The Beni-Ourtilane-Tachaouaft fault and Seismotectonic aspects of the Babors region (NE of Algeria)

A shallow moderate (Ms=5.7) but damaging earthquake shook theregion of Beni-Ourtilane located about 50 km NW of Setif and 390 kmNE of Algiers (Central Eastern Algeria). The main shock caused the deathof 2 peoples, injured 50 and caused sustainable damage to about 3000housing units. The main shock was preceded by 2 foreshocks and followedby many aftershocks which lasted for many days. Analysis of historicalseismicity including the localisation of epicenters, the trend of isoseismalmaps of some historical events, the localisation of the November 10, 2000main shock (Ms=5.7) and the November 16, 2000 aftershock(Ms=4.5) as well as the shape of the area of maximum intensity ofthe November 10, 2000 earthquake suggest that the Tachaouaft fault of20 km of length is the activated geological structure. Although, there isno clear surface breaks associated with this earthquake, the localisation ofgeological disorders, such as ground fissures, during the Beni-Ourtilaneearthquake, which are remarkably located near the fault, may have atectonic meaning. Geomorphological analysis through Digital ElevationModels (DEMs) allowed us to identify a clear fault scarp related likely tostrong earthquakes occurred in the past. Among geomorphologicalevidences of this active fault there are the uplift and tilt of alluvial terraceson the hanging wall and the diversion of the drainage pattern. Based onthe quality of constructions and field observations an intensity I0 = VII (MSK scale) is attributed to the epicentral area,which is striking NE-SW in agreement with the focal mechanism solutionand the seismotectonic observations. In the other hand the amount ofdamage is due rather to the bad quality of constructions than to theseverity of ground motion. The Tachaouaft fault with the Kherrata fault isthe main source of seismic hazard in the Babors region.