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NEW ASPECTS ON ESTIMATING EARTHQUAKE RISK AROUND MAKKAH REGION

 

AL-FURAIH, ALI A., AL-ASWAD, AHMAD A., and KEBEASY, RASHAD M.

 

Seismologcal-Geophysical Observatory, King Saud University,

P.O. Box 2455, Riyadh 11451, Saudi Arabia

 

 

ABSTRACT

Due to the worldwide importance of Makkah region, available information of historical and recent earthquakes occurred within 500 km around it was collected from local and international sources. The Geology of the region was also reviewed.

In this work two seismogenic zones were identified to be the source of earthquake generation in the region. The first one was found in the Central Red Sea. It is characterized by large active faults capable of generating large and shallow earthquakes. Recurrency studies show that the maximum intense earthquake can reach 7.4 on Richter scale with return period of one time every one hundred years. The effect of such earthquake on :Makkah region will be completely masked by the thick rock salt layer present along the Red Sea Coast. Rock salts in this region are mostly saturated with water and acts as visco-elastic damper. Evidences are the 1967 and 1993 Red Sea earthquakes with body wave magnitudes of 6.7 and 5.8 respectively. Although their epicenters were about 100 krn from the coast, the two events were not felt, at all, in any coastal cities. Thus the risk is minimal.

The second seismogenic zone is located between Jeddah, Makkah and Taif and characterized by several sets of faults and lineaments oriented mainly either parallel to the Red Sea or northeast-southwest, Micro-to small earthquakes are observed along these faults. The largest recorded event had 4.1 unit of magnitude and was felt strongly due to its shallow focus. Recurrency could not be determined for this zone due to the inhomogeneity of the data. However, in the period between 640 to 1994 A.D. we found

 
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