of EgyptSeismicityof EgyptSeismicity

BadawyAhmed

National Research Institute of Astronomy and Geophysics

,Cairo, EGYPTHelwan11421 -

Abstract.

Spatial distribution of earthquake epicenters indicates that Egypt has been suffered from both interplate and intraplate earthquakes. Most earthquakes activity has been concentrated in northern Egypt along the geologically documented borders of Sinai subplate (northern Red Sea and its two branches Suez rift and Aqaba –Dead Sea transform). Away from this relatively high seismic zone, inland seismic active dislocations (e.g. Dahshour, Aswan, Abo-Dabbab and Abo-Simple dislocations) are also reported. As a matter of fact the installation of new Egyptian National seismic Network (ENSN) since the end of 1997has significantly enhanced the old picture of seismic activity across Egypt and Red Sea. New seismically active zones have been discovered for the first time (e.g. southeast Beni-Suef and west Sohag). Moreover, old aseismic areas are now appearing as seismically active regions. For example, what used to be determined a seismic gap in seismicity between the southern Red Sea and northernmost Red Sea; there is a significant observed seismicity off-axis in the central and northern Red Sea along and outside of the rift axis. The extensive ENSN seismicity database will help to constrain hypotheses for the role of plate boundaries across the Middle East region (Sea floor spreading in southern Red Sea, Zagros collision and subduction in the

Hellenic trench) ..

Focal mechanism solutions and neotectonic studies including recent GPS observations indicate that the Egyptian territory is characterized extensional deformation. The present-day stress field in Egypt, as inferred from earthquake focal mechanisms and borehole breakouts, is dominated by transtensional stress regime, the trajectory of which changes with depth, at least in the Gulf of Suez. This means that Red Sea rift is an important tectonic zone in the region and represents to some degree the reactivation of old Precambrian sutures

Conclusions

Historical and instrumental earthquake catalogs have been used to re-evaluate the seismicity of Egypt. Focal mechanism solutions of 76 earthquakes have been used for better understanding of Seismotectonics of Egypt as well. The following conclusions can be drowning:

•A general concentration of the historical activity is quite clear around the Nile Valley and Nile Delta. These areas are densely inhabited, but the presence of thick sediments is likely the main cause of the widespread felt areas.

•The historical and instrumental earthquakes are, concentrated in northern Egypt, distributed in relatively similar ways indicating the same areas have been active since many centuries.

•Egypt has suffered from both interplate and intraplate earthquakes. Indeed, the intraplate earthquakes are less frequent, however, they are greatly expand the region of seismic hazard in Egypt (e.g. 12th October, 1992).

•Earthquake focal mechanisms reveal normal faulting with strike-slip component reflecting that the present-day stress filed in Egypt has dominated by transtensional stress regime with maximum horizontal extensional NNE stress axis.

•Earthquake catalogs of Egypt may having all information for earthquakes of M3.5 from 1900 up to present, however, for smaller events (M<3.0) only the last two decades should be considered.

Finally the deployment of local networks (Aswan, ENSN) are highly enhanced the old seismicity picture, particularly for micoearthquakes. New seismic active areas are discovered in Egypt for the first time (e.g. west Sohag, southeast Beni-Suef).

Egyptian National Seismological Network (1997)

Seismic stations in Egypt before 1997

Historical seismicity of Egypt from 2200BC to 1899AD

Small and Moderate earthquake distribution in Egypt before 1997 (A) and after 1997 (B) Focal mechanisms of selected earthquakes in Egypt from

1900-2003

Seismicity of Egypt and Eastern Mediterranean region from 1964-2003 (ISC data file)

Micro-earthquake activity in Egypt pre-and post ENSN installation, A,B respectively.

National Research Institute Of Astronomy and Geophysics (NRIAG)