Determination of earthquake focal mechanisms and crustal structure in eastern Australia using surface waves

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The surface waves from two medium sized ( M ∼ 5.5) earthquakes in southeast Australia (1966 Mt. Hotham and 1982 Wonnangatta) were examined to determine average S-wave crustal structure for the region and the focal mechanisms of the two earthquakes. The S-wave models were determined from an analysis of the group velocities of the Rayleigh waves. For all paths a two-layer crust overlying a Moho at about 40 km is required to fit the observations. In the upper layer, down to about 25 km, the velocity gradients are very small and the S-wave velocities are in the range 3.4–3.6 km/s. In the lower crust the S-wave velocities increase gradually to about 4.6 km/s at the Moho where the velocity gradients are again very small. The region where the S-wave velocities increase significantly with depth corresponds to the region of the lower crust which generates strong seismic reflections (Mathur, 1983). The thin crust postulated beneath Bass Strait and Tasmania (Johnson, 1973) is not substantiated by these data and a crustal thickness of about 40 km is also required for this region. For the longer profiles (~ 1800 km) to the north and northwest of the earthquakes, the velocity step at the Moho is smaller than for the shorter profiles (~ 700 km). However, because the surface waves are less dispersed for the shorter profiles, the resolution is poorer and the models less reliable. For the 1982 Wonnangatta earthquake, the surface-wave synthetics from a focal mechanism similar to that obtained by Denham et al. (1985) fit the observations at five regional stations very well, and confirm the NW-SE compressive stress regime found by other workers. However, for the 1966 Mt. Hotham earthquake, the surface wave synthetics generated from the normal faulting solution, which was obtained from an analysis of P-wave first motions (Denham et al., 1985), do not agree with the Rayleigh wave-trains observed at Australian stations. Rather, the surface wave observations support a thrust fault type mechanism with the pressure axis having a similar direction to that obtained from the 1982 Wonnangatta earthquake and other earthquakes in the region.