The 1992 Cairo earthquake: A case study of a small destructive event

Abstract

On 12 October 1992, an earthquake, magnitude mb=5.9 and Ms=5.2, hit the City of Cairo, Egypt. It was this century's largest earthquake in northern Egypt with related destruction in the City of Cairo, the Nile Valley and the Nile Delta areas. Our source parameter determinations show that the 1992 earthquake had a normal faulting mechanism, seismic moment Mo=5.2 × 1017 Nm, centroid depth of 23 km and a source time function duration of 3 seconds. The mechanism is compared with those corresponding to two other events that occurred in the northern Red Sea. The similarity between the mechanisms as well as the spatial distribution of the geological faults around Cairo suggest seismic activity along the extension of the stress field of the Red Sea rift system to the area around the City of Cairo. This situation affects the level of seismic hazard in the Cairo area. The 1992 earthquake belongs to an unusual class of relatively small, Mw < 6.0, yet destructive earthquakes. The damage caused by these events is usually attributed to their shallow focal depth, < 5 km, and to amplification of seismic waves in the local soil beneath the damaged structures. However, the Cairo earthquake deviates from other events of this class since the focal depth was determined to be 23 km. We calculated synthetic accelerograms for the 1992 earthquake with the loose sediments observed in the Nile Valley, and show that this enhanced the amplitude of the acceleration by a factor of two. However, the determined accelerations, about 0.5 m s-2, cannot alone explain the relatively large amount of damaged structures. Hence, a major cause to the destruction is likely the poor state of construction of the Cairo buildings.