Sp phases from the Australian upper mantle

Abstract

Routinely published source mechanisms of large earthquakes greatly facilitate the selection of data that may be suitable for the study of S to P conversions from the upper mantle. This is demonstrated for three stations of the Global Digital Seismograph Network located at Narrogin (NWAO), Charters Towers (CTAO) and Tasmania University (TAU) in Australia. Of ten events that were selected, eight showed precursors to S, SKS and ScS on long-period vertical-component seismograms at epicentral distances between 70° and 91°. The precursors are interpreted as Sp conversions from the upper mantle beneath Australia and surrounding areas. Synthetic seismograms that were calculated for the PREM model of Dziewonski and Anderson (1981) show good agreement with many details of the data. The main precursor arrivals are compatible with S to P conversions from a seismic discontinuity at 400 km depth. Relative to conversions from the ‘400-km’ seismic discontinuity, Sp phases from the ‘670-km’ discontinuity have much smaller amplitudes both in the synthetics and data in the epicentral distance range from 75° to 85°. It is suggested that Sp phases from the ‘670-km’ discontinuity can best be observed at epicentral distances beyond 89°. This is confirmed by one observation at Δ = 90.6° made at CTAO. Within the limits of resolution of long-period data, there is no indication for strong lateral variations in the upper mantle discontinuity at 400 km between various geological provinces of Australia and surrounding areas. There are indications for the presence of S to P converted phases from a depth near 220 km. Their appearance is variable so that they are difficult to explain by S to P conversions at a seismic discontinuity at this depth.