Investigation of upper mantle discontinuities near Northwestern Pacific Subduction Zones using precursors to sSH

Abstract

Teleseismic long‐period World‐Wide Standard Seismograph Network (WWSSN) tangential component recordings of deep and intermediate depth earthquakes are analyzed for the presence of sS precursors, denoted sxS, signifying underside reflections 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 in the vicinity of subduction zones. The clearest precursor is usually the smS phase, the underside reflection from the Moho, which is observed for a number of events in the northwestern Pacific for paths reflecting under the Sea of Okhotsk and North Korea. The amplitude and timing of smS relative to sS are modeled using synthetic seismograms to determine the shear wave impedance contrast 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. smS is strong for continental reflection points under North Korea, which has laterally varying 31 to 36 km crustal thickness and 31.2% to 18% shear wave impedance contrast at the Moho. In the northern Sea of Okhotsk region the crustal thickness is around 21 to 29 km with 26% to 28% shear wave impedance contrast at the Moho. Stacked smS 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. smS is not clearly separated from sS for oceanic paths under the Izu and Japan regions, and our procedure cannot resolve oceanic Moho properties unless broadband data are used. Other precursors, sxS, are much weaker than smS and are difficult to identify in individual waveforms. We use slant stacking to enhance the signal to noise and search for precursors with various slownesses, using recordings from 13 deep events. Four SH wave reflectors may exist below the Moho in our study area. The shallowest is the “80‐km” discontinuity, which varies in depth laterally, being near 80 to 85 km below the Sea of Okhotsk, near 66 km beneath Izu Japan, and near 90 km under North Korea. A reflector near 200 to 210 km depth is indicated by data from Izu Japan, but there is no evidence for any shear wave impedance contrast around this depth under North Korea or the Sea of Okhotsk. Weak evidence is found for a “330‐km” discontinuity in these regions, with the depth of a few percent impedance contrast varying from 325 to 335 km. Relatively strong and consistent arrivals indicate the presence of a “400‐km” discontinuity, with the depth varying from 380 to 400 km, perhaps indicating slight elevation of the olivine→ β phase transformation near the slabs.