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Seismicity and seismic hazard in Alexandria (Egypt) and its surroundings

机译：亚历山大（埃及）及其周围地区的地震活动和地震灾害

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摘要

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 (347degrees), dip (29degrees) and slip (125degrees). The second subevent occurred at a shallower depth (24 km) and has a relatively different focal parameter (strike 334degrees, dip 60degrees and slip 60degrees). 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).
机译：在历史和近代，亚历山大市都遭受了远近地震的巨大破坏。有时，此类损害的来源尚不清楚。红海，亚喀巴湾-死海希腊弧，苏伊士-开罗-亚历山大，东地中海-开罗-Faiyoum和埃及沿海地区等地震区位于该城市附近。埃及沿海地区的地震活动度最低，因此其构造背景尚不为人所知。1998年埃及沿海地区地震是中等复杂的震源。它由相隔4秒的两个子事件组成。第一次子事件始于28 km的深度，并导致走向（347度），倾角（29度）和滑移（125度）破裂。第二个子事件发生在较浅的深度（24 km），并且具有相对不同的震源参数（打击334度，倾角60度和滑移60度）。可用的震源机制有力地支持了从希腊弧到亚历山大近海地区的复杂应力状态的体现。在本研究中，采用了一种数值建模技术来估算亚历山大地区的定量地震危险。在地震灾害方面，无论是地方性地震还是边远地震都对该城市产生巨大影响。离岸地区发生M-s = 6.7级的局部地震，使地面峰值加速度高达300 cm / sec（2）。预计此类地震造成的总晃动时间约为3秒。地面加速度的傅立叶振幅谱表明，最大能量是由地震波的一部分低频（1-3 Hz）承载的。亚历山大市最大的响应频谱在此频带内。尽管持续时间长（长达3分钟），但由于希腊弧，红海和亚喀巴湾大地震造成的地面加速度计算值很小（小于10 cm / sec（2））。

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