Correlation of ground motion and intensity for the 17 January 1994 Northridge, California, Earthquake

We analyze the correlations between intensity and a set of ground-motion parameters obtained from 66 free-field stations in Los Angeles County that recorded the 1994 Northridge earthquake. We use the tagging intensities from Thy-wissen and Boatwright (1998) because these intensities are determined independently on census tracts. rather than interpolated from zip codes, as are the modified Mercalli isoseismals from Dewey et al. (1995). The ground-motion parameters we consider are the peak ground acceleration (PGA), the peak ground velocity (PGV), the 5%-, damped pseudovelocity response spectral (PSV) ordinates at 14 periods from 0. 1 to 7.5 sec, and the rms average of these spectral ordinates from 0.3 to 3 see. Visual comparisons of the distribution of tagging intensity with contours of PGA, PGV, and the average PSV suggest that PGV and the average PSV are better correlated with the intensity than PGA. The correlation coefficients between the intensity and the ground-motion parameters bear this out: r = 0.75 for PGA, 0.85 for PGV, and 0.85 for the average PSV. Correlations between the intensity and the PSV ordinates, as a function of period, are strongest at 1.5 sec (r = 0.83) and weakest at 0.2 sec (r = 0.66). Regressing the intensity on the logarithms of these ground-motion parameters yields relations I infinity mlog theta with 3.0 less than or equal to m less than or equal to 5.2 for the parameters analyzed, where m = 4.4 +/- 0.7 for PGA, 3.4 +/- 0.4 for PGV. and 3.6 +/- 0.5 for the average PSV.