Postseismic deformation at El Asnam (Algeria) in the seismotectonic context of northwestern Algeria

Abstract

We use a combination of seismicity, tectonic features, focal mechanisms, seismic strain and postseismic movement to study the western part of North Algeria, the El Asnam region and its surrounding area in particular. A seismotectonic map of this part of Algeria, delimited by the Mediterranean Sea in the north and the Tellian mountains in the south, was built from available geological and seismological data. An examination of this map shows that the most significant earthquakes are concentrated along tectonic features and quaternary basins elongated in an east-west direction, suggesting NNW-SSE compressional movements. During the large El Asnam earthquake of 1980 October 10, Mw = 7.1, vertical movement was measured along a 40 km northeast-southwest thrust fault. These movements were determined geodetically in 1981 with reference to a basic network previously measured in 1976. In order to control postseismic movement and to ensure the monitoring of the seismic area, a dense geodetic network has been regularly measured since 1986, both in planemetry and altimetry. The results of the altimetric remeasurements show significant vertical movements. The elevation changes of the benchmarks have been deduced from precise levelling measurements: a remarkable uplift (5.1 ± 1.9 mm yr-1) of the northwestern block, during the 1986-91 period has been observed, whereas the southeastern block is seen to be relatively stable. The Sar El Maârouf anticline, situated along the central segment of the El Asnam surface breaks, appears to be growing with a maximum postseismic slip rate of (9.6 ± 1.4 mm yr-1). The mean uplift rates computed for the northwestern block support the view that the 1954 earthquake did not occur on the same reverse fault as the 1980 event.