Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopmagnitude 5.6 event near Newcastle on 28 December 1989 (McCue and others, 1990). Parameterssuch as ‘a’ and ‘b’ of computed recurrence relations for identified seismic zones in the SydneyBasin are in the same range as those computed for other source zones throughout Australia (Brownand Gibson, 2000), they are essentially the same given the uncertainty in calculating them from theshort period of instrumental data compared with the return period of potentially damagingmagnitude 5 + earthquakes.A recognised measure of the hazard is the computed peak ground acceleration in a 475 yr period asshown on maps adopted by Standards Australia for incorporation into various editions of theAustralian Loading Code (see for example McCue and others, 1993). Earthquake hazard in Australiais similar to that of other intraplate regions of the World, not negligible, but considerably lower thanthe hazard in any of the interplate regions (Giardini and others, 1999). By any recognised standard,the Sydney Basin setting is classically intraplate.ATTENUATION IN THE SYDNEY BASINValuable strong motion data have been collected in Australia over the last 15 years since the JointUrban Monitoring Program to install accelerographs in urban areas with more than 50 000 peoplewas initiated after the 1989 earthquake near Newcastle NSW. As yet there is insufficient data todetermine attenuation rates in Western, Central or Eastern Australia, three regions that many expectshould exhibit different rates of attenuation because of their different ages.There is however a wealth of intensity information summarised in several hundred isoseismal mapsfor earthquakes throughout Australia (Everingham and others 1982, Rynn and others 1987 andMcCue 1996) and Geoscience Australia Annual Seismological Reports post 1989). It is instructiveto compare just two of these important earthquakes only weeks apart, one beneath the Sydney Basinsediments of Mesozoic age, and a similar sized event in southwestern WA in the Archaean craton(McCue and Gregson, 1993). The mechanisms of the earthquakes were very similar; thrusts withnodal planes dipping at intermediate angles under horizontal compression. Attenuation of seismicenergy through the basin and craton might be expected to be as different as between any twogeological settings in Australia. One factor which affects intensity estimates is time of day but bothof these earthquakes occurred near midday local time so this factor can be discounted.Pertinent details of the earthquakes are given in table 1 below along with characteristicmeasurements taken from the isoseismal maps, the equivalent circular radii of the areas enclosingintensities MM5, 4 and 3 or higher. It might seem surprising that the assigned maximum intensity ishigher in Newcastle at MM8 than in Meckering MM6 at similar epicentral distances, especiallygiven the shallower focus of the Meckering event. This is probably partly a result of goodengineering practice. Following the magnitude Ms 6.8 earthquake that all but destroyed thetownship of Meckering in 1968, the town was relocated and rebuilt, the local council ensuringbuilders used earthquake design provisions for all new buildings. By comparison, no buildings inNewcastle had the benefit of incorporating earthquake design provisions, indeed most of the highlydamaged buildings had severe flaws such as poor fastenings, missing beams, corroded wallconnections and soft lower storeys. In addition the worst damage in Newcastle occurred in an areaunderlain by soft sediments, much of it hydraulic fill that no doubt contributed to the seismic loadand assessed intensity.59