Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopGround motions in Sydney from an Earthquakeon the Lapstone StructurePAUL SOMERVILLE* ,# AND ROBERT GRAVES**Pasadena Office, URS CorporationTel: 626-449-7650, Email: paul_somerville@urscorp.com# Deputy Director, Risk FrontiersTel: 02-9850-4416, Email: psomervi@els.mq.edu.auINTRODUCTIONThe Lapstone Structure is thought to be a north-striking fault that dips westward at a shallow angle.If an earthquake were to occur on this structure, the ground motions in the Sydney basin areexpected to be stronger than predicted by empirical ground motion attenuation relations, due to twoeffects. First, rupture on the fault would be expected to occur at depth, and propagate up dip on thefault. This updip and easterly rupture propagation would focus energy toward the east into theSydney basin, due to the rupture directivity effect. Second, this energy would become trappedwithin the thickening western margin of the Sydney basin, causing further amplification of theground shaking level in the Sydney basin. We have used earthquakes in the Los Angeles region ofCalifornia to illustrate the potential impact of these two effects on strong ground motion amplitudes.RUPTURE DIRECTIVITY EFFECTAn earthquake is a shear dislocation that begins at a point on a fault and spreads at a velocity that isalmost as large as the shear wave velocity. The propagation of fault rupture toward a site at avelocity close to the shear wave velocity causes most of the seismic energy from the rupture toarrive in a single large pulse of motion that occurs at the beginning of the record (Somerville et al.,1997). This pulse of motion represents the cumulative effect of almost all of the seismic radiationfrom the fault. The radiation pattern of the shear dislocation on the fault causes this large pulse ofmotion to be oriented in the direction perpendicular to the fault, causing the strike-normal groundmotions to be larger than the strike-parallel ground motions at periods longer than about 0.5seconds. To accurately characterize near fault ground motions, it is therefore necessary to specifyseparate response spectra and time histories for the fault-normal and fault-parallel components ofground motion.Forward rupture directivity effects occur when two conditions are met: the rupture front propagatestoward the site, and the direction of slip on the fault is aligned with the site. The conditions forgenerating forward rupture directivity effects are readily met in strike-slip faulting, where therupture propagates horizontally along strike either unilaterally or bilaterally, and the fault slipdirection is oriented horizontally in the direction along the strike of the fault. The conditionsrequired for forward directivity are also met in dip slip faulting. The alignment of both the rupturedirection and the slip direction updip on the fault plane produces rupture directivity effects at siteslocated around the surface exposure of the fault (or its updip projection if it does not break thesurface).Rupture directivity effects were clearly evident in the strong motion recordings of the 1994Northridge, California earthquake, as illustrated in Figure 1. The top part of the figure shows thesurface projection of the dipping fault plane, whose top edge (to the north) lies at a depth of 6 kmand whose bottom edge (to the south) lies at a depth of 20 km. Rupture propagated updip from thehypocenter, shown by a star, which was located near the bottom edge of the fault. The recordedpeak ground velocities were much larger over the top edge of the fault than above the epicentral84