THE UPPER-MANTLE S-VELOCITY AND P-VELOCITY STRUCTURE BENEATH NORTHERNAUSTRALIA FROM BROAD-BAND OBSERVATIONS

Good coverage of seismic wave propagation through the upper part of th e mantle can be obtained for sources in the earthquake belt through In donesia and New Guinea recorded at broad-band seismometers in the Nort hern Territory of Australia. The midpoints of the propagation paths li e along the northern margin of the Australian continent. Most of the o bservations come from a Guralp CMG3 seismometer at the Warramunga Arra y, but these have been supplemented by field deployments using compara ble instrumentation. Two groups of sources, (1) along the Flores Are, Indonesia, with propagation under northwestern Australia, and (2) in N ew Guinea, with paths to the NNE of the array, have been used to const ruct composite record sections for P, SV and SH waves over the distanc e 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 path s whose turning po;nts are separated by about 1000 km. The S waves rec orded on the radial (SV) and tangential (SH) components are of compara ble quality because the hard-rock recording sites minimise the influen ce of coupling to P on the radial component. As the P and S sections a re prepared from the same events, we are able to make direct estimates of P/S velocity ratios for the mantle transition zone. This ratio inc reases from 1.77 at the top of the mantle to 1.85 at the 410 km transi tion, indicating that the S-wave velocity gradients are rather lower t han might be expected. However, through the transition zone down to 66 0 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 differen ce in the frequency content of P and S waves beyond 2000 km. The P wav es remain high frequency but the S waves returned from the transition zone and below are of intermediate period (0.25-0.3 Hz), associated wi th significant attenuation zone for S beneath 210 km.