A plausible model for the present day seismicity and tectonic activity in the Hindukush complex zone

The Hindukush zone is situated on the northern boundary of the Indian Plate along its northwestern flanks. Fifteen years (1974-1988) of seismic data published by United States Coast and Geodetic Surveys (USCGS) and focal mechanism solutions of the subducted lithosphere, reported by different scientists, have been used to model the seismic and tectonic characteristics of the Hindukush complex zone. The data indicate that the zone can be separated into two seismic segments, OP and PQ, with a V-shaped pattern, striking E-W and NE-SW, respectively. The two segments do not represent well-defined subduction zones, as the depth and frequency of events is not uniformly distributed. The E-W striking segment extends from 68.2°E to 70.5°E at about 36.5° N, whereas the northeast-southwest segment extends from 36.5°N, 70.5°E to 38°N, 73°E. The frequency and depth of earthquakes are greatest at 36.5°N, 70.5°E decreasing towards the W and NE. Focal mechanism solutions (thrust and normal faulting) for the Hindukush zone suggest that the directions of stress may be grouped into: 1. Compressive forces acting NE-SW for the east-west striking segment. For the NE-SW striking seismic segment compressive forces are directed towards the NW; 2. Compressive forces are also acting along the strike of the two seismic segments; and 3. The direction of the tensile stress is different in each segment and extends to a depth of up to 221 km. A model is proposed to explain the distribution of earthquake foci in terms of the V-shaped subduction of oceanic lithosphere, with tensile, and compressive stresses in the two seismic belts being due to the upper mantle flow of the Indian Plate and the Tibetan Plateau. The sinking of the mantle in the Hindukush zone is responsible for the stretching of the seismic slab in a vertical direction, giving rise to the present earthquake frequency and focal depth distribution pattern.