Abstract

Regional broadband waveforms of deep- and intermediate-depth earthquakes written at College, Alaska, and Matsushiro, Japan, show large SsPmp phases that may be the largest phase seen on the seismogram. SsPmp is created when the direct, upgoing SV wave converts to a P wave at the free surface and subsequently becomes trapped in the crust because of postcritical P-wave reflection at the Moho. SsPmp may arrive between the P- and S-wave phases or it may arrive after S, depending on source depth and distance. Thus, it can be used as an additional constraint on source depth and location. It is important to recognize the existence of this phase since it is easily confused with the S-wave arrival, resulting in erroneous S-wave arrival time picks, or it may be interpreted as a split shear wave, incorrectly implying shear-wave anisotropy in the medium. Such a wave propagation effect will also be important for a crustal source for structures that contain low-velocity layers near the surface.

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