Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopSeismicity of the Sydney Basin RegionGary Gibson, Seismology Research Centre, ES&S.SEISMIC MONITORING IN THE SYDNEY REGIONIntroductionOur knowledge of past earthquakes in any region depends on the recorded history of the inhabitants,and on the recordings made by seismic monitoring instruments, especially those installed within theregion. We have no information of any of the earthquakes that occurred in the Sydney region duringthe period of aboriginal habitation prior to European colonisation. The earliest known Australianearthquake was felt in Sydney on 22 June 1788, a few months after the colony was established. Untilseismograph coverage was established, the location of larger earthquakes could be approximated bydetermining the location of maximum ground motion intensity, the earthquake depth estimated bythe relative area that experienced high intensity relative to low intensity, and the magnitudeestimated from the area or radius of perceptibility (McCue, 1980).Purposes of Seismic MonitoringMost seismic monitoring is done in order to investigate earthquake hazard. The dominant earthquakehazard is ground vibration, and it is this motion that is usually monitored. Other hazards includesurface rupture, liquefaction, landslides and tsunami. Earthquake ground motion monitoring is usedto determine information about the earthquake source (location, magnitude, mechanism, etc),seismic wave travel path (reflections, refractions, attenuation with distance), and the effects of thesurface geology and topography at the recording site (amplification by resonance, attenuation ofhigh frequency motion).1. The Earthquake SourceThe seismicity of an area includes the location of earthquakes within the area, their magnitude, andtheir recurrence rates. This includes delineation of active faults, if possible. Earthquake locationsand origin times can be determined from measurements of the precise times that seismic waves fromthe earthquakes arrive at seismographs. A network of at least three seismographs, but preferably fiveor more, is required to locate the earthquakes that occur within the network. Earthquakes outside thenetwork cannot be located as accurately.Earthquake depths can only be determined accurately if there is a seismograph near to the epicentre,within a horizontal distance not exceeding about twice the earthquake depth. In the Sydney areaearthquakes occur at depths from about 2 to 20 km.2. The Seismic Wave Travel PathThe attenuation of seismic wave amplitudes with distance from an earthquake depends on theproperties of the rocks within the earth’s crust in the area, particularly by its temperature.Attenuation in cold, hard, old rock depends primarily on geometric spreading, while the reduction ofamplitude with distance is higher in hot, soft, young rock. Absorption of energy within rocks isfrequency dependent, and attenuation of high frequency motion is much more rapid than for lowfrequency motion.3. The Site EffectsThe effects of surface sediments and topography can significantly modify ground motion. Softsurface sediments give amplification of ground motion at their natural frequency by resonance,while reducing amplitudes at high frequencies by absorption. Site effects depend on near-surfaceunconsolidated sediments and on topography, and they can vary rapidly from place to place at scales46