MULTIPLES AS PRECURSORS TO S, SKS AND SCS

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 degrees to 70 degrees. If interpreted as an S-P conversion from upper mantle discontinuities, the lead time to mantle S of about 30s 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 degrees 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.