Late Holocene earthquake sequences on the El Asnam (Algeria) thrust fault

Abstract

Geological analysis of eight trench exposures through the El Asnam fault zone, reactivated during the October 10, 1980 earthquake ( M = 7.3), shows various tectono-sedimentary structures associated with large seismic events in the past. Situated on the southeastern flank of the active Sara El Maarouf fold, the trenches cut across the main 1980 fault traces and also across a secondary normal fault (bending-moment or extrados fault). The erosional action of the Cheliff and Fodda rivers on the active fault, during the late Quaternary, formed a large gorge and for each coseismic incremental growth of the anticline, a thrust fault scarp rises in its path and stops the rivers flow. An earthquake-induced flood, caused by damming of the rivers, occurred just after the 1980 event. A stratigraphic section [7] in the flood area exhibited 6 flood horizons, each of which may represent an earthquake of magnitude greater than 7. Close to this area, buried fault scarps with cumulative vertical movements and 14C ages provide an average uplift rate of 0.6 mm y −1. Vertical displacements in 1980 were not constant along the fault trend; according to the trench locations, the estimated uplift rate may be considered as a minimum value for the El Asnam fault. Tectonic and sedimentary data suggest that eight large unevenly distributed earthquakes occurred prior to 1980, and characterize the late Holocene seismic activity. Clusters of large seismic events with M > 7 appear around 4000 years B.P. and also during the last 1000 years, which is in agreement with previous evaluations [7]. The recurrence interval of large earthquakes is about 720 years, but during seismic sequences this recurrence interval decreases and varies between 300 and 500 years.