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Origin of precursors to teleseismic S waves

LEV. P. VINNIK^* and BARBARA A. ROMANOWICZ

LABORATOIRE DE SISMOLOGIE INSTITUTE DE PHYSIQUE DU GLOBE, 4 PLACE JUSSIEU, TOUR 14, 75252 PARIS CEDEX 05, France

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.

Footnotes

^* On leave from: Institute of Physics of the Earth, Soviet Academy of Sciences, Bolshaya Gruzinskaya 10, Moscow, U.S.S.R.

Present address: Seismographic Station, University of California at Berkeley, Berkeley, California 94704.

This article has been cited by other articles:

				G. BOCK Comment on "Origin of precursors to teleseismic S waves" by L. P. Vinnik and B. A. Romanowicz Bulletin of the Seismological Society of America, August 1, 1992; 82(4): 1973 - 1976. [PDF]