Source-side splitting of S waves from Hindu Kush-Pamir earthquakes

Abstract

We present eight new measurements of source-side S shear wave splitting parameters derived from 20 earthquakes in the Hindu Kush region. For each event-station pair we corrected for receiver-side splitting in order to isolate the source-side contribution to observed splitting. The receiver correction splitting parameters were taken from published values, but we confirmed these values to ensure their accuracy, when possible, via our own observations of SKS splitting at the stations. For two events we found source-side splitting parameters that were identical within errors after correction for receiver splitting at two distant stations with different receiver splitting parameters. Fast polarization directions, ø, vary systematically both geographically along the Hindu Kush and Pamir subducted slabs, and with depth: at around 100 km depth in the transition region between the two slabs, ø's trend north; at >200 km depth in the Hindu Kush slab, ø's trend east-northeast, approximately parallel to the strike of the slab; one measurement at 120 km depth in the Pamir slab trends within 20° of the local slab strike. Delay times, δt, range from 2.3 to 3.7 s, indicating strong upper mantle deformation and alignment of olivine, and/or long source-side travel paths through the anisotropic medium. We interpret our results in the context of two subducted slabs of opposite dip (Hindu Kush slab dipping north, Pamir slab dipping southeast) and shear and compressional flattening around the two slabs caused by the India-Eurasia collision. Thus, north-trending ø's at 100 km depth in the transition zone between the two slabs probably represent shear-alignment of upper mantle olivine along the western boundary of the Indian continental lithosphere, now indenting Eurasia along reactivated Mesozoic sutures such as the Chaman Fault. ENE-trending ø's from events deeper in the Hindu Kush slab represent flattening and mantle flow along the slab below 200 km. A similar, horizontal slab-parallel flow or olivine alignment may occur beneath the Pamir slab, on the basis of our one measurement there.