Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopThe Sydney Basin is more of a transition region between continent and margin, so we would expectstress drops to be a little lower than for a continental region. Rates of historical seismicity are low,so earthquake recurrence intervals will be long, again indicative of high stress drops (Kanamori &Allen, 1986).Given that there is no Australian-specific scaling relation available, and few reliable determinationsof stress drop, we need to use scaling relations from other similar tectonic environments. Anappropriate self-similar seismologically based scaling relation for continental regions can be derivedfrom the Eastern North America (ENA) data set of Somerville et al. (1987). Their data areearthquake source durations and seismic moments, measured by matching synthetic with actualseismograms. The source duration can then be interpreted in terms of rupture dimension. A selfsimilarregression of seismic moment on source duration, weighted according to the estimateduncertainties in both quantities, giveslog (M o ) = 23.810.13 + 3.0 log (), (1)where M o is the seismic moment in dyne-cm and is the source duration in seconds. To deriverupture length we follow Cohn et al. (1982), who show that the rupture duration that we measure iscomprised of three terms: the time to rupture the fault plane, a travel time delay, and the rupture risetime. The time to rupture the fault is simply L/0.8 for a unilateral rupture and L / 2 /0.8 for bilateral,where is the shear wave velocity and L the fault length. The travel time delay is zero, on average,for a unilateral rupture and 0.64 L / 2 / for bilateral. To estimate the rise time (slip duration) we usethe moment-duration relation of Somerville et al. (1999, their Table 7): r = 2.0310 -9 M o1/3(2)and the corresponding moment-area (A) relation:A = 2.2310 -15 M o2/3(3)to get a rise time of 0.266 L/ for unilateral ruptures and 0.266 L / 2 / for bilateral, with total rupturedurations of 0.43L and 0.31L respectively, if = 3.5 km/s.It has recently been shown that earthquake ruptures tend to be unilateral, rather than bilateral(McGuire et al., 2002). They found that 80% of ruptures tend to be unilateral and that this was trueof both strike-slip and subduction zone thrust events. Given that duration, = 0.31L for bilateralruptures and = 0.43L for unilateral, we use a linear combination of the logarithm of these factors toobtain a moment-length scaling relation that takes account of bilateral ruptures:log (M o ) = 22.63 0.13 + 3.0 log (L), (4)where L is the rupture length in km and the uncertainty in the intercept value has been derived fromthe regression intercept uncertainty and an additional uncertainty related to the ratio of unilateral tobilateral ruptures. The scaling relation has a fixed slope of 3.0, in agreement with self-similarscaling, because the derived value was insignificantly different from 3.0.We then use the relationship between seismic and geodetic moment:M o = L W D, (5)69