The 21 May 2003 Zemmouri (Algeria) earthquake Mw 6.8: Relocation and aftershock sequence analysis

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 double-difference ( 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 less than or equal to Mw less than or equal to 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 55degrees-60degreesE trending and 45degrees-55degreesSE 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 57degrees, 44degreesSE 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.