Seismological aspects of the Cairo earthquake, 12th October 1992

Abstract

On 12 October 1992 a significant earthquake (MB = 5.8) occurred southwest of Cairo in the vicinity of the Dahshour region. For the mainshock, an average moment release M 0 = 9.70E17 n-m; a fault length = 13.8 km; an average displacement = 0.22 m; a stress drop = 0.760 MPa and maximum spectral magnitude = 5.78 were obtained. Spectral magnitude calculations were used in this analysis. The distribution of the well-recorded aftershocks over 15 months using a temporary seismic network installed immediately after the mainshock shows a zone of concentrated activity. Three composite focal mechanism solutions are constructed, using P wave polarity data for 30 earthquakes. These solutions have a mechanism, involving normal and right lateral strike slip motion along E-W to ENE-WSW trending fault plane. The P-wave polarity data of the individual 30 earthquakes are inverted to determine the stress tensor. The stress field estimated is extensional with s3 in the direction of NNE. The stress pattern determined from the inversion is in good agreement with the one estimated from the three composite fault plane solutions, while the neotectonic situation in Northern Egypt supports ENE-WSW extensional movement. However, the region of study has suffered both extensional tectonic activity of the Northern Red Sea rift zone and the compressional tectonic activity along the Hellenic arc. The inconsistency of the principal tension directions may belong to the interaction between the extensional and compressioal tectonics.