Lithospheric structure in the Australian region - a synthesis of surface wave and body wave studies

Abstract

The configuration of earthquake belts around Australia provides a wealth of events at suitable distances to be used as probes into the seismic structure of the upper mantle. The few permanent seismic stations have been supplemented with extensive deployments of portable broadband stations for periods of a few months at each site. The broad band records have been used in a variety of studies of 3-D structure. Surface wave tomography is based on matching seismic waveforms on individual paths and then mapping the path-specific constraints on shear structure into a 3-D model. Higher frequency body wave arrivals are refracted back from the variations in structure in the mantle and are particularly sensitive to discontinuities in structure. Observations out to 3000 km provide coverage of the structures down through the transition zone. For northern Australia, the combination of short-period and broadband observations provides detailed information on both P and S wavespeeds and attenuation structure. There is a complex pattern of 3-D structure beneath the Australian region. The cratonic region in the centre and west is underlain by a thick mantle lithosphere extending to around 200 km depth with fast wavespeeds (especially for S waves). However, the mobile belt in Central Australia has comparatively low wavespeeds to at least 75 km depth with fast lithospheric material beneath. In the asthenosphere the S wavespeeds diminish and there is significant attenuation. Beneath the eastern zone with Phanerozoic outcrop the lithosphere is generally thinner (less than 140 km) and the asthenosphere has a pronounced low velocity zone for S again with high attenuation.