Seismic ground-motion hazard mapping incorporating site effects for Los Angeles, Orange, and Ventura Counties, California: A geographical information system application

The seismic ground motion hazard with site conditions incorporated is mapped for a 10% probability of exceedance in 50 years for the three counties affected by the 1994 Northridge earthquake (Mw 6.7). The source model of Petersen et al. (1996) was used to calculate peak horizontal acceleration (pga) and 5% damped spectral acceleration (SA) levels at 0.3- and 1-sec periods for alluvium, soft-rock, and hard-rock site conditions. A geologic map that differentiates the three site conditions was produced using shear-wave velocity maps of Tinsley and Fumal (1985) and extended into other areas that were not mapped by Tinsley and Fumal using 1:250,000 scale geologic maps. Ground-motion maps for each site condition were overlaid with the geologic map to produce the resulting hazard maps. The effect of geology on the hazard is observed only slightly in the peak ground acceleration map, more in the 0.3-sec SA map, and significantly more in the 1-sec SA map. The uncertainties for ground motions represented on these three maps ranges from 0.1 to 0.3 g for pga and 0.2 to 1.0 for SA; in general, 95% confidence Limits are about +/- 50% of the mean ground-motion value.