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Department of Earth Sciences
University of Southern
California
Los Angeles, California, 90089-0740
(E.H.F.)
Southern California Earthquake Center
University of Southern
California
Los Angeles, California, 90089-0740
(SCEC)
This article presents an overview of the Southern California Earthquake Center (SCEC) Phase-III effort to determine the extent to which probabilistic seismic hazard analysis (PSHA) can be improved by accounting for site effects. The contributions made in this endeavor are represented in the various articles that compose this special issue of BSSA.
Given the somewhat arbitrary nature of the site-effect distinction, it must be carefully defined in any given context. With respect to PSHA, we define the site effect as the response, relative to an attenuation relationship, averaged over all damaging earthquakes in the region. A diligent effort has been made to identify any attributes that predispose a site to greater or lower levels of shaking. The most detailed maps of Quaternary geology are not found to be helpful; either they are overly detailed in terms of distinguishing different amplification factors or present southern California strong-motion observations are inadequate to reveal their superiority. A map based on the average shear-wave velocity in the upper 30 m, however, is found to delineate significantly different amplification factors. A correlation of amplification with basin depth is also found to be significant, implying up to a factor of two difference between the shallowest and deepest parts of the Los Angeles basin. In fact, for peak acceleration the basin-depth correction is more influential than the 30-m shear-wave velocity. Questions remain, however, as to whether basin depth is a proxy for some other site attribute.
In spite of these significant and important site effects, the standard deviation of an attenuation relationship (the prediction error) is not significantly reduced by making such corrections. That is, given the influence of basin-edge-induced waves, subsurface focusing, and scattering in general, any model that attempts to predict ground motion with only a few parameters will have a substantial intrinsic variability. Our best hope for reducing such uncertainties is via waveform modeling based on first principals of physics.
Finally, questions remain with respect to the overall reliability of attenuation relationships at large magnitudes and short distances. Current discrepancies between viable models produce up to a factor of 3 difference among predicted 10% in 50-yr exceedance levels, part of which results from the uncertain influence of sediment nonlinearity.
This article has been cited by other articles:
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  F. Pettenati and L. Sirovich Validation of the Intensity-Based Source Inversions of Three Destructive California Earthquakes Bulletin of the Seismological Society of America, October 1, 2007; 97(5): 1587 - 1606. [Abstract] [Full Text] [PDF]
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J. B. Scott, M. Clark, T. Rennie, A. Pancha, H. Park, and J. N. Louie A Shallow Shear-Wave Velocity Transect across the Reno, Nevada, Area Basin Bulletin of the Seismological Society of America, December 1, 2004; 94(6): 2222 - 2228. [Abstract] [Full Text] [PDF]
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W. J. Lutter, G. S. Fuis, T. Ryberg, D. A. Okaya, R. W. Clayton, P. M. Davis, C. Prodehl, J. M. Murphy, V. E. Langenheim, M. L. Benthien, et al. Upper Crustal Structure from the Santa Monica Mountains to the Sierra Nevada, Southern California: Tomographic Results from the Los Angeles Regional Seismic Experiment, Phase II (LARSE II) Bulletin of the Seismological Society of America, April 1, 2004; 94(2): 619 - 632. [Abstract] [Full Text] [PDF]
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B. Savage and D. V. Helmberger Site Response from Incident Pnl Waves Bulletin of the Seismological Society of America, February 1, 2004; 94(1): 357 - 362. [Abstract] [Full Text] [PDF]
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Empirical Corrections for Basin Effects in Stochastic Ground-Motion Prediction, Based on the Los Angeles Basin Analysis Bulletin of the Seismological Society of America, August 1, 2003; 93(4): 1679 - 1690.
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J. Gomberg, B. Waldron, E. Schweig, H. Hwang, A. Webbers, R. VanArsdale, K. Tucker, R. Williams, R. Street, P. Mayne, et al. Lithology and Shear-Wave Velocity in Memphis, Tennessee Bulletin of the Seismological Society of America, June 1, 2003; 93(3): 986 - 997. [Abstract] [Full Text] [PDF]
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D. M. Boore Phase Derivatives and Simulation of Strong Ground Motions Bulletin of the Seismological Society of America, June 1, 2003; 93(3): 1132 - 1143. [Abstract] [Full Text] [PDF]
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Tests of Source-Parameter Inversion of the U.S. Geological Survey Intensities of the Whittier Narrows 1987 Earthquake Bulletin of the Seismological Society of America, February 1, 2003; 93(1): 47 - 60.
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Updated Near-Source Ground-Motion (Attenuation) Relations for the Horizontal and Vertical Components of Peak Ground Acceleration and Acceleration Response Spectra Bulletin of the Seismological Society of America, February 1, 2003; 93(1): 314 - 331.
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Ground Motions at Memphis and St. Louis from M 7.5-8.0 Earthquakes in the New Madrid Seismic Zone Bulletin of the Seismological Society of America, April 1, 2002; 92(3): 1015 - 1024.
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Unexpected Values of Qs in the Unconsolidated Sediments of the Mississippi Embayment Bulletin of the Seismological Society of America, April 1, 2002; 92(3): 1117 - 1128.
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Nonlinearity at California Generic Soil Sites from Modeling Recent Strong-Motion Data Bulletin of the Seismological Society of America, March 1, 2002; 92(2): 863 - 870.
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H. Magistrale, S. Day, R. W. Clayton, and R. Graves The SCEC Southern California Reference Three-Dimensional Seismic Velocity Model Version 2 Bulletin of the Seismological Society of America, December 1, 2000; 90(6B): S65 - S76. [Abstract] [Full Text] [PDF]
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J. H. Steidl and Y. Lee The SCEC Phase III Strong-Motion Database Bulletin of the Seismological Society of America, December 1, 2000; 90(6B): S113 - S135. [Abstract] [Full Text] [PDF]
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E. H. Field A Modified Ground-Motion Attenuation Relationship for Southern California that Accounts for Detailed Site Classification and a Basin-Depth Effect Bulletin of the Seismological Society of America, December 1, 2000; 90(6B): S209 - S221. [Abstract] [Full Text] [PDF]
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E. H. Field and M. D. Petersen A Test of Various Site-Effect Parameterizations in Probabilistic Seismic Hazard Analyses of Southern California Bulletin of the Seismological Society of America, December 1, 2000; 90(6B): S222 - S244. [Abstract] [Full Text] [PDF]
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