Seismicity and Tectonics of the Red Sea and Western Arabia

Abstract

Search in a number of seismological sources revealed that in the period 1913–86 some 135 earthquakes (3 ⩽ M ⩽ 6.9) occurred in the Red Sea and western Arabia between latitudes 14° and 27.2° N. In the same period, 49 earthquakes (M ⩽ 6.9) and 247 earthquakes (M ⩽ 4.9) are reported to have occurred in the Gulfs of Suez and Aqaba respectively. Twenty-three historical earthquakes are reported to have been felt in the region with intensities IV-IX and a few more with unassigned intensities, during the period AD 627-1884. the epicentral distribution of both instrumental and historical data and their characteristics show a general correlation with the regional geology and tectonics. Sixty per cent of the total energy released from all instrumental earthquakes is released from earthquakes epicentred within the spreading zone. Concentrations of activity are seen where the spreading zone is intersected by the NE transform faults and where this zone shifts in direction. Much of the seismicity of this region is of the swarm type and volcanic-related. Both instrumental and historical data show an apparent lower seismicity in the northern Red Sea region between latitudes 22° and 27.2° N, but the whole region is of a noticeable seismic risk that is mostly associated with regional land faults, some of which are continuations of the Red Sea transform faults. the instrumental seismicity of the whole region conforms with the relation: log N = 4.23 - 0.53M. When historical data are considered, both datasets conform with the relation: logN = 4.1 -0.47M. the b constant correlates with the shield-type of environment and seems to increase gradually northwards to attain a value of 0.71 in the Gulf's region. This may be caused by either heterogeneity in the lithosphere or most probably asymmetry of the regional stress field in the vicinity of latitude 27.5° N or both causes.