Shear-wave velocity structure of Australia from Rayleigh-wave analysis

The elastic structure beneath Australia is shown by means of S-velocity maps for depths ranging from zero to 400 km, determined by the regionalization and inversion of Rayleigh-wave dispersion. The traces of 233 earthquakes, occurred from 1990 to 2010, have been used to obtain Rayleigh-wave dispersion data. These earthquakes were registered by 65 seismic station located in Australia and the surrounding area. The dispersion curves were obtained for periods between 5 and 250 s, by digital filtering with a combination of MFT (Multiple Filter Technique) and TVF (Time Variable Filtering), filtering techniques. Later, all seismic events (and some stations) were grouped to obtain a dispersion curve for each source-station path. These dispersion curves were regionalized and inverted according to the generalized inversion theory, to obtain shear-wave velocity models for a rectangular grid of 2.5 degrees x2.5 degrees mesh size. The shear-velocity structure obtained through this procedure is shown in the S-velocity maps plotted for several depths. These results agree well with the geology and other geophysical results previously obtained. The obtained S-velocity models suggest the existence of lateral and vertical heterogeneity. The zones with consolidated and old structures present greater S-velocity values than those in the other zones, although this difference can be very little or negligible in some case. Nevertheless, in the depth range of 15 to 50 km, the different Moho depths present in the study area generate the principal variation of S-velocity. A similar behaviour is found for the depth range from 65 to 180 km, in which the lithosphere-asthenosphere boundary generates the principal variations of S-velocity. Finally, it should be highlighted a new and interesting feature was obtained in this study: the definition of the base of the asthenosphere, for depths ranging from 155 to 280 km, in Australia and the surrounding area. This feature is also present in the continents: South America, Antarctica and Africa, which were part of the same super-continent Gondwanaland, in the early Mesozoic before fragmentation.