Hindu Kush intermediate-depth seismicity: A possible explanation for the preferential asymmetry displayed by river catchments within the corresponding epicentral area

Abstract

Some conceptual models proposed for explaining the Hindu Kush intermediate-depth seismicity assumed that a subhorizontal fracture was migrating laterally, from east toward west, along the strike of a subducted tectonic plate. The present study has identified a distinct cluster of earthquake hypocenters, likely corresponding to the advancing tip of the subhorizontal fracture. Those hypocenters belonged to a set of fairly moderate earthquakes (5.6 ≤ M ≤ 6.5), which had not been, so far, explicitly addressed in the Hindu Kush seismo-tectonic interpretations, and which were proven to be virtually the only events displaying a failure regime characteristic to a laterally propagating breakoff – namely, reverse-fault focal mechanisms with subhorizontal compression axes parallel to the slab strike. A separate, upper-positioned hypocenter cluster was noticed to include all the Mw ≥ 7 intermediate-depth earthquakes: it was conjectured, relying on a previously published numerical model, that those strong events could be triggered by upward transfer of the viscoelastic stress induced by the slab pull increase caused by an episode of lateral breakoff advance. At the same time, as the inferred breakoff fracture enables the deeper segment of the plate to detach from the shallower segment, the plate region situated above the severed region is relieved of the gravitational pull that was imposed by the deeper, and currently detached, plate segment. In response to the resulting crustal rebound, the surface topography is expected to exhibit down-tilting to the west. Such an anticipation seems to be confirmed by the catchments of several streams which flow roughly perpendicular to the intermediate-depth earthquakes epicentral domain: relevant geomorphic indices (the swath profile average and maximum elevation curves; the transverse hypsometric integral – THi; the transverse topographic symmetry factor – T-vector) computed for the concerned valleys consistently suggested larger uplift rates in the east, as compared to those manifest further to the west.