Abstract

The El Asnam earthquake (MLH = 7.2; MLV = 7.3; MPV = 6.5) occurred at 12:24:24 G.M.T. on 10 October 1980 in the central part of the Chelif valley in Algeria affecting a rather densely populated region of about 900 000 within 7000 km2. (Ambraseys 1980). It destroyed at least 25 000 housing units and rendered 300 000 people homeless. The main shock, which killed about 2500 and injured more than 7000 people, was felt as far away as Spain 550 km distant. It was followed by a damaging aftershock 3 hr later (MLV = 6.1; MPV = 5.9). No foreshocks were recorded, but other phenomena have been reported which could have been premonitory (Fig. 1a).

The El Asnam earthquake is the largest event in the Atlas since 1790, and in the Chelif valley since 1716. For the Atlas as a whole, the moment rate calculated from long-term observations is only 1025 dyn-cm/year. This corresponds to a slip rate of only 0.7 mm/year which is a small fraction of the value calculated from plate motions, implying that much of the deformation of the Atlas occurs aseismically and that the tectonic activity of the region is not fully reflected by its short-term seismicity. This is a serious handicap in the assessment of the earthquake hazard in the Atlas. The frequency distribution of events above magnitude 4 3/4 was found to be f(M) = 2.2 exp(-2.2M), with a probable cut-off at about M = 7 1/4 to which corresponds a return period

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