Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopdirectivity channels large amplitude pulses of motion directly into the Los Angeles basin, whichthen propagate southward as basin surface waves. This complexity is further apparent in comparingthe broadband ground velocity time histories simulated at downtown Los Angeles and Long Beach(Figure 6). The waveform at Los Angeles is dominated by rupture directivity, which produces astrong, concentrated pulse of motion. At Long Beach, which is located across the Los Angeles basinfrom the rupture, the waveform is dominated by late arriving longer period surface waves. Thesesurface waves are generated in the near fault region as the direct waves from the rupture are trappedwithin the sediments of the basin.Figure 6: Broadband ground velocity time histories simulated at downtown Los Angeles (top)and Long Beach (bottom). The waveform at Los Angeles is dominated by rupture directivity,which produces a strong, concentrated pulse of motion. At Long Beach, which is located onthe other side of the Los Angeles basin from the rupture, the ground motion is dominated bylate arriving longer period surface waves. These surface waves are generated in the near faultregion as the direct waves from the rupture are trapped within the sediments of the basin.The high density of sites used in the calculation allows us to construct detailed maps of variousground motion parameters. An example of this displays the peak ground velocity determined fromthe suite of simulated time histories (Figure 7). The largest motions occur along the top edge of therupture plane, and are the result of rupture directivity effects. These strong pulses of motion arechanneled directly into the sediments of the Los Angeles basin and become trapped as basingenerated surface waves. These large amplitude surface waves travel southward across the basin,giving rise to an elongated pattern of elevated peak velocity that extends across the basin.IMPLICATIONS FOR LAPSTONE EARTHQUAKE GROUND MOTIONS IN SYDNEYThe configuration of the Lapstone Structure and the Sydney Basin is equivalent to the configurationof the Puente Hills Blind Thrust and the Los Angeles Basin. Rotation of Figures 5 and 7anticlockwise by about 110 degrees would orient the Los Angeles geometry to match the geometryin Sydney. The thick black line would represent the surface expression of the Lapstone Structure,and the dashed rectangular region to the left of this line would represent the surface projection of aLapstone fault that dips to the west at a shallow angle. The region to the right of the thick black line90