Seismic structure in the mantle beneath Australia
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 limited number of permanent high-fidelity seismic stations on the continent has been supplemented with extensive deployments of portable broadband stations for periods of a few months at each site. The combination of a long term of recording at the permanent stations and the broad spatial coverage of the portable stations provides an excellent resource for studies of the mantle. A wide range of techniques can be used to gain information on the three-dimensional structure in the mantle, which exploit different aspects of seismograms. The large-amplitude surface waves in the later part of the seismogram travel nearly horizontally and can be used in a tomographic inversion to determine the 3-D variations in shear-wave speed. This approach relies on matching the waveforms on individual paths and then mapping of the path-specific constraints on shear structure into a 3-D model. In contrast, the 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 and for the region below northern Australia, the combination of short-period and broadband observations has provided detailed information on both P and S wave speeds and attenuation structure. Further information on lateral variations in structure can be extracted from the patterns of travel-time residuals. The combination of the different classes of results reveal a complex pattern of 3-D structure beneath the Australian region. The cratonic region in the centre and west of Australia is underlain by a thick mantle lithosphere extending to around 210 km depth with fast wave speeds (especially for S waves). In the asthenosphere below, the S wave speeds diminish and there is significant attenuation and also some level of seismic anisotropy. 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. The variations in seismic-wave speeds extend through the upper mantle with noticeable differences in the transition zone. There is also evidence for pervasive small-scale heterogeneity (scale lengths of 100–200 km) superimposed on the broader scale variations that can be imaged using tomographic methods.