Probabilistic earthquake risk maps of Australia

Abstract

New earthquake risk maps, based on the Cornell‐McGuire methodology, have been prepared for the Australian continent and continental margins. The risk is depicted as contours of peak ground velocity, acceleration and ground intensity with a 10% probability of being exceeded in 50 years. The earthquake process was assumed to be Poissonian and consequently foreshocks and aftershocks were eliminated. The estimated maximum earthquake magnitude likely to be expected in each zone was assumed to be half a magnitude unit greater than the largest recorded Richter magnitude for that zone. For areas outside the defined source zones, normalized background seismicity levels were used in the risk estimates. Separate relationships for the attenuation of ground intensity with hypocentral distance using a scaling law of the Kanai form were obtained for western, southeastern and northeastern Australia and for Indonesian earthquakes felt in northern Australia. These were based on the determination of mean isoseismal radii from isoseismal maps of Australian earthquakes and hence reflect average site conditions. Conversions of ground intensity (I, on the Modified Mercalli scale) to peak ground acceleration (A, in m/s2) and velocity (v, in mm/s) were carried out using the empirical relations log A=I/3.1 — 2.3 and 2 I= 7ν/5. Recordings of peak ground acceleration were used where available to check the resultant acceleration and velocity attenuation functions. This re‐evaluation of seismic risk in Australia may be used as the basis for a revision of the seismic zone map in the current Australian Building Code.