2145 Bulletin of the Seismological Society of America, Vol. 92, No. 6, pp. 2145–2162, August 2002 Instrumental Intensity Distribution for the Hector Mine, California, and the Chi-Chi, Taiwan, Earthquakes: Comparison of Two Methods by Vladimir Sokolov and David J. Wald Abstract We compare two methods of seismic-intensity estimation from ground- motion records for the two recent strong earthquakes: the 1999 (M 7.1) Hector Mine, California, and the 1999 (M 7.6) Chi-Chi, Taiwan. The first technique utilizes the peak ground acceleration (PGA) and velocity (PGV), and it is used for rapid gener- ation of the instrumental intensity map in California. The other method is based on the revised relationships between intensity and Fourier amplitude spectrum (FAS). The results of using the methods are compared with independently observed data and between the estimations from the records. For the case of the Hector Mine earthquake, the calculated intensities in general agree with the observed values. For the case of the Chi-Chi earthquake, the areas of maximum calculated intensity cor- respond to the areas of the greatest damage and highest number of fatalities. How- ever, the FAS method producees higher-intensity values than those of the peak am- plitude method. The specific features of ground-motion excitation during the large, shallow, thrust earthquake may be considered a reason for the discrepancy. The use of PGA and PGV is simple; however, the use of FAS provides a natural consideration of site amplification by means of generalized or site-specific spectral ratios. Because the calculation of seismic-intensity maps requires rapid processing of data from a large network, it is very practical to generate a “first-order” map from the recorded peak motions. Then, a “second-order” map may be compiled using an amplitude– spectra method on the basis of available records and numerical modeling of the site- dependent spectra for the regions of sparse station spacing. Introduction Seismic intensity (or severity of earthquake ground mo- tion) is widely used throughout the world as a useful and simple quantity describing the damage due to earthquakes. The building codes of several countries are based on the intensity values assigned to a given seismic region (Paz, 1994), and seismic-hazard maps are often constructed in terms of modified Mercalli (MM) or Medvedev–Sponhauer– Karnik (MSK) intensity (e.g., Denham and Smith, 1993; Schenk et al., 1996; Slejko et al., 1996; Dowrick et al., 1998). At the same time, intensity distribution patterns pre- dicted for future destructive earthquakes are widely used for loss estimation (Shah et al., 1991; Corsanego, 1995; Yong et al., 1996; Spence et al., 1998). With the density of seismic networks increasing, it becomes possible to generate the in- tensity maps rapidly after earthquake for public consumption via the World Wide Web (Wu et al., 1997; Wald et al., 1999a,b). The fast computation of intensity distribution for an earthquake on the basis of ground-motion parameters (so- called “instrumental intensity map”), as well as the hazard assessment for future events, requires a regression relation- ship between intensity and strong-ground-motion parame- ters. Many attempts have been made to correlate intensity with recorded ground motions. The correlations of MM or MSK intensity with peak amplitudes (e.g., Trifunac and Brady, 1975; Murphy and O’Brien, 1977; Chernov and So- kolov, 1983; Krinitzsky and Marcuson, 1983; Aptikaev and Shebalin, 1988; Chernov, 1989; Spence et al., 1992; Wang, 1995) typically show large scatter. At present, there is no doubt that seismic intensity is an expression of the ampli- tude, duration, and frequency content of ground motion. Therefore, several attempts have been made to find relation- ships between intensity and a combination of amplitude, pe- riod, and duration (Chernov and Sokolov, 1983; Chernov, 1989), response spectra (Spence et al., 1992; Atkinson and Sonley, 2000; Atkinson, 2001; Boatwright et al., 2001), or duration-dependent ground-motion parameters, for example, the “modified root of mean square of acceleration” (Wang, 1995). The recent results obtained by Wald et al. (1999a) show that the MM intensity (I MM ) displays a correlation with peak ground acceleration (PGA) for the intensity range V  I MM  VIII and with peak ground velocity (PGV) for V 