Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopGEOLOGICAL SETTING OF THE SYDNEY REGIONSydney is located in the Sydney basin (Herbert & Helby, 1980)(Figure 1), which is characterised byup to 5 km thickness of Permian and Mesozoic, terrestrial and marine sedimentary rocks. Theserocks rest unconformably on Paleozoic rocks that are much more deformed. At the western marginof the Sydney basin Permian coal measures lap onto the Paleozoic rocks of the Lachlan Fold Belt.Sydney basin deposits consist of non-marine coal measures grading up into Triassic and possiblyearly Jurassic terrestrial and estuarine sandstone and siltstone (Herbert, 1980). The absence ofyounger Mesozoic rocks is attributed, by Ollier (1982) to uplift and doming as a precursor to theopening of the Tasman Sea which occurred from 80-55 Myr ago (Shaw, 1978). Thin Cainozoic-agerocks occur within the Sydney basin, most notably in the Penrith basin adjacent to the LapstoneStructural Complex (LSC) (Mauger et al., 1984) where several tens of metres of conglomerate, sandand clay represent ancient fluvial terrace and overbank flood deposits.From a global tectonics perspective Australia is regarded as an intraplate setting, thousands ofkilometres from an active plate boundary. However, as other areas on the margins of stablecontinental interiors such as eastern USA and India there are thick post-craton basins that arecharacterised by greater seismic activity than in the plate interiors. The Sydney area therefore isintraplate from a global tectonics point-of-view, but seismically more active than the continentalinterior, and with rock velocities and seismic attenuation that is more akin to active margin settings.Most earthquakes that occur within the Sydney Basin are within Paleozoic basement rocks that arelocated beneath the Mesozoic sediments, more than 3-4 km below ground surface. However, rare,shallow events such as the M L 4.4 1999 Appin earthquake do occur within the Sydney basinsediments. Prior to the current study faults within the Sydney basin rocks were considered unusual,Figure 1 Mapped structural features of the Sydney region prior to this study. Moststructural data are from Herbert (1980), and Sherwin & Holmes (1986).62