Integrated geologic and geophysical studies of north unstable shelf seismicity, Egypt
The large impact of moderate earthquakes in unstable shelf of north Egypt on human society provides a strong motivation to study and understand the systematics of their occurrence. In this study, we examine the correlation of north Egypt unstable shelf earthquakes using five geologic and geophysical data sets: a newly compiled age-province map, Bouguer gravity data, aeromagnetic anomalies, tectonic stress field, and GPS velocity rate measurements. Based on the qualitative and quantitative interpretation of these five data sets, we inferred that (1) although surface features disappeared of unstable shelf crust, Phanerozoic crust shows clear correlation of crustal age and earthquake frequency. (2) The seismic ages are during Precambrian and Paleozoic unstable shelf crust. (3) Seismicity is correlated with the major tectonic events in the geologic history of Egypt. Unstable shelf seismicity mainly (1) follows the NW–SE lineaments and (2) forms clusters at what have been termed stress concentrators (e.g., intersecting faults and igneous intrusions) at EN–WS and E–W structural trends. The correlation of seismicity with NW–SE-oriented lineaments implies that the unstable shelf seismicity is related to the accretion and rifting processes that have formed the Red Sea and Gulf of Suez which still records active rifting. An analysis of hypocentral depths for unstable shelf earthquakes shows that the frequency and moment magnitude of events are nearly uniform for the entire 5–30 km depths over which crustal earthquakes extend. We appreciate that the deep structure of the crust, in particular the existence of deeply penetrating faults, is the controlling parameter, rather than lateral variations in rheology or high pore pressure. We conclude that the distribution of the unstable shelf earthquakes in north Egypt is consistent with the existence of deeply penetrating crustal faults that have been reactivated in the present stress field. Future earthquakes may occur anywhere along the geophysical lineations that we have identified. This implies that seismic hazard is more widespread in the Nile Delta Basin and Cairo province of north Egypt than indicated by the distribution of limited and inaccurate historical seismicity.