Open AccessMultiples as precursors to S, SKS and ScS
Author(s) -
Bock Günter
Publication year - 1994
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.1994.tb00132.x
Subject(s) - seismogram , classification of discontinuities , amplitude , geology , transition zone , mantle (geology) , superposition principle , multiple , seismometer , seismology , discontinuity (linguistics) , seismic wave , geodesy , geophysics , physics , optics , mathematics , mathematical analysis , arithmetic , quantum mechanics
SUMMARY Complete theoretical P‐SV seismograms calculated for the radially symmetrical IASP91 model with the reflectivity method show a wide variety of seismic phases in the time window between P and mantle S at teleseismic distances. One particular phase, which appears as precursor to S mainly on vertical‐component seismograms, clearly stands out over the distance range from 62° to 70°. If interpreted as an S‐P conversion from upper mantle discontinuities, the lead time to mantle S of about 30 s would indicate a conversion depth of 220 km. Clearly, this would be a false interpretation because the IASP91 model does not contain a discontinuity at this depth. Synthetic seismograms calculated with the Gaussian beam method indicate that the precursors can be explained by superposition of P and S multiples between the surface and the transition zone from the upper to the lower mantle. Surface multiples of this kind appear as S ‐wave precursors, up to about 90° distance, and may have amplitudes that are comparable to those of S‐P converted waves from the upper mantle transition zone. Interference between surface multiples and mode‐converted phases may lead to highly variable amplitudes of S ‐wave precursors. An observation from the long‐period Global Seismograph Network station NWAO (Narrogin, Western Australia) suggests that surface multiples are actually observed as precursors to S .