Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopA)B)Figure 5 Fault data from tunnels logs. a) Frequency histogram of fault strikes and b)Plot of fault displacement versus distance to closest adjacent fault.DERIVATION OF FAULT SCALING RELATIONS FOR THE SYDNEY REGIONIn general, the parameters of earthquake rupture length, width, displacement, and seismic momentscale in quite predictable ways. These parameters can be related by scaling relations. Scalingrelations enable the prediction of, for example, the length and magnitude of an earthquake rupturegiven its displacement. The observed field data in Sydney is fault offset and we have interpreted thisin terms of a number of distinct earthquakes by using a displacement-length scaling relationship. Adisplacement-magnitude relationship is used to predict the range of mean magnitudes associatedwith our range of mean SED valuesMany different earthquake scaling relationships have been developed over the past several decades,the most well-known of which are those derived by Wells and Coppersmith (1994). Theirrelationships are now recognised as having some limitations (e.g. Stirling et al., 2002), the mostsignificant for this study being that they are based on linear, rather than bilinear regressions. It has67
Seismic Hazard in SydneyProceedings of the one day workshopbeen recognised for some time that large shallow earthquakes rupture the entire crustal thickness andare thus width-limited. Such earthquakes scale differently to smaller events (e.g. Scholz, 1982,Shimazaki, 1986). In this study, where displacements are relatively small and associated rupturewidths and lengths are also modest, scaling relations that are self-similar, i.e. length, width, anddisplacement all increase in proportion to seismic moment, are appropriate. Seismologically basedrelations, rather than geological ones, are best in this situation as surface geological data are sparsefor small earthquakes as few rupture to the surface.The parameter that controls a self-similar displacement-length relationship is static stress drop(Mohammadioun & Serva, 2001). Stress drops, although certainly not a measure of absolute stresslevels in the crust, are often used as a guide. A simple division between high stress drops forintraplate earthquakes compared with lower stress drops for interplate earthquakes has beenrecognised (Scholz et al., 1986). Kanamori and Allen (1986) noted an almost equivalent differencebetween high stress drops for earthquakes occurring on crustal faults with long recurrence intervalsversus low stress drops on short recurrence interval faults. These values have been determined fromgeological observations of large earthquakes world-wide.For continental Australia we would expect high stress drops, it being a typical intraplateenvironment with relatively low rates of seismicity and long recurrence intervals. Earlydeterminations of stress drops for continental Australian earthquakes by Denham et al. (1981) andMumme (1984) found very low values of 10 bars or less. In these studies, rupture lengths werederived from aftershock distributions. In a following section we discuss how aftershockdistributions systematically overestimate rupture lengths for world-wide data. It seems that forcontinental Australian earthquakes this effect is particularly pronounced. For example, in theMeckering earthquake, the secondary faulting, accompanied by aftershocks, was seen to developover a period of weeks after the mainshock and, in one case, over 22 months (Gordon & Lewis,1980). Seismological studies of the Meckering earthquake show that it was, in fact, a high stressdrop event. Teleseismic body wave modelling by Vogfjord and Langston (1987) determined asource duration of 3–5 s, corresponding to a stress drop of ~100 bars. In their elastic dislocationmodelling the subsurface fault length was assumed to be 20 km, significantly shorter than the curved37 km length of the surface fault scarp. A detailed study of aftershock depths by Langston (1987)also concluded that the crust was very strong in continental Australia and that associated crustalstresses were high.The Tennant Creek earthquake comprised three main events with each being associated withseparate episodes of surface faulting (Crone et al., 1992). Such an expansion of faulting with timewould also be expected to be accompanied by an expansion of the aftershock distribution asobserved in the Meckering earthquake. Such expansion is now understood in terms of stresstriggering (e.g. King et al., 1994) whereby the initial dislocation triggers slip on adjacent faults. Theprevalence of such expansion or triggering is more an indication of high stresses within the crustrather than low stresses.Other evidence that the crust in continental regions is in a critical state of stress comes from deepborehole data (Townend & Zoback, 2000, Zoback & Townend, 2001), which show that hydrostatic,rather than lithostatic, pore pressures exist in the brittle crust. The lower pore pressures mean thatthe crust is much stronger than it otherwise would be, and thus capable of withstanding high stresslevels from tectonic driving forces. Areas of continental crust can thus behave as rigid plates overtime scales of tens to hundreds of millions of years.68
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Landslides in the Sydney Basin - Geoscience Australia

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