Earthquake-induced flooding and paleoseismicity of the El Asnam, Algeria, fault-related fold

Late Quaternary activity of the El Asnam thrust fault (responsible for the October 10, 1980, M(s) = 7.3 earthquake and 36-km coseismic faulting) is explored through diverse paleoseismological field methods, We trenched through the main and secondary 1980 coseismic scarps, made an analysis of earthquake-induced flood deposits, conducted a geomorphic study of leveling profiles and offset geological units across thrust and normal scarps, and considered the geomorphology of the active fold. Twenty-five radiocarbon ages from colluvial and alluvial deposits, along with conventional (fossils and artefacts) dating of geological units, show the timing of successive net vertical displacements on the thrust-related fold and bending-moment faults, Previous work showed that eight pre-1980 earthquakes occurred in clusters during the late Holocene. Cumulative net vertical slip that amounts to 3 times the 1980 displacement is observed on multiple thrust fault scarps and offsets of geological units on bending-moment faults, Maximum cumulative vertical displacements, which are visible in different zones along the fault segments, can reach 10 to 25 times the 1980 coseismic displacements, The late Quaternary uplift rate on the El Asnam fault is 0.6 mm/yr, and taking into account slip variation along the fault, this rate may range between 0.25 - 1.6 mm/yr, from which we obtain a shortening rate of 0.17 - 1.2 mm/yr, Because the Tell Atlas mountains exhibit comparable seismogenic fault-related folds with parallel and sequential structures of active faults, a total average shortening rate of 2.05 mm/yr during the late Quaternary represents part of the convergence accommodated along the Africa-Eurasia plate boundary.