Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopwhen compared to the return period of large earthquakes, and earthquakes always cluster in spaceand time. There are still uncertainties regarding the activity to the south of the area, including thesource of the Appin and Robertson earthquakes.EARTHQUAKE HAZARD IN THE SYDNEY AREAThe most common measure of earthquake hazard at a location is a ground motion recurrence plot,either giving motion as a function of return period, or giving response spectra for one or more returnperiods. This is done using the Cornell method, where earthquake sources are defined andquantified, then attenuation functions are used to integrate probabilities of ground motion for allpossible earthquakes in longitude, latitude, depth and magnitude.The earthquake catalogue of known events is a key input, but when the duration of the catalogue isvery much shorter than the average return period for medium to large earthquakes this gives a verylimited sample of possible earthquakes. In stable continental regions like Australia the cataloguemay have good coverage for 50 years and limited coverage for 100 or 200 years, while the averagereturn periods for particular moderate to large earthquakes may be from thousands up to hundreds ofthousands of years.A seismotectonic model is used to include all possible earthquake sources. This usually consists ofarea or volume source zones within which it is assumed that earthquakes can occur at any point withthe same probability. It is possible to include variations in probability within the source, andvariation of the earthquake magnitude recurrence with depth can be included. As well as seismicityinput, the seismotectonic model normally incorporates geological and geophysical data in thedefinition of the zones. Seismotectonic models with only area or volume source zones give a lowresolutionaverage ground motion that only varies on a scale comparable to the size of the zones.Since all shallow moderate to large magnitude earthquakes occur on pre-existing faults with rupturedimensions of kilometres or larger, it is possible to include active faults in the seismotectonic model,each with a specified location and earthquake magnitude recurrence rate. When an active fault isincorporated in the model, it is normal to reduce the earthquake activity rate in the local area orvolume source zone by a compensating amount. As more and more active faults are delineated andquantified, the seismotectonic model gains in resolution, giving variations in estimated hazarddepending on the proximity of faults.Brown and Gibson (2000, 2004) have developed a series of seismotectonic models, designatedAUS1 to AUS6, to compute ground motion recurrence as a measure of earthquake hazard. These aresubject to continuous updating as more data comes available. Because earthquakes in Australiaoccur at a relatively limited rate, most of the model developments are currently the result ofgeological and geophysical inputs.Figure 5 shows the earthquakes in the Sydney area to the end of 2001 and the source zones formodel AUS5. In defining the source zones, the Sydney Basin was taken as a geological entity that isa significant feature on the scale of the upper crust, where earthquakes occur in this region.The earthquake catalogue shows considerable variation in activity over the Sydney Basin, so it wasdivided into three zones. The high level of activity under the Blue Mountains in the south-westformed the West Sydney Basin zone. The Lapstone Fault, assumed to be a west dipping reversefault, is along the eastern edge of the West Sydney Basin zone, and a high proportion of the WestSydney Basin activity may be on the Lapstone Fault.54