Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopFigure 1: Incised valleys of the Blue Mountains Plateau and locations mentioned in the text.Structure according to Bembrick et al. (1973).Landslide failure triggered by earthquakes has been widely reported internationally, especially intectonically active regions (e.g. Keefer, 1984; Keefer, 1994; Dadson et al., 2003). Despite beinglocated on a plate centre, earthquakes have been recorded in Australia, although events are of acomparatively lower frequency and lower magnitude than those at plate boundaries (Sinadinovski etal., 2000). McCue (1999) relates the distribution of earthquakes across Australia to the dominantnorth – south and east – west intraplate stress patterns (Figure 2) created through the northwardmovement of the Indo-Australian plate and the eastward movement of the Pacific plate. He showsthe development of north-west and north-east oriented shear zones across the Australian continent,aligned to areas of seismicity, uplift and earlier volcanism. The centre of Australia is dominated by anorth-west shear zone which extends from the east coast around Sydney. A north-east shear zoneoccurs parallel to the east coast and includes the south-eastern highlands. The Sydney Basin, and inparticular the southern end of the Blue Mountains Plateau which includes the Nattai River, isrevealed as an area of earthquake activity, the largest of which were recorded at Robertson in 1961and Burragorang in 1973 with magnitudes of 5.6 and 5.5 respectively (Drake, 1976; Reynolds,1976). The occurrence of earthquakes and intersection of shear zones across the Sydney Basin posesan important question. Could the landslides in the incised valleys of the Blue Mountains Plateau be101
Seismic Hazard in SydneyProceedings of the one day workshoptriggered by seismicity? In this paper we explore the spatial relationship between landslideoccurrence and earthquakes to assess if such a link exists.Figure 2: Distribution of earthquakes across Australia (magnitude > 4) showing the relationship tonorth-west and north-east oriented shear zones. The Sydney Basin is located at the intersection ofthe central Australian (north-west) and east coast (north-east) shear zones (indicated by shading).(Source: McCue, 1999)LANDSLIDE OCCURRENCE AND DISTRIBUTION IN THE BLUE MOUNTAINS PLATEAUEarly geology reports from the Wollondilly and Nattai valleys comment on accumulations of largeblocks of sandstones at the base of hillslopes, covered with debris and dense vegetation (Etheridge,1893; McElroy and Relph, 1958). These blocks were sourced from the vertical cliff line aboveformed in the Hawkesbury and Narrabeen Group rocks. Reconnaissance investigations revealed thatlarge mounds of blocks are widespread on the lower hillslopes and in the valley floor particularlyalong the lower Wollondilly and Nattai Rivers. Several large landslides were discovered in theNattai and Wollondilly catchments which pre-date European settlement. For example, TumbledownLandslide in the Nattai valley 5.5 km upstream of the confluence with the Wollondilly River,Bimlow Terrace on the Wollondilly River 5 km upstream of the confluence with the Coxs River andLacys Landslide on Lacys Creek 10 km upstream of the confluence with the Wollondilly (Figure 1).Recent investigation of the Tumbledown Landslide found it to consist of a rock avalanche deposit of1.5 M m 3 overlying an older, more extensive rotational landslide (Taylor, 2005). More recentsmaller landslides along the Burragorang Walls at North Nattai have been associated with coalmining (Pells et al., 1987; Cunningham, 1988). Mapping of the geology of the lower Wollondillyvalley by McElroy and Relph (1958) shows additional areas of slumping which may representlandsliding but these areas are yet to be ground truthed.Other large landslides in the Blue Mountains Plateau have been found in the Wolgan Valley alongCarne Creek, a major tributary (Figure 1). The Carne Landslide is approximately 30 M m 3 and is102
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Landslides in the Sydney Basin - Geoscience Australia

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