Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopHowever, at periods longer than one second, seismic waves have wavelengths that are much longerthan 30 meters, and their amplitudes are controlled by geological structure having depths ofhundreds or thousands of meters which in many cases, such as in sedimentary basins, is nothorizontally layered. If the wave enters a basin through its edge, it can become trapped within thebasin if post-critical incidence angles develop. The resulting total internal reflection at the base ofthe layer is illustrated at the top right of Figure 2. In the lower part of Figure 2, simple calculationsof the basin response are compared with those for the simple horizontal layered model. In each case,a plane wave is incident at an inclined angle from below. The left side of the figure shows theamplification due to impedance contrast effects that occurs on a flat soil layer overlying rock(bottom) relative to the rock response (top). A similar amplification effect is shown for the basincase on the right side of the figure. However, in addition to this amplification, the body waveentering the edge of the basin becomes trapped, generating a surface wave that propagates across thebasin. Most empirical ground motion attenuation relations do not distinguish between sites locatedon shallow alluvium and those in deep sedimentary basins, and tend to underestimate basin groundmotions.Recent work by Dolan et al. (2003) and Shaw et al. (2002) have identified the seismic potential ofthe Puente Hills Blind Thrust Fault system, which lies directly beneath downtown Los Angeles(Figure 3). The M w 6.0 Whittier Narrows earthquake of 1987 evidently occurred on a segment ofthe Puente Hills Blind Thrust. The intensity distribution of this earthquake, shown in Figure 4, isdominated by strong intensity values south-southwest of the epicenter, due to the combined effectsof rupture directivity to the south, and the trapping of this energy by the thickening Los Angelesbasin.Figure 3. South – North (B-B 2 ) cross section showing the Puente Hills blind thrust and thelocation of the 1987 Whittier Narrows earthquake. Downtown Los Angeles is located near“SYL.” Source: Shaw et al., 2002.87
Seismic Hazard in SydneyProceedings of the one day workshopThe rupture dimensions of the Whittier Narrows earthquake were approximately 10 km x 10 km. Toassess the potential ground shaking hazard posed by a larger earthquake on the Puente Hills system,we examined a scenario where a “Northridge-type” earthquake occurs on the Los Angeles segmentof the Puente Hills system. The Los Angeles segment of the Puente Hills system shares many of thecharacteristics of the fault that ruptured during the 1994 Northridge earthquake. Both are blindthrust faults with similar dimensions (20 km by 25 km), both dip at moderate angles (40 deg. forNorthridge, 28 deg. for Puente Hills), and both terminate a few kilometers below the Earth’s surface(6 km for Northridge, 3 km for Puente Hills).Figure 4. Instrumental ground shaking intensity in the 1987 Whittier Narrows earthquake,whose epicenter is shown by a star. Source: TriNet.We calculated broadband (0-10 Hz) ground motions for this earthquake scenario using the hybridsimulation methodology of Graves and Pitarka (2004). The simulation covers an area 110 km by150 km, which spans most of the greater Los Angeles metropolitan region. This region is dividedinto a uniform grid of sites with a spacing of 500 m, yielding a total of 66,000 sites. To obtain thefull broadband response, the calculation is carried out independently in two frequency bands, andthen summed together. At low frequencies (f < 1 Hz), motions are calculated deterministically usinga finite-difference approach, which includes a detailed representation of the 3D subsurface structurethroughout the model region. Nearly 400 million grid nodes are used to represent the FD model.The calculation is carried out on the HPCC Linux Cluster at USC and requires only about 8 hours torun using 120 CPU’s. At high frequencies (f > 1 Hz), motions are calculated using a stochasticapproach for each of the 66,000 locations. The model for earthquake rupture is taken from thedescription of the Northridge earthquake given by Hartzell et al. (1996).The complexity of the propagating wave field is readily apparent in snapshots of the broadband (0-10 Hz) ground velocity simulated for the Puente Hills rupture scenario (Figure 5). Strong rupture88
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Landslides in the Sydney Basin - Geoscience Australia

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