Abstract

Two molassic basins corresponding to the Basse and Moyenne Medjerda have been individualized at the front of the Alpine mountain chain of Tunisia. In these basins, the recent tectonics is manifest in the compressive structures locally reactivated during the historic period. These structures are consistent with a tectonic model corresponding to a submeridien shortening. The Quaternary to historic tectonics is still active today as it can be seen in the relatively important seismic activity which characterizes these basins during the 1920-1992 period of recordings. A microseismic campaign realized on the border of the molassic basin of the Basse Mejerda (Ichkeul Lake) has shown the existence of a relatively weak but permanent seismicity in a region which had, otherwise, known some important historical seismic events. The seismicity around the Ichkeul Lake is diffuse, and earthquakes happen on a system of faults with various directions. However, we distinguish two types of seismicity, probably due to the tangential faults around the thrust sheet and a seismicity linked to the subvertical faults of the autochthonous area. The depth of the hypocentres of the earthquakes of the Ichkeul Lake is often less than 8 kms, but some hypocentres can be at a depth exceeding 15 kms. Most of the hypocentres cluster below the subvertical faults which seem to affect as a whole the sedimentary autochthonous series and to extend to the brittle part of the crust. The focal mechanisms determined in the region of the Ichkeul Lake are often associated with the visible faults on the surface. They show a direction of compression NW-SE to NNE-SSW coherent with the global compressive movement that characterizes the actual bringing together of the Africa-Europe plates.

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