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Preliminary seismic hazard assessment for Los Angeles, Ventura, and Orange counties, California, affected by the 17 January 1994 Northridge earthquake

California Department of Conservation Division of Mines and Geology, 801 K St. MS 12-31, Sacramento, California 95814

Abstract

The seismic ground motion hazard is assessed for a 10% probability of exceedance in 50 years for the three counties (Los Angeles, Ventura, and Orange) impacted by the 1994 Northridge earthquake (M_w 6.7). The earthquake source model of the Southern California Earthquake Center has been modified with additional slip-rate information for mapped faults and blind thrusts and incorporated into seismic hazard maps that will be used for making regional hazard and risk mitigation decisions by state and local government agencies. Peak horizontal ground acceleration (pga) and 5% damped spectral acceleration (SA) (0.3 and 1 sec) were calculated with three equally weighted attenuation relationships of Boore et al. (1993), Campbell and Bozorgnia (1994), and Sadigh (written comm., 1994). The results of this assessment indicate high hazard over the entire tri-county area with ground motions exceeding 0.4 g (pga), 1.0 g (0.3-sec SA), and 0.5 g (1-sec SA) nearly everywhere. A Monte Carlo uncertainty analysis is described for two sites located in Los Angeles and Northridge. This analysis yields 95% confidence limits for peak ground acceleration at the two sites that range between ±0.1 and ±0.2 g. Our calculations indicate that the uncertainty in the magnitude-rupture length relations, magnitude distribution, moment-magnitude relation, attenuation relation, and slip-rate contribute most to the hazard uncertainty at these sites and that the highest uncertainties in the mapped ground motion are associated with the strongest anticipated ground motions.

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