emphasis goes to parameter b, for it is a tectonic factor that is correlated to tectonic structure in constructed regional seismic zones in terms of b.Experimental work by Mogi (1962) using rock specimens showed that the parameter b is related to the degree of heterogeneity of material under stress.Scholz (1968) indicated frequency of microshocks is a function of the area of rupture and the applied stress.The purpose of this paper is to construct three quantitative seismicity maps for the Arabian Peninsula.The a-value map , the b-value map, and the maximum magnitude map.These three maps are studied in relation to tectonics of the region, and then compared with the seismicity data for this region, as published by other workers.

P
-ISSN 1991-8941 E-ISSN 2706-6703 Journal of University of Anbar for Pure Science (JUAPS) Open Access 2007,(1), (2 ) :102-110 103 through west Iran and northeast Iraq to Turkey.The second belt extends from the central Red sea region south to Afar and then east through the Gulf of Aden.The third belt extends from the northern tip of the Red sea in a northeasterly direction through the Gulf of Aqabah, Dead sea, Lebanon, Syria, and terminates in southern Turkey.The Dead sea transform system connects active spreading centers of the Red sea to the area where the Arabian plate is converging with Eurasia in southern Turkey.The Gulf of Aqabah in the southern portion of the rift system has experienced left-lateral strike-slip faulting with a 110Km offset since early Tertiary to the present.Earthquakes location provide evidence for continuation of faulting regime from the Gulf north eastward in land beneath thick sediments, suggesting that the northern portion of the Gulf is subjected to more severe seismic hazard compared to southern portion (Al-Amri et.al.,1991).The majority of earthquakes and tectonic activity in the Red sea region are concentrated along a belt that extends from the central Red sea region south to Afar and then east through the Gulf of Aden .There is little seismic activity in the northern part of the Red sea.Instrumental seismicity of the northern Red sea shows that 68 earthquake (3.8< mb <6.0) were reported to have occurred in the period from 1964 to 1993 (Al-Amri, 1995a).Instrumental seismicity of the southern Red sea shows that 170 earthquake (3.0< mb< 6.6) were reported to have occurred in the period (1965-1994).The Arabian plate boundary extends eastnortheast from the Afar region through the Gulf of Aden and into the Arab sea and Zagros folded belt.The boundary is delineated by teleseismic epicenters, although there are fewer epicenters bounding the eastern third of the Arabian plate south Oman.Most seismicity occurs in the crustal part of the Arabian plate beneath the Zagros folded belt (Jakson and Fitch, 1981).The Zagros is prolific source of large magnitude earthquakes with numerous magnitudes 7 and above even occurring in the last few decades.The overall lack of seismicity in the interior of the Arabian peninsula suggests that little internal deformation of the Arabian plate is presently occurring.There is widespread Quaternary volcanism along the Red sea coast, with at least one documented historical eruption in 1256 A.D (Barazangi.1981).Some seismicity was associated with that eruption.Seismicity may also be related to transform faults in the Red sea continuing onto land.To date, few on-land epicenters are accurately located and there is little mechanism available.DATA USED The Arabian peninsula area under study extending between 10 o N and 35 o N, and 30 o E and 60 o E. Catalogs are prepared for earthquakes (magnitude >4) using the epicentral data from the Earthquake Database of the Arabian peninsula issued at (1999) by Seismic Studies Center-King Saud University, Saudi Arabia kingdom.Ambrasey's (1988) catalogue was the main source of seismic data prior to 1965, and from 1965 on wards on the PDE,ISC and Ambraseys (1988).A time period of observation of approximately 100 years (1900-1999) for the seismic data was used in the calculations of the required seismicity parameters.The number of earthquakes falling in each 2 o by 2 o grid during this period is determined.Data completeness test in the study area was not conducted since some source areas used in the study do not have sufficient number of events.P-ISSN 1991-8941 E-ISSN 2706-6703 Journal of University of Anbar for Pure Science (JUAPS) occurrence of earthquakes in any area for a fixed interval of time can be expressed as a function of magnitude: Log Nc = a -b M……………………….(1)Where Nc is the cumulative number of earthquake with magnitudes equal to and greater than M, and a and b are constants.The constant a exhibits significant variations from study to study as it depends on the period of observation, as well as on the area of the region under investigation, while constant b is characteristic of the region, is correlated to the tectonic structures and to the seismic activity in the given area.Miyamura (1962) re-evaluated the seismicity of the world and found out that parameter b is related to tectonic structure.Udias (1977) Correlated the low b -value with heterogeneous physical conditions in the corresponding crustal source region.Wyss (1973) pointed out that the bvalue increases as the degree of symmetry of the applied stress decreases.The values of the seismicity parameters (a , b) were determined in space in a given period of time.In this regard, the maximum likelihood method from Utsu (1965) and Aki (1965) was applied as follows: b = log (e)/ (Ma -Mo)………………… ………… ( 2 ) a = log (N) + b Mo…………………………….. = the number of events equal and larger than Mo in a set data. e = the base of natural logarithm.
Figure 2 represents a seismotectonic map of the Arabian Peninsula comprising a -value contours.This map brings out a number of a -value highs, these highs are associated with the seismotectonic belts of the Arabian Peninsula: Zagros Folded zone, Dead Sea system and Gulf of Aden.In Dead Sea rift system, there is a localized high zone centered around Gulf of Aqabah has a maximum a -value of 7.5 and enclosed by the contour of 6.5 .In Gulf of Aden high zone, there are two localized high, firstly, is centered around (12.8 o N, 47.33 o E) and enclosed by contour of 5, and secondly, is centered around (15.2 o N, 56.73 o E) and enclosed by contour of 6.

a
given area and to seismic activity.The constant a which can be considered as an index of seismicity gives the number of events in a given sample with given b and minumum magnitude and therefor, a function of b.Since the parameter a is dependent on parameter b, it is expected that the contour shapes shall be quite similar.Referring to our a and b value maps (Figs.2 and 3), we find that there are similar shapes in most seismic zones such as zagros seismic zone, Gulf of Aden and Arab sea, Red sea and Dead sea transform.

Fig. 2 .Fig. 3 .
Fig.2.Quantitative seismicity map of Arabian peninsula based on a values contour values represent a corresponding to a 20 X 20 area and 100-year earthquake observation period.

Fig. 4 .
Fig.4.Quantitative seismicity map of Arabian peninsula based on estimated maximum magnitude.