VARIATIONS IN UPPER-MANTLE STRUCTURE UNDER NORTHERN AUSTRALIA

Temporary array deployments of short-period seismometers in northern A ustralia have been used to build up composite record sections for wave s interacting with the upper mantle. Stable measures of the seismic wa vefield are provided by stacking the complex envelopes of all the seis mic waveforms falling in a 10 km distance interval away from the sourc e. Two groups of sources (a) along the Flores Arc, Indonesia with prop agation under northwestern Australia, and (b) in New Guinea with paths to the NNE of the array, have been used to construct composite record sections for both P and SV waves over the distance range 1300-2800 km . The timing and amplitude distributions for P waves from the two regi ons show noticeable differences. Detailed modelling of the record sect ions yields velocity models with significant variation in velocity for the two sets of propagation paths for which the midpoints are separat ed by about 1000 km. The short-period SV-wave sections indicate effici ent propagation of high-frequency S waves in a lithosphere extending d own to 210 km. Arrivals from the deeper mantle cannot be correlated wi th confidence because of a loss in high-frequency content revealed by broad-band observations. This requires a significant attenuation zone for S beneath 210 km.