Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopequivalent tectonic environment and (ii), as already discussed, moment release rate is dominated bythe largest events.With the foregoing decisions and calculations we are able to calculate the moment release rate overall dip-slip structures in a 25 km × 10 km area. This moment release rate was based on the pattern ofmapped strike-slip and dip-slip faulting in coalfield data in the South Sydney Basin. The tunnel logdata for the North Sydney Basin is relatively complete for the dip-slip structures (Figure 5). Thesedata show an average spacing of 5.7 faults/km. Reactivated reverse faults are also identified in thetunnel logs. These have an average spacing of 0.29 faults/km. Other dip-slip faults may also bereactivated, however, but if their net offset is still normal they will not be identified. We can thuspartition the moment rate by using the dip-slip fault density measured in an east-west direction andassuming that these structures, or structures sub-parallel to them, accommodate all the necessarydip-slip motion within the 25 km × 10 km sample region.Few of the tunnels in the north Sydney Basin have any significant north-south extent. The bestexample is the New Southern Railway tunnel, in which 89% of logged faults are in the NE-SWquadrant leaving only 11% as possibly being in the category of NW-SE oriented strike-slip faults.These data support our interpretation, based on the coalfield data to the south, that the strike-slipfaults are fewer in number and more major through-going structures than the dip-slip faults.In partitioning the moment release rate among the dip-slip faults consideration was given to thelikelihood that already reactivated faults would take up future compressional stress or whetheranother of the normal faults will be reactivated with reverse slip for the first time. In theprobabilistic calculations these decisions introduced large uncertainty to derived slip rate andrecurrence estimates for the existing array of reverse faults.In the final step on this part of the logic tree we need to calculate the slip rate for the reverse faults.Thus far we have determined the moment release rate on dip-slip structures and we understand theeast-west density of those structures. As mentioned above, we assume that in the along-strikedirection these structures continue, or are replaced by sub-parallel structures. This is a safeassumption because (i) dip-slip faulting has been shown to be pervasive throughout the region and(ii) tectonic strain needs to be accommodated over the whole length of the region. Effectively then,we can think of the dip-slip portion of the moment rate being released by an array of faults of length25 km, width 18 km, at an appropriate density in the east-west direction. Slip rate for each of thesefaults can then be calculated using Equation 5, where M o is replaced by moment rate and D isreplaced by the slip rate. This slip rate is the average sub-surface rate, applies along the entire fault,and is thus the rate we need for any single fault.In terms of the regional seismicity, the total moment release rate and slip rate for the fault array ismade up of characteristic earthquakes on the any particular fault (whose length is calculated on theother branch of logic tree) plus a range of various-sized earthquakes over the rest of that fault array.These combine to produce the appropriate Gutenberg-Richter distribution of sizes and rates for thearea.Having calculated average subsurface SED (and the associated earthquake magnitude) and mean sliprate, then the mean recurrence interval for any fault of interest can now simply be calculated bydividing the individual SED values by the associated slip rate.
Seismic Hazard in SydneyProceedings of the one day workshopRESULTSIn this section we first present the results from our preferred model of rupture length, earthquakemagnitude, slip rate, and surface rupture recurrence interval for distributions of surface SED. Wethen undertake sensitivity tests, varying certain model assumptions to judge the effect on the results.Outputs from the preferred modelTypical outputs for a reverse fault in the 20-30 km coastal part of the Sydney basin with meansurface single event displacement of 30 cm from our preferred probabilistic model are summarisedin Table 5, where we list the mean, median, 16 th , and 84 th percentiles for each quantity.Table 5 Preferred values of logic tree output84% MEDIAN MEAN 16%Surface SED (cm) 16 25 30 46Subsurface SED (cm) 23 41 50 74Rupture length (km) 2.5 4.0 4.9 7.5M w 5.0 5.5 5.5 6.0Slip rate (mm/year) 0.00003 0.00015 0.00057 0.00082Mean recurrence interval (Ma 1 ) 0.53 3.31 13.13 20.911 Millions of years, the surface SED value in bold is the input parameterSensitivity testsTo investigate which of the input or derived parameters have the greatest effect on the target hazardparameters of rupture recurrence and associated earthquake magnitude, a series of sensitivity tests ofthe probabilistic model was carried out. For each sensitivity test the effect on mean recurrenceinterval is listed in Table 6.In the section “Derivation of fault parameters and earthquake size from fault offset” we discussedthe partitioning of moment rate between strike-slip and dip-slip structures in the Sydney Basin. Forthe preferred model, the majority of the moment rate was released on strike-slip structures. To putall the moment release on dip-slip structures is not a realistic option because strike-slip structures arelargest in the region and in some cases also show signs of reactivation. The strike-slip structures arealso well-oriented to slip in the current regional east-west compressive stress field. As a sensitivitytest then, we put equal amounts of moment release on the strike-slip and dip-slip structures. Theresult is to significantly shorten the mean recurrence interval of surface rupture compared to thepreferred model (Table 6), although the mean recurrence interval is still very long. The resultingmean recurrence interval distribution has been to move the distribution towards shorter recurrenceintervals rather than change its shape. There is no effect on the magnitude distribution.In the section “Derivation of fault parameters and earthquake size from fault offset” we discussedthe partitioning of moment rate between the dip-slip structures in the Sydney Basin to obtain the sliprate of a reactivated fault. For the preferred model, slip was partitioned according to a mix of thefault density of reactivated reverse structures, and of all of the dip-slip faults. As a sensitivity testwe used a fault density appropriate to just the logged reactivated reverse structures. This is quite anextreme test because faults are only logged as being reactivated if they have net reversedisplacement. The resulting recurrence interval distribution is shortened by about a factor of 3, andthe distribution is much more peaked. There is no effect on the magnitude distribution.78
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Landslides in the Sydney Basin - Geoscience Australia

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