Seismicity and seismotectonics of the Red Sea Region

Abstract

The historical seismicity of the Red Sea region between latitudes 13 and 30° N and longitudes 32 and 46° E has been revised. It is found that during the last 1400 years, some 88 historical earthquakes have occurred with magnitudes M ≥ 5.6. Only two M = 7.0 are reported, one in each of the northern and southern Red Sea regions, followed by two and one 6.8 in the northern and southern regions, respectively. During the period 1960–2010, some 1310 instrumental earthquakes have occurred in the region with M ≥ 4.0. The largest (M = 7.1) occurred in the Gulf of Aqaba, followed by a 6.9 in the entrance of the Gulf of Suez, followed by a 6.7 and a 6.4 that occurred in the southern Red Sea region. All others had M ≤ 6.2. More than 90 % of the seismicity has occurred in the form of sequences and swarms and are believed to be volcanic related. Thus, the seismic hazard of the study region remains quite limited. Both northern and southern parts of the Red Sea region are of comparable seismic activities, while that of the central part is of lower activity. The historical and instrumental seismicity data show excellent correlation with the general tectonics of the study region. These also show that earthquake activity has fluctuated with time from one region to the other. In the Red Sea proper, more than 83 % of the seismic moment is associated with the regional faults of the axial trough with noticeable concentration of activity where the NE trending strike-slip faults cross this trough. Published fault plane solutions for the largest instrumental earthquakes of the last few decades show that both normal and strike-slip faults are active in support of a divergent environment between the Arabian and African plates. Shallow seismicity is a characteristic feature of this region. The data indicates that lithospheric deformations beneath the gulfs area are restricted to the upper part of the crust, while beneath the Red Sea south of latitude 27.8°, these appear to concentrate partially within the crust down to 10 km depth and much more within the uppermost mantle, mostly from 20.1 to 40 km depths. Very little earthquakes occur at depths deeper than 60 km. Frequency-magnitude results of both historic and instrumental data gave the expected b value close to 1.0. The calculated recurrence periods for earthquakes with M = 6, 6.5 and 7 are 30 ± 2 years, 150 ± 10 years and 600 ± 20 years, respectively. The data show that the threshold magnitude for the study region is 4.0, and slightly more for the axial trough.