Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopcomposed of Narrabeen Group sandstones sourced from the upper cliff line. Carne Creek has incisedthrough into the Permian sediments which now form the valley floor and lower slopes. The landslideitself probably blocked Carne Creek, which is now manifested as an over steepened reach in thecreek bed longprofile, with the drainage line pushed against the western valley side. The exact ageof the landslide is unknown. However, material from a partly infilled depression in the toe-zone, butwell away from any fluvial influence, was dated by Optically Stimulated Luminescence (OSL-SARmethod using multiple grain quartz; University of Melbourne, Luminescence Laboratory) as 13.6 +2 ka, indicating at least a Late Pleistocene age for the landslide (Macris, 2002).It is likely that large landslides are more widespread in the incised valleys of the Blue MountainsPlateau but have remained undiscovered due to the rugged terrain and difficulty of access. Air photointerpretation is hampered by a thick vegetation cover and also by “shadowing” from high cliffs andsteeper valley sides. Thus known or strongly suspected landslides (e.g. Tumbledown, Lacys andBimlow) were detected where the vegetation had been partly cleared and by way of drainagedisruption and anomalous contouring on topographic maps (Rib and Liang, 1978).SEISMICITY IN THE SOUTH-WEST SYDNEY BASINThe distribution of earthquakes recorded in the Sydney Basin since 1872 is shown in Figure 3. Thereis a distinct cluster of earthquakes in the south-west of the basin at the location of the WollondillyRiver (Warragamba to Upper Burragorang) and also on the western margin around Lithgow, and thesouthern margin near Mittagong and Wollongong. The south-west cluster, shown in more detail inFigure 4, includes 16 earthquakes and 291 aftershocks measured since 1960 (Table 1). The majorityof the 16 earthquakes were between magnitude 1 and 3 and occurred at mid-crustal depths with theexception of 3 earthquakes which occurred at the surface and could be mining related (blasting). Thelargest earthquake was the Burragorang / Picton earthquake (M 5.5) which is discussed below.The Burragorang earthquake (also known as the Picton earthquake) occurred on 9 March 1973 underthe southern end of Lake Burragorang in the Wollondilly valley (Drake, 1974). The earthquakeregistered a magnitude 5.5 and was followed by 23 aftershocks which continued to occur for severalmonths after the main shock (Mills and Fitch, 1977). The epicentre of the earthquake was located 6km east of Yerranderie, although the area of seismicity extended in a north-west to south-eastdirection for 25 + 4 km (MWSDB, 1974). The earthquake is thought to have resulted frommovement along a steeply dipping thrust fault which extends 8 – 24 km depth below the surfacewithin the Palaeozoic basement rocks, and approximately 8 km laterally along a north-west to southeaststrike (Mills and Fitch, 1977). The earthquake was felt over an area of 60,000 km 2 , however,structural damage in the township of Nattai 17 km north-east of the epicentre was reported to be noworse than at Wollongong 65 km away, suggesting that ground movement at the surface in thevicinity of the epicentre was probably minor (Denham, 1976).DISCUSSIONThe overlap in the distribution of earthquakes and landslides in the lower Wollondilly and Nattaivalleys suggests a possible cause and effect link. Alternatively, it may be a coincidence given thepresence of landslides in the Wolgan Valley to the north where very few seismic events haveoccurred since 1872. However, the problem with this analysis is that the Carne Landslide occurredwell outside the modern seismic record and it is impossible to determine if seismicity was a triggeror contributing factor. Similarly, the ages of the Wollondilly and Nattai landslides are at least pre-European and therefore outside the period of record. The only landslides known to have occurredwithin the period of the seismic record are those at North Nattai, but these are attributed to groundsubsidence caused by pillar extraction for coal mining (Pells et al., 1987; Cunningham, 1988).Nonetheless analysis of the timing of failures at North Nattai (Pells et al., 1987) shows some103
Seismic Hazard in SydneyProceedings of the one day workshoppossible overlap with recorded seismic events (Table 1) in 1973, 1976 and 1979, but not for the late1965, December 1983 and early 1984 failures. Additionally, the Burragorang earthquake showedthat moderate earthquakes can impact on structures tens of kilometres away, which suggests thatslopes at or close to the angle of stability in neighbouring valleys might be affected. Therefore, weconclude that seismicity has been recorded within the areas where landslides occur, but we canneither confirm nor rule out a seismic link at this stage.Figure 3: Seismic events recorded in the Sydney Basin since 1872. Landslides indicated as C –Carne, B – Bimlow, L – Lacys, T – Tumbledown, N – North Nattai (Source: Geoscience AustraliaEarthquake Database http://www.ga.gov.au/oracle/quake/quake_online.jsp)The magnitude of earthquakes may be a key factor in determining if a landslide is triggered alongthe steep sides of the incised valleys in the Blue Mountains Plateau. Reynolds (1976) reports that amagnitude 5.5 earthquake is not likely to fracture hard rock at the surface or within 1 km of theepicentre, whereas a magnitude 6.5 earthquake is very likely to fracture hard rock at the surface. Thestrong joining patterning in the Triassic sandstones suggests that the vertical cliff lines couldfracture and fail under a magnitude 6.5 earthquake or greater. Based on early records, Drake (1976)estimates that the return interval of a magnitude 5.5 earthquake in the south-west Sydney Basin isapproximately 30 years, whilst the return interval of a magnitude 6.5 earthquake is approximately300 years. More recent analysis of earthquake magnitude recurrence by Berryman and Stirling(2003) for the western Sydney Basin, which includes the LSC and Blue Mountains Plateau, indicatesslightly longer return intervals. For example, a magnitude 6 earthquake is predicted to have a returnof around 1,000 years and earthquakes of magnitude 7 to 7.5 may have return intervals of 10 – 25kyr. Using the Tumbledown and Carne Landslides as indicators of the volume of rock that could beremoved by landslides from the incised Nattai valley, the estimated return period of a 1.5 M m 3104
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Landslides in the Sydney Basin - Geoscience Australia

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