Investigation of upper mantle discontinuities near northwestern Pacific subduction zones using precursors to sSH

recordings of deep and intermediate d pth earthquakes areanalyzed for the presence of sS precursors, denoted sxS, signifying underside r flections from discontinuities at depth x below the sS reflection point above the source. These sS precursors can be used to place constraints on upper mantle discontinuities n the vidnity of subducfion zones. The clearest precursor is usually the sins phase, the underside r flection from the Moho, which is observed for a number of events in the northwestem Pacific for paths reflecting under the Sea of Okhotsk and North Korea. The amplitude and timing of s,•S relative to sS are modeled using synthetic seismograms to determine the shear wave impedance ontrast at the Moho and the crustal thickness in these regions. The results are compared with previous work on pP precursors reflected from the Moho for similar paths, with consistent crustal thickness being found. s,•S is strong for continental reflection points under North Korea, which as laterally varying 31 to 36 km crustal thickness and 31.2 % to 18 % shear wave impedance ontrast at the Moho. In the northem Sea of Okhotsk region the crustal thick-ness is around 21 to 29 km with 26 % to 28 % shear wave impedance ontrast a the Moho. Stacked sins waveforms for the Sea of Okhotsk vary from strong, isolated arrivals in the north to weak, poorly defined precursors toward the south, reflecting a transition from continental to oceanic crust. s,•S is not clearly separated from sS for oceanic paths under the Izu and Japan regions, and our procedure cannot resolve oce