Sam Wimpenny , Timothy J Craig, Alice Blackwell

Earthquakes beneath the foreland basins of the Andes and Tibet follow a simple pattern, with normal-faulting events from 0--20 km depth and reverse-faulting events from 30--50 km depth. The switch in faulting style with depth suggests that the elastic stresses generated by flexure within these forelands are large enough to break faults, with opposite senses of horizontal strain either side of a neutral fibre in the mid-crust. In this study, we document a 31 km-deep Mw 5.2 normal-faulting earthquake in the forelands of the Algerian Atlas Mountains near Biskra. The Biskra earthquake is of interest, as it indicates that the lower crust of the Atlas forelands is seismogenic and in extension at the same depth that the Tibetan and Andean forelands are in compression. In order to match the shape of the gravity anomaly and the depth of normal faulting in the Algerian foreland, we find that models of lithospheric flexure require the neutral fibre to be >35 km deep in places and at least the top 5--10 km of the lithospheric mantle supports elastic stresses without yielding. The differences in the depth-extent of normal-faulting earthquakes between the forelands of Tibet, the Andes and the Algerian Atlas can be explained solely by differences in the buoyancy forces acting between these mountain ranges and their lowlands that place the foreland lithosphere into varying amounts of net compression. The upper mantle beneath cratonic foreland lithosphere may therefore support bending stresses of the order of 10's of MPa, likely because it is cool and the strain rates associated with bending are low.