Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopThe North Sydney Basin was defined as the remainder of the Sydney Basin zone, characterised by avery low level of earthquake activity. Compared with other Australian cities, Sydney experiencesvery few local earthquakes that are less than magnitude 3 but which are felt over a limited area. TheNorth Sydney zone might be considered as a downthrown region between reverse faults that aredipping away from the zone – the Lapstone to the west and the Hunter-Mooki Fault system to thenortheast.Note that the activity on a reverse fault is under the upthrown block, offset at distances up to 25kilometres from the surface outcrop.CONCLUSIONSThe higher density seismograph network operating over the past decade has significantly improvedthe resolution of earthquake locations in the Sydney region. The majority of earthquakes in theSydney area are to the west, under the Blue Mountains, and probably on the active Lapstone Fault orrelated faults. The depth range of earthquakes under the Sydney Basin is limited, with few veryshallow events in the Mesozoic sediments near the surface, and a maximum depth of about 20 km inbasement rock.It seems that attenuation for earthquake waves in the Sydney area is complicated by higherattenuation in the Mesozoic sediments of the Sydney Basin than in basement rocks. Earthquakes orblasts near the surface, including the only moderate magnitude earthquake in the area since the newnetwork was installed (Ellalong near Cessnock, in 1994, magnitude ML 5.1), have near surfaceattenuation effects at both source and seismograph, so are difficult to analyse with present data.Some additional well-located events larger than about magnitude 3.0, particularly deeper events,will be required before local attenuation can be determined.ACKNOWLEDGEMENTSThe author wishes to thank the many people involved in Australian seismograph monitoring whohave contributed data over many years, especially staff from the Riverview Observatory, AustralianNational University, Seismology Research Centre, Sydney Water and Geoscience Australia.Lawrence Drake, David Denham and Kevin McCue have made significant contributions in analysisof the data.REFERENCESBrown, Amy & Gary Gibson, 2000: Reassessment of earthquake hazard in Australia, Proc 12 thWorld Conf on Earthquake Engineering, Auckland, paper 751, 8 pages.Brown, Amy & Gary Gibson, 2004: A multi-tiered earthquake hazard model for Australia,Tectonophysics, 390, 25-43.Burke-Gaffney, T.N., 1951: Seismicity of Australia, Proc. Royal Soc. of NSW, Vol 85, 47-52.Clarke, W.B., 1869: On the Causes and Phenomena of Earthquakes, Especially in relation to ShocksFelt in New South Wales and in other Provinces of Australasia, Trans. Royal Soc. N.S.W., 2,51-86Denham, D. (ed), 1973: Seismicity and Earthquake Risk in Eastern Australia, Bureau of MineralResources Bulletin 164, Aust. Govt. Publishing Service, CanberraDenham, D., 1980: The 1961 Robertson earthquake - More Evidence for Compressive Stress inSoutheast Australia, BMR Journal Vol 5, No 2, 153-156.Denham, David, Günter Bock & Ron S. Smith, 1982: The Appin (New South Wales) Earthquake of15 November 1981, BMR Journal Vol 7, No 3, 219-223.56