Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopThe pattern of faulting for a 10 km 25 km portion of the Southern Coalfield is shown in Figure 3.This region is part of the coastal zone which is bounded to the west by the South Coast Monocline.Faults in this region comprise two main sets that strike approximately northwest and north to northnortheast.The northwest striking faults are regional structures with typical trace lengths of 5 to >10km, maximum apparent throws of 25 to 90 m (Lohe et al., 1992), and predominantly strike-slipmotion. The spacings of regional faults in this set are often 1–3 km. The more northerly strikingfault set has predominantly dip-slip offsets and is dominated by two clustered fault arrays, which arespaced at 3–4 km (Figure 3.) and have a maximum length of 10 km. In the Wollongong–PortHacking area there is a tendency for faults striking north-northeast to have displacements of lessthan 15 m, whereas faults striking west-northwest and northwest can reach 100 m in displacement(Sherwin & Holmes, 1986). The northwest striking faults appear to have accommodated the highestdisplacements and can be regarded as the dominant structures in the Southern Coalfield.Although net displacements on the north-northeast striking fault sets are normal, small-scale fielddata indicates that many of these faults experienced late-stage reverse reactivation (Shepherd, pers.comm., 2002). A component of reverse displacement on north to north-northeast striking faults andsinistral strike-slip on northwest striking faults is consistent with contemporary strain data which aredominated by a northeast to east principal horizontal shortening direction (Hillis et al., 1999). Thusboth fault sets could be active in the contemporary strain field. Therefore, in our calculations ofmoment release rate we use both fault sets. Given that the northwest fault set is greater in number, inmaximum apparent throw, and in trace length, we have assigned a greater component of the momentrelease to this set than to the more northerly-striking faults.Northern tunnel log dataSome of the most complete fault data in the Sydney region, with respect to spacing, sense of slip,and displacement, are from tunnels, which, on the scale of the basin, produce one-dimensionalsamples of the fault population. Here we use data from six tunnels located in central and northSydney trending in east-west (five tunnels including two from the M5 motorway) and north-south(one tunnel) directions (Figure 4). For each tunnel, fault locations, orientations, and netdisplacements were recorded. The fault data for these tunnels are summarised in Table 1. Thesedata permit the spacing between adjacent faults to be estimated and these values were used forpartitioning moment release among the dip-slip structures.In each tunnel, faults were recorded where they offset beds within the Hawkesbury SandstoneFormation. A total of 133 faults were recorded in all tunnels with the main fault strikes beingapproximately north to northeast and northwest (Figure 5a). Approximately 128 faults have netnormal displacement, while the remainder have net reverse displacement. Displacements range up to9 m with most faults (110) carrying ≤ 1 m (Figure 5b).Fault spacing was calculated for both reverse and normal faults and for northwest and northnortheaststriking fault sets. We calculate an average normal fault spacing of 140 m. This valueincreases to 175 m if we consider only faults with a north to northeast strike. These results appear tobe independent of tunnel trend.Determining the spacing of faults that have experienced reverse reactivation is more difficult. This isbecause net displacements provide no indication of reverse movement on a reactivated fault if themagnitude of the reverse movement was less than earlier normal slip on the fault. Therefore, themaximum spacing for reverse faults is approximately 3.5 km, while the minimum could be as littleas 180 m, the spacing of faults with net normal displacement. This lack of knowledge is handled asalternative branches with assigned weights in the probabilistic analysis.65
Seismic Hazard in SydneyProceedings of the one day workshopFigure 4 Locations of tunnels from which faults were recorded.Table 1 North Sydney Basin tunnel fault dataFAULT DATA TUNNEL SUMMARYMALABAR NORTH BONDI NEW M5 M5 TOTAL MEANHEAD OUTFALL S. RLY E-BOUND W-BOUNDLogged distance (m) 4746 4120 1920 2230 2895 2973 18884Total offset (m) 25.4 29.6 12.0 18.3 11.4 6.1 103Mean fault spacing (m) 115.0 129.5 159.25 96.2 166.2 177.4 140.6Throw (m/km) 5.4 7.2 6.3 8.2 3.9 2.0 5.5N&NE fault spacing (m) 173.3 166.9 159.3 101.0 230.0 221.7 175.4# of reverse faults 2 1 0? 1 1? 1 5-666
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