TECTONIC ACTIVITY AND HIGH SITE EFFECT NEAR CAIRO CITY, EGYPT

A. El-Sayed, R. Arvidsson & Ota Kulh nek (Seismological Department, Uppsala University Box 2101, S 750 02 Uppsala, Sweden)

 

On October 12, 1992, an earthquake with magnitude mb = 5.9 and Ms = 5.2 occurred close to Cairo city, Egypt. It was one of the largest earthquakes in Egypt during this century. Destruction associated with this earthquake is concentrated to Cairo city, the Nile Valley and the Nile Delta areas. Similar destruction due to near as well as rather distant earthquakes were reported in Cairo city in the historical time. The return period for events with mb > 5 in Cairo area is 100-200 years. The 1992 earthquake shows a normal faulting mechanism with a strike slip component. The mechanism is compared with those corresponding to two events that occurred previously at the most northern part of the Red Sea.. The similarity between the mechanisms as well as the spatial distribution of the geological faults around Cairo city suggest an existence of seismic activity along the extension of the Red Sea rift system to the neighbourhood of Cairo. Consequently, the level of hazard in Cairo is relatively high. Synthetic accelerograms calculated for the 1992 earthquake, show maximum acceleration of 0.55g at an epicentral distance of 10 km, but attenuate rather rapidly with distance. The value of 0.55g is relatively large with respect to the size of the earthquake and is a clear effect of loose sedimentary layer on top of the crust beneath the meizoseismal area. By ignoring the loose sedimentary layer, the maximum peak ground acceleration dropped to 0.35g at the same distance. The similarity between areas of strong shaking for the 1303 and 1992 Cairo 1992 earthquakes lead to a conclusion of proximity of hypocenters of the two events.