SEISMOTECTONICS IN THE TELL-ATLAS OF ALGERIA - THE CAVAIGNAC (ABOU-EL-HASSAN) EARTHQUAKE OF 25.08.1922 (M(S)=5.9)

We have studied the Tenes-Abou El Hassan (TAEH) active fold and its relationships with the 1922 earthquake (M = 6.0) in terms of repeated coseismic deformations in compressional domains. The Abou El Hassan village (formerly Cavaignac) experienced a destructive but moderate-sized earthquake, which was registered in several seismic stations, and generated surface breaks and rock falls located on the active fold. Recent field observations reveal the existence of a N060 degrees-striking fault scarp, with a vertical displacement of 1 m affecting young alluvial terraces. Parallel to the fault, the NE-SW-striking active fold with an asymmetrical shape and transport direction to the southeast, displays on his southeastern flank a 23 km long reverse faulting with flexural slip. A complex drainage pattern including the Oued Allalah river which predates the anticline, marks the successive tectonic movements (uplifted late Quaternary terraces), and yield an uplift rate of 0.5 mm yr(-1). Kinematic aspects of broken folds are illustrated showing that the TAEH active fold conforms the fault-bend folding model. A seismic moment M(0) = 1.36 10(25) dyn cm of the 1992 main shock, which triggered aftershocks for more than one year, is estimated from the geological parameters. Three segments are identified along strike, indicating some evidence for the occurrence of possible large earthquakes in the past. From the seismic hazard point of view, the TAEH fault segments could produce a larger earthquake magnitude M>6.0 as is derived from the coseismic faulting, the total fault length and vertical displacement of young terraces. Such seismogenic structure is very comparable to the southern one at El Asnam that produced the large 1980 earthquake (M(s) = 7.3).