Seismic Hazard <strong>in</strong> <strong>Sydney</strong>Proceed<strong>in</strong>gs of <strong>the</strong> one day workshopFigure 7: Map of peak ground velocity from <strong>the</strong> broadband simulation of <strong>the</strong> Puente Hillsrupture scenario. The largest simulated motions occur along <strong>the</strong> top edge of <strong>the</strong> rupture plane,and are <strong>the</strong> result of rupture directivity effects. These strong pulses of motion are channeleddirectly <strong>in</strong>to <strong>the</strong> sediments of <strong>the</strong> Los Angeles bas<strong>in</strong> and become trapped as bas<strong>in</strong> generatedsurface waves. These large amplitude surface waves travel southward across <strong>the</strong> bas<strong>in</strong>, giv<strong>in</strong>grise to an elongated pattern of elevated PGV that extends across <strong>the</strong> bas<strong>in</strong>. This patternresembles <strong>the</strong> pattern of ground shak<strong>in</strong>g <strong>in</strong>tensity from <strong>the</strong> 1987 Whittier Narrows earthquakeshown <strong>in</strong> Figure 4.would represent <strong>the</strong> <strong>Sydney</strong> Bas<strong>in</strong>. Figures 5 and 7 show that <strong>the</strong> strongest ground motions wouldbe located around <strong>the</strong> surface trace of <strong>the</strong> Lapstone structure, due to rupture directivity effects. Theyalso show that <strong>the</strong>re would be large ground motion amplitudes <strong>in</strong> <strong>the</strong> <strong>Sydney</strong> Bas<strong>in</strong>, due to <strong>the</strong>trapp<strong>in</strong>g of <strong>the</strong>se strong directivity pulses by <strong>the</strong> thicken<strong>in</strong>g edge of <strong>the</strong> <strong>Sydney</strong> Bas<strong>in</strong>.Particularly strong ground motion amplification can occur at <strong>the</strong> edge of a bas<strong>in</strong>, if <strong>the</strong> bas<strong>in</strong> has anabrupt edge caused by dip-slip fault<strong>in</strong>g. The severe damage caused by <strong>the</strong> 1995 Kobe earthquake91
Seismic Hazard <strong>in</strong> <strong>Sydney</strong>Proceed<strong>in</strong>gs of <strong>the</strong> one day workshopoccurred on <strong>the</strong> bas<strong>in</strong> side of <strong>the</strong> fault, about 1 km from <strong>the</strong> fault that caused <strong>the</strong> earthquake. Thisnarrow band of damage has been attributed to bas<strong>in</strong> edge effects (Pitarka et al., 1998) caused by <strong>the</strong>abrupt change <strong>in</strong> depth to crystall<strong>in</strong>e basement (about 1 km) that occurs across <strong>the</strong> fault. An abrupt<strong>in</strong>crease <strong>in</strong> depth to bedrock across <strong>the</strong> Santa Monica fault caused a similar bas<strong>in</strong> edge effect dur<strong>in</strong>g<strong>the</strong> 1994 Northridge earthquake (Graves et al., 1998). If <strong>the</strong>re is an abrupt <strong>in</strong>crease <strong>in</strong> depth tobedrock across <strong>the</strong> Lapstone Structure, <strong>the</strong>n it could cause bas<strong>in</strong> edge amplification effects <strong>in</strong> a zonewith<strong>in</strong> a few km to <strong>the</strong> east of <strong>the</strong> Lapstone Structure.REFERENCESDolan, J.F., S.A. Christofferson, and J.M. Shaw (2003). Recognition of paleoearthquakes on <strong>the</strong>Puente Hills Bl<strong>in</strong>d Thrust Fault, California. Science 300, 115-118.Graves, R. and A. Pitarka (2004). Broadband time history simulation us<strong>in</strong>g a hybrid approach.Proceed<strong>in</strong>gs of <strong>the</strong> 13 th World Conference on Earthquake Eng<strong>in</strong>eer<strong>in</strong>g, Vancouver, Canada,August 1-6, 2004, Paper No. 1098.Graves, R. W., A. Pitarka, and P. G. Somerville (1998). Ground motion amplification <strong>in</strong> <strong>the</strong> SantaMonica area: effects of shallow bas<strong>in</strong> edge structure, Bull. Seism. Soc. Am., 88, 1224- 1242.Graves, R.W. (1996). Simulat<strong>in</strong>g earthquake ground motions <strong>in</strong> 3D elastic media us<strong>in</strong>g staggeredgridf<strong>in</strong>ite-differences, Bull. Seism. Soc. Am. 86, 1091-1106.Hartzell, S., Liu, P., Mendoza, C. (1996). The 1994 Northridge earthquake: <strong>in</strong>vestigation of rupturevelocity, risetime, and high frequency radiation. J. Geophys. Res. 101, 20,091-20,108.Pitarka, A., K. Irikura, T. Iwata and H. Sekiguchi (1998). Three-dimensional simulation of <strong>the</strong> nearfaultground motion for <strong>the</strong> 1995 Hyogo-ken Nanbu (Kobe), Japan, earthquake. Bull. Seism.Soc. Am., 88, 428-440.Shaw, J.H., A. Plesch, J.F. Dolan, T.L Pratt, and P. Fiore (2002). Puente Hills bl<strong>in</strong>d-thrust system,Los Angeles, California. Bull. Seism. Soc. Am. 92, 2946-2960.Somerville, P.G., N.F. Smith, R.W. Graves, and N.A. Abrahamson (1997). Modification ofempirical strong ground motion attenuation relations to <strong>in</strong>clude <strong>the</strong> amplitude and durationeffects of rupture directivity, Seismological Research Letters, 68, 180-203.92