Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopUnfortunately the measurements of ground movement did not extend far enough to the west to allowthe extent of the zone of en-masse movement to be determined. In particular, it is not known howthe ground moved west of Harris Creek, immediately to the west of the bridges (see Figure 3). Thiscreek flows through a narrow ravine to its junction with the Nepean River and could well be locatedon a north trending geological fault inferred from the results of mine exploration.Hatherly et al. (2000) also discuss the issues of triggered seismicity caused by mining as discussedby McGarr and Simpson (1997). Triggered seismicity is produced when an engineering activity suchas mining and the filling of reservoirs causes small but sufficient changes to the overall stress fieldto facilitate ground movement. For example, around the mature coal mining areas of countries suchas in Poland and Britain, there have been increases in the regional seismic activity. In the case of theAppin area, the Geoscience Australia website 1 also shows that there was a small cluster of smallseismic events in the Appin area between 1971 and 1978 but at that time, the mining was not asextensive as at present.There has been no mining north of LW17 since 2000, and the new Douglas Colliery is currentlymining further to the east. The situation with regards to ground movement and seismic activity in thevicinity of the gorges of the Cataract and Nepean Rivers has stabilized. However, the question ofexactly how the mining induced changes to the stress field interacted with the tectonic stress field inthe period of 1999 to 2000 remains unanswered. If mining were to recommence in this area, theground movements and seismic activity could well recommence.ACKNOWLEDGEMENTSThis paper is published with the permission of BHP Billiton, Illawarra Coal. ANSIR is thanked formaking available the short period seismographs.REFERENCESHatherly, P., Poole, G., and Luo, X., 2000. Seismicity in the Appin area of NSW and its associationwith mining activities. V.H. Jensen and B. Butler, editors. Conference of the AustralianEarthquake Engineering Society. 15-16 November 2000, Hobart, pp. 6-1 to 6-6.Hatherly, P., Gale, W., Medhurst, T., King, A., Craig, S., Poulsen, B. and Luo, X., 2003. 3D stresseffects, rock damage and longwall caving as revealed by microseismic monitoring. End ofgrant report. Australian Coal Association Research Program. Project C9021.Hebblewhite, B., Waddington, A. and Wood, J., 2000. Regional horizontal surface displacementsdue to mining beneath severe surface topography. 19 th Intl Conference on Ground Control inMining, Morgantown, West Virginia, August 2000, pp. 149-157. (see alsohttp://www.mines.unsw.edu.au/Publications/publications_staff/Paper_Hebblewhite_MineSubsidence_2001.htm)McGarr, A. and Simpson, D., 1997. A broad look at induced and triggered seismicity.Gibowicz and Lasocki, editors, Rockbursts and Seismicity in Mines, Balkema,Rotterdam, pp. 385-396.MacGregor, S., 2003. Definition of stress regimes at borehole, mine and regional scale in theSydney Basin through breakout analysis. A.C. Hutton, B.G. Jones, P.F. Carr, B. Ackermanand A.D. Switzer, editors, Proceedings if the 35 th symposium on “Advances in the study ofthe Sydney Basin”, September 29-30, 2003 University of Wollonging, pp. 223-232.Vasundhara, Watts, E., Hatherly, P., Poole, G. and Hebblewhite, B., 2000. Investigation ofseismicity near Appin NSW, and its association with mining geology and tectonics. 19 th IntlConference on Ground Control in Mining, Morgantown, West Virginia, August 2000.1 http://www.ga.gov.au/oracle/quake/quake_online.jsp113