Observations of shear‐coupled P waves

Abstract

Teleseismic SV waves couple to P waves in the lithosphere in several ways. P waves converted from discontinuities in the lithosphere precede the main SV arrival as small amplitude Sp precursors. Large amplitude P waves follow the main SV arrival after conversion by reflection at the free surface, and post‐critical reflection from a discontinuity in the lithosphere. SPdiff denotes a converted P wave diffracted along the underside of the crust when a teleseismic SV wave strikes it from below near the critical angle of incidence.We observed several of these shear‐coupled P waves on seismograms of deep earthquakes recorded at broadband seismic stations (RSTN) in North America. For these paths, large amplitudes relative to SV characterize the lithospheric multiple near 50°, with amplitudes decreasing rapidly beyond 60°. Calculation of synthetic seismograms indicate that a post‐critical reflection from a discontinuity at 70‐80 km depth where compressional velocity increases abruptly to about 8.5 km/s produce the largest amplitude P arrival. The observations of these shear‐coupled P waves presents a previously untapped source of information about the fine structure of the upper mantle.