Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopLandslides in the Sydney Basin: Is there aseismic link?KERRIE M. TOMKINS, GEOFF S. HUMPHREYS, JOHN MACRIS AND PAUL P. HESSEMACQUARIE UNIVERSITY, NORTH RYDE NSW 2109ABSTRACTSeveral large landslides have been found in the incised valleys of the Blue Mountains Plateau,located on the western margin of the Sydney Basin, where the valleys have cut into weaker strata ofthe Permian Shoalhaven Group. The landslides highlight the potential importance of massmovement processes in valley widening and hillslope denudation, yet the cause of failure in thesevalleys is unknown. The main triggers of landslides elsewhere (e.g. Europe, Asia and USA) areoften earthquakes or severe rainfall events. In this paper we consider earthquakes as a possibletrigger for the landslides and assess whether there is a seismic link between earthquakes and failureof the cliff lined upper slopes in the Blue Mountains Plateau. Seismic records for the Sydney Basinshow a clustered distribution of earthquakes particularly in the south-west of the basin where severalof the large pre-European landslides and smaller recent landslides have been found. The distributionof earthquakes and landslides indicates a possible seismic link. However, due to the difference in thetime period of records (i.e. pre-European age of the older landslides versus the contemporary seismicrecord) and the impact of mining on the more recent landslides, we are not able to link earthquakeevents to failure. We suggest that the recorded earthquakes were not significant enough to triggerslope failure (maximum magnitude 5.6), but that the larger, older landslides may have been triggeredby higher magnitude earthquake events.Keywords: rock fall, dissection, landscape evolution, seismicity, BurragorangINTRODUCTIONExamination of relief and geology maps of the Sydney Basin (Brunker and Rose, 1967; 1979)reveals several important features of the landscape: (i) a lower lying coastal sandstone plateau(Hornsby Plateau to the north and Woronora Plateau to the south) separated by a low relief basin onWiannamatta Group shales (Cumberland Plain); (ii) a higher sandstone plateau to the west (BlueMountains Plateau) bounded to the east by the Lapstone Structural Complex (LSC) (Branagan andPedram, 1990); and (iii) deeply incised valleys that stem from the LSC to the west and south-westchanging from narrow gorges to wide valleys upon intersecting the underlying Permian sediments.The incised valleys of the Blue Mountains Plateau are the focus of this paper (Figure 1), especiallythose erosion processes that lead to valley widening once river incision has reached the Permianrocks. Recent investigations in the Nattai River valley, which drains into the Wollondilly River 55km upstream of the LSC, suggests that valley widening, and presumably scarp retreat, is takingplace by mass movement events, such as rock fall and debris flows (Tomkins et al., 2004b).Previous mapping of the geology of the Wollondilly - Nattai area by McElroy and Relph (1958)indicates extensive areas of large scale surface slumping (landslides) but none of these have beenconfirmed. In contrast, landslides in Sydney Basin sediments have been identified along theIllawarra Escarpment near Wollongong where high relief coupled with high rainfall (1200 - 1500mm a -1 ) facilitates mass slope failure (Walker, 1963; Young, 1977). Further west, where rainfall ismuch lower (< 1000 mm a -1 ), it appears that landslides have not been considered even though rockfalls are omnipresent. However, the recognition of a large rotational landslide in the Wolgan valleyto the north (Macris, 2002) has helped to change this perception.100