Active tectonics of the Algerian Atlas Mountains‐evidence from aftershocks of the 1980 El Asnam earthquake

Summary The El Asnam (Algeria) earthquake of 1980 October is the largest earthquake to have occurred in North Africa since instrumental records began (Ms = 7.3). It was caused by movement on a segmented reverse fault which is part of the fault zone that separates the Chelif alluvial plains from the coastal range of the Atlas Mountains. The coastal range is the actively deforming plate boundary between Africa and Eurasia. In this paper, the results of an aftershock study carried out using a 28‐station portable network in the epicentral area in the 5 weeks after the main shock are presented. A total of 4517 aftershocks have been located, using both P‐ and S‐phases. Of these 4517 events, 1279 pass a set of quality criteria, and are used to discuss the detailed tectonics of the aftershock zone. The southwestern half of the aftershock zone has a relatively restricted band of seismicity running parallel to the main fault. In cross‐section, this seismicity is observed to lie almost exclusively in the footwall of the main fault, and indicates both antithetic reverse faulting and extension parallel to the fault zone. The only exception to this pattern occurs at a junction of the main fault segments, where a distinct cluster of events with very variable focal mechanisms is located in the hangingwall. This cluster may be equated with a barrier that impeded the rupture of the main shock. Beyond the southwest end of the main fault, a mixture of strike‐slip and reverse faulting occurred, and suggests a mechanism whereby this end of the fault can act as an asperity for earthquake generation. Aftershocks in the northeastern half of the study area indicate movement on a stack of listric reverse faults. The focal mechanisms of these events show that the N‐dipping nodal planes (assumed to be the fault planes) flatten with depth. We suggest that the faults sole into a low‐angle (20°) decollement at about 8‐10 km depth, at the base of the aftershock zone. We argue that the northeastern part of the 1980 aftershock zone is typical of the actively deforming coastal belt of northwest Algeria. Subduction of the western Mediterranean basin beneath the north African margin does not seem to occur. Instead, the margin is absorbing the motion, with old normal faults being reactivated as thrusts.