A seismotectonic study of northeastern Tunisia

Two molassic basins corresponding to the Basse and Moyenne Medjerda have be en individualized at the front of the Alpine mountain chain of Tunisia. In these basins, the recent tectonics is manifest in the compressive structure s locally reactivated during the historic period. These structures are cons istent with a tectonic model corresponding to a submeridien shortening. The Quaternary to historic tectonics is still active today as it can be seen i n the relatively important seismic activity which characterizes these basin s 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 impor tant historical seismic events. The seismicity around the Ichkeul Lake is d iffuse, and earthquakes happen on a system of faults with various direction s. However, we distinguish two types of seismicity, probably due to the tan gentiel faults around the thrust sheet and a seismicity linked to the subve rtical faults of the autochthonous area. The depth of the hypocentres of the earthquakes of the Ichkeul Lake is ofte n 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 th e 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 o f compression NW-SE to NNE-SSW coherent with the global compressive movemen t that characterizes the actual bringing together of the Africa-Europe plat es.