The upper-mantle S and P velocity structure beneath northern Australia from broad-band observations

Mathematics – Logic

Scientific paper

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Australia, Earth Mantle, P Waves, Propagation Velocity, S Waves, Seismic Waves, Seismology, Wave Propagation, Broadband, Geological Faults, Indonesia, Island Arcs, New Guinea (Island), Seismographs

Scientific paper

Good coverage of seismic wave propagation through the upper part of the mantle can be obtained for sources in the earthquake belt through Indonesia and New Guinea recorded at broad-band seismometers in the Northern Territory of Australia. The midpoints of the propagation paths lie along the northern margin of the Australian continent. Most of the observations come from a Guralp CMG3 seismometer at the Warramunga Array, but these have been supplemented by field deployments using comparable instrumentation. Two groups of sources, (1) along the Flores Arc, Indonesia, with propagation under northwestern Australia, and (2) in New Guinea, with paths to the NNE of the array, have been used to construct composite record sections for P, SV and SH waves over the distance range 1250-3600 km. The choice of the two source zones was based on earlier short-period studies, and the broad-band observations confirm the presence of noticeable differences in P velocity profiles for paths whose turning points are separated by about 1000 km. The S waves recorded on the radial (SV) and tangential (SH) components are of comparable quality because the hard-rock recording sites minimise the influence of coupling to P on the radial component. As the P and S sections are prepared from the same events, we are able to make direct estimates of P/S velocity ratios for the mantle transition zone. This ratio increases from 1.77 at the top of the mantle to 1.85 at the 410 km transition, indicating that the S-wave velocity gradients are rather lower than might be expected. However, through the transition zone down to 660 km the S-wave gradients increase and the P/S velocity ratio is less than 1.81 below the 660 km transition. There is a significant difference in the frequency content of P and S waves beyond 2000 km. The P waves remain high frequency but the S waves returned from the transition zone and below are of intermediate period (0.25-0.3 Hz), associated with significant attenuation zone for S beneath 210 km.

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