Seismic Moments of Intermediate-Depth Earthquakes Beneath the Hindu Kush: Active Stretching of a Blob of Sinking Thickened Mantle Lithosphere?

摘要

Intermediate-depth seismicity beneath the Hindu Kush may reveal the Earth's best example of a stretching blob of mantle lithosphere sinking through the asthenosphere. Seismically, this region is the Earth's most active at intermediate depths (70-300km). Fault plane solutions show nearly vertical stretching of the region in which the earthquakes occur, and a summation of seismic moment tensors implies that on average material at a depth of 300km moves downward at similar to 40mm/year relative to the overlying crust. As shown by Kufner et al. (2017, ) and Zhan and Kanamori (2016, ), the central part of the zone stretches so rapidly that this rate there is similar to 100mm/year, much faster than the present-day convergence rate of similar to 15mm/year at the surface and across the Hindu Kush. The pattern of stretching, with maximum strain rates in the middle depths of the intermediate-depth range, resembles that beneath the southeastern Carpathians, where Lorinczi and Houseman (2009, ) have shown that seismicity is consistent with a blob of mantle lithosphere stretching rapidly and sinking into the asthenosphere, though more slowly than occurs beneath the Hindu Kush.