Seismicity and Seismic Hazard in Alexandria (Egypt) and its Surroundings

Abstract

Alexandria City has suffered great damage due to earthquakes from near and distant sources, both in historical and recent times. Sometimes the source of such damages is not well known. Seismogenic zones such as the Red Sea, Gulf of Aqaba-Dead Sea Hellenic Arc, Suez-Cairo-Alexandria, Eastern-Mediterranean-Cairo-Faiyoum and the Egyptian costal area are located in the vicinity of this city. The Egyptian coastal zone has the lowest seismicity, and therefore, its tectonic setting is not well known.The 1998 Egyptian costal zone earthquake is a moderate complex source. It is composed of two subevents separated by 4 sec. The first subevent initiated at a depth of 28 km and caused a rupture of strike (347°), dip (29°) and slip (125°). The second subevent occurred at a shallower depth (24 km) and has a relatively different focal parameter (strike 334°, dip 60°and slip 60°). The available focal mechanisms strongly support the manifestation of a complex stress regime from the Hellenic Arc into the Alexandria offshore area.In the present study a numerical modeling technique is applied to estimate quantitative seismic hazard in Alexandria. In terms of seismic hazard, both local and remote earthquakes have a tremendous affect on this city. A local earthquake with magnitude M s = 6.7 at the offshore area gives peak ground acceleration up to 300 cm/sec 2 . The total duration of shaking expected from such an earthquake is about three seconds. The Fourier amplitude spectra of the ground acceleration reveals that the maximum energy is carried by the low frequency (1-3 Hz), part of the seismic waves. The largest response spectra at Alexandria city is within this frequency band. The computed ground accelerations due to strong earthquakes in the Hellenic Arc, Red Sea and Gulf of Aqaba are very small (less than 10 cm/sec 2 ) although with long duration (up to 3 minutes).

Effects of surface geology on the ground-motion at New Borg El-Arab City, Alexandria, Northern Egypt

“…These are referred to as the Baltim, Temsah and Rosetta trends, respectively (Abdel Aal et al, 1994). Also, the study area is located near to the Nile Delta cone which is considered as a large hinge zone that consists of several southward half-grabens (El-Sayed et al, 2004) as shown in Fig. 4.…”

“…About 14 of these earthquakes were located mainly in the Hellenic Arc region with relatively large magnitudes. The other nine earthquakes are located off-shore of Alexandria City with moderate magnitude (El-Sayed et al, 2004). In terms of seismic risk, the unconsolidated sediment materials could amplify ground motion during earthquakes and this causes building on these grounds to become more vulnerable to earthquake damage than those built above the hard rocks.…”

Simulation of tsunami generation, propagation and coastal inundation in the Eastern Mediterranean

“…Papadopoulos and Chalkis, 1984;Papazachos and Papazachou, 1998;Soloviev et al, 2000;Papadopoulos, 2003;El-Sayed et al, 2004;Tinti et al, 2004;Papadopoulos and Fokaefs, 2005;Stefatos et al, 2006;Papadopoulos et al, 2014). The Aegean Sea and its surrounding areas, in particular, are not only the most active Mediterranean regions in terms of seismicity and tectonic movements, but their coastlines have also experienced numerous tsunami events in recent, historic and pre-historic times.…”

nhess-2018-62 Reply to RC2

“…Major historical tsunamis in the eastern Mediterranean region which affected northern Egypt are triggered by large earthquakes (Papadopoulos et al, 2014). However, there is a possibility of landslide triggered tsunamis associated with local earthquakes (El-Sayed et al, 2004;Tinti et al, 2005). Yalciner et al (2014) estimated from modelling that a landslide with a volume up to 500 km 3 may have caused a tsunami with a wave height ranging from 0.4 to 4 m offshore of the Nile Delta.…”