Seismic Phases Recorded in South Australia and their Relation to Crustal Structure

Abstract

Summary The phases recorded from local earthquakes in South Australia are discussed. A simple single-layered crustal model, put forward by Doyle & Everingham (1964) from recordings of atomic explosions, has provided very consistent interpretations of all the prominent phases. The Mohorovicic discontinuity appears to be sufficiently sharp to produce the converted reflection SM P at frequencies near 2 c/s. Normal supercritical reflections and a surface converted reflection sPM P are also regularly recorded. The converted reflections are very useful for the determination of focal depths; their recorded amplitudes give added weight to the hypothesis of almost constant crustal velocities. They illustrate the usefulness of later phases in studies of crustal structure and the advantages of using relatively long wavelengths, particularly for studying reflections. The amplitude data for P waves were found to give little useful information but satisfactory agreement was obtained between the observed amplitudes of SM S, SM P and sPM P and the amplitudes calculated for these phases using ray theory and plane wave reflection coefficients.