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

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 (70–300 km). 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 300 km moves downward at ~40 mm/year relative to the overlying crust. As shown by Kufner et al. (2017, https://doi.org/10.1016/j.epsl.2016.12.043) and Zhan and Kanamori (2016, https://doi.org/10.1002/2016GL069603), the central part of the zone stretches so rapidly that this rate there is ~100 mm/year, much faster than the present-day convergence rate of ~15 mm/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, https://doi.org/10.1016/j.tecto.2008.05.024) 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.