Status of the crustal stress in Egypt as inferred from earthquake focal mechanisms and borehole breakouts

Abstract

The recent stress field has been investigated by the analysis of the directions of maximum horizontal stress ( σ H) as derived from earthquake focal mechanisms and borehole breakouts in Egypt. The results indicate that, in the southern part of the Gulf of Suez, the shallow (borehole breakouts) stress directions are not consistent with the deep (earthquakes focal mechanisms) stress directions. Analysis of 30 borehole breakouts measurements indicate WNW–ESE alignment of the maximum horizontal stress with a vector mean of 110°±14°(±P.E.). However, the earthquake studies, including orientation of P axes, reflect ENE–WSW alignment of σ H with a vector mean of 66°±30°. Thus, the stress directions are not constant throughout the crust. Along the Gulf of Aqaba, the southern part of the Dead Sea Fault, the focal mechanism solutions indicate that the maximum horizontal stress is presently NW–SE oriented corresponding to strike-slip mechanism in concert with the geological evidence. In northern Egypt, the nine earthquake focal mechanisms for the Nile Delta, Cairo-Bitter lakes region scatter in orientation, varying from NE–SW to NW–SE for σ H. About 45% from these mechanisms indicate NW–SE, 22% WNW–ESE, 22% ENE–WSW and 11% NE–SW alignment of the maximum horizontal stress. While in southern Egypt it is relatively uniform and aligned to nearly E–W direction with a vector mean of 95°±3°.