Origin of precursors to teleseismic S waves

Abstract

Abstract In the literature, there are descriptions of precursors to teleseismic S waves that are polarized as teleseimic P. They are commonly regarded as phases converted from SV to P underneath the seismographic stations. We present observations of precursors at the broadband digital GEOSCOPE stations in the distance range from 45° to 95° and at periods around 10 sec. These precursors are polarized as the teleseismic P but cannot be interpreted in terms of conversion underneath the station. We propose that they are formed by conversion and scattering from S to P at the free surface and scattering of the resulting P in the lithosphere of the region between the source and the receiver. The apparent velocity of S in the region of scattering is usually around 7 km/sec, which implies a long wavepath of the converted P in the continental crust. Apparently, S-to-P and P-to-P scattering play an important role in forming teleseismic wave fields even in the relatively long-period band around 10 sec. A possibility of observing true S-P converted phases originating in the lithosphere underneath the station is certainly not ruled out. However, our analysis is a warning that scattered waves sometimes can be erroneously taken for converted phases.