[1] A strong earthquake (Mw 6.8) struck the coastal region east of Algiers and the Tell Atlas of Algeria on 21 May, 2003 and was responsible of severe damage and about 2400 casualties. The coastal mainshock was followed by a large number of aftershocks, the largest reaching Mw 5.8 on 27 May 2003. We study the mainshock, first major aftershocks and about 900 events recorded by temporary seismic stations using master-event approach and doubledifference (DD) methods. Although the seismic station array has a large gap coverage, the DD algorithm provides with an accurate aftershocks location. The mainshock hypocenter relocation is determined using three major aftershocks (5.0 Mw 5.8) chosen as master events. The new mainshock location shifted on the coastline (36.83N, 3.65E) at 8 -10 km depth. Seismic events extend to about 16-km-depth and form a N 55°-60°E trending and 45°-55°SE dipping fault geometry. Up to now, it is the unique among the recently studied seismic events of the Tell Atlas of Algeria. Mainshock and aftershocks relocation, the thrust focal mechanism (Harvard CMT: N 57°, 44°SE dip, 71 rake) and the seismic moment 2.86 10 19 Nm, infer a 50-km-long fault rupture that may appear at the sea bottom at 6 to 12 km offshore north of the coastline. The Zemmouri earthquake occurred along the complex thrust-and-fold system of the Tell Atlas and provides with new constraints on the earthquake hazard evaluation in northern Algeria.
Site response characteristics at seismic stations were investigated using horizontal-to-vertical (H/V) spectral ratios calculated from a seismic-motion dataset of the 2003 Boumerdes earthquake, and transfer functions were evaluated from soil profile data. Although high peak ground acceleration (PGA) values were recorded at some sites, the nonlinear effect at these stations was not clear. The H/V spectral ratios calculated from weak and strong motion events did not show a clear difference in the predominant period and amplitudes, and the shapes of the H/V ratios were flat for some stations. These observations are characteristic of the presence of firm to hard layers under the stations; however, one station was located on Quaternary deposits showed a remarkable amplification at the predominant period and a high PGA value.
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