Kinematic and Elastic Modeling of Fault-Related-Folds: A Comparison Between El Asnam and the Sahel Active Structures

Active fault-related folds with surfacing, hidden, or blind thrust are tectonic structures capable of generating large-magnitude earthquakes (Mw > 6.5). We study active folds using geological and geomorphological field observations coupled with seismic parameters of moderate to large earthquakes of the central Tell Atlas (Algeria). Case studies are taken from the El Asnam fault-related fold responsible for the 1980 large earthquake (Mw 7.1) and the coastal Quaternary Sahel anticline with an active crustal structure tectonic geomorphology. Field investigations in Plio-Quaternary geology and tectonic geomorphology are conducted using high-resolution Pleiades correlated with the active tectonic features of each folded structure. Fault parameters are taken from the fault-related fold dimensions and geometry at the surface and depth using the seismicity distribution and focal mechanisms. El Asnam case study provides data on (i) coseismic surface folding and its relationships to the 1980 earthquake faulting at depth and (ii) coseismic rupture geometry and related seismogenic depth using the mainshock and aftershock distributions. The coastal Sahel active fold affects late Quaternary marine and alluvial terraces and may be related to a blind or hidden fault. The folding and faulting analysis compares the elastic modeling in isotropic homogeneous half-space (Okada, Bull Seismol Soc Am 82:1018–1040, 1992) with the kinematic modeling (Allmendinger, Tectonics 17(4):640–656, 1998). Modeling approaches suggest 1 ± 0.2 mm/year shortening across folds, and the comparative study of the fault rupture at depth and surface shows the seismogenic potential of active folds.