Seismic structure in the mantle beneath Australia

The exposed geology of the Australian continent is composed of an assemblage of crustal blocks that can be broadly grouped into the Precambrian western and central cratons and the Phanerozoic eastern province (Figure 1). Structural differences between the Precambrian shield and eastern Australia are inferred from surface-wave dispersion (Muirhead & Drummond 1991; Denham 1991) and teleseismic travel-time residuals (Drummond et al. 1991) whose origin is due to structures that certainly extend below 100 km depth. The extensive seismic activity in the earthquake belt that runs through Indonesia, New Guinea and its offshore islands, Vanuatu, Fiji and the Tonga–Kermadec zone provides an abundant source of seismic probes for the structure in the lithosphere and the upper mantle beneath. A wide range of studies has exploited the different aspects of the seismic wave train from the P and S body waves refracted back from the velocity structure in the upper mantle through to the large amplitude surface-wave trains which travel nearly horizontally on their path from source to receiver. The Australian continent itself has a fairly low level of seismicity and only a few high-quality seismographic stations. The study of structure beneath the continent has therefore depended on the deployment of portable seismic instrumentation. Up to 1993 the experiments involved the deployment of vertical-component seismometers (with natural frequency around 1 Hz), with firstly analogue recording, and later low-power digital reel-to-reel tape recorders (Figure 2a). The first portable broadband instruments were used in 1992, and from 1993 the emphasis has been on continent-wide coverage using a mobile network of stations (Figure 2b). In the SKIPPY experiment from 1993 to 1996 (van der Hilst et al. 1994), stations were deployed across the whole continent at approximately 400 km spacing. Because only a limited number of broadband seismometers and highfidelity recorders were available, the continental coverage was achieved using between 8 and 12 stations at a time, installed for about 5 months in each location. This period is sufficient to get good coverage of the regional seismicity but is a little short for some classes of studies that depend on less-frequent teleseismic events at specific distance Geol. Soc. Australia Spec. Publ. 22, and Geol. Soc. America Spec. Pap. 372 (2003), 7–23