Landslides in the Sydney Basin - Geoscience Australia

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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
Seismic Hazard in SydneyProceedings of the one day workshopTable 1: Parameters of the 1989 Newcastle NSW and 1990 Meckering WA earthquakesPLACE DATE TIME ML DEPTH ISOSEISMAL RADIUS*UTC km MM5 MM4 MM3km km kmNewcastle NSW 28 December 1989 06:38 5.6 13 ± 2 105 185 300Meckering WA 17 January 1990 23:26 5.5 6 25 150 260* note that the scale bar of figure 9 in McCue and Gregson ( 1993) is incorrectly drawn.The magnitudes measured by GA of the two earthquakes were similar, the Meckering event only 0.1magnitude units smaller than the earlier earthquake at Newcastle. The table shows a much largerarea of intensity MM5 in the Sydney Basin indicating that the focal depth of the WA earthquakemay have been as shallow as 2 km or less which would support the lower peak intensity inMeckering but the MM5 radii also lend no support for an assumption of higher attenuation in theeast relative to the west. The similar radii for the lower intensities clearly contradict the assumptionof a higher attenuation in eastern Australia relative to the west, indeed, an opposite outcome mightbe deduced.There are several isoseismal maps of other well documented pairs of earthquakes in the SydneyBasin and SW WA which could and should be analysed to check the universality of theseobservations.CONCLUSIONSFirstly, the Sydney Basin is a classic intraplate setting as measured by both the seismicity and hazardparameters. Secondly, the evidence from a comparison of the isoseismal maps of two very similarearthquakes on opposite sides of the continent challenges assumptions that the ground attenuation inthe Sydney Basin is special or different from elsewhere in Australia.REFERENCESBrown, A., and Gibson, G., 2000. Reassessment of Earthquake Hazard in Australia. 12 th WorldConference on Earthquake Engineering. Auckland NZ, Paper 0751, 7 pp.Doutch, W., H., 1982 (Chairman). Plate-Tectonic Map of the Circum-Pacific Region, SouthwestQuadrant. AAPG, Tulsa Oklahoma, USA.Everingham, I.B., McEwin, A.J., and Denham, D., 1982. Atlas of isoseismal maps of Australianearthquakes. Bureau of Mineral Resources, Australia, Bulletin 214.Giardini, D., Grunthal, G., Shedlock, K., and Zhang, P., 1999. The GSHAP Global Seismic hazard Map.Annali di Geofisica, Vol 42, No. 6, pp 1225-1228.McCue, K.F., Wesson, V., and Gibson, G., 1990. The Newcastle New South Wales, earthquake of 28December 1989. BMR Journal of Australian Geology and Geophysics, 11, 4, pp 559-568.McCue, K., (compiler), Gibson, G., Michael-Leiba, M., Love, D., Cuthbertson, R., and Horoschun, G.,1993. Earthquake Hazard Map of Australia - 1991.McCue, K., and Gregson, P., 1993. Australian Seismological Report, 1990. BMR Record, 1993/52,58pp.McCue, K.F., (Compiler), 1996. Isoseismal Atlas of Australian earthquakes, Part 3. AGSO Record,1996/44.Rynn, J.M.W., Denham, D., Grenhalgh, S., Jones, T., Gregson, P.J., McCue, K. and Smith, R.S.,1987. Atlas of isoseismal maps of Australian earthquakes. Bureau of Mineral Resources,Australia, Bulletin 222.60
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