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EVALUATION OF SITE EFFECTS DURING THE 1999 CHI-CHI EARTHQUAKE AND ITS AFTERSHOCKS

GRADUATE RESEARCH ASSISTANTS: Mustafa Kockar, M. Cem Ozbey

FUNDED BY: USGS National Earthquake Hazard Reduction Program

DESCRIPTION
This US Geological Survey (USGS)-sponsored project involves the study of the strong ground motions recorded during the 1999 Chi-Chi earthquake in Taiwan and its aftershocks. Over 300 motions were recorded during the mainshock across a diverse range of soil conditions. Additionally, approximately 1000 motions were recorded during 5 aftershocks with magnitude greater than 5.0. These data represent a unique opportunity to evaluate the effect of soil conditions on ground motions during large and moderate earthquakes.

Nonlinear regression techniques were used to develop event-specific attenuation relationships for peak ground acceleration (PGA) and spectral acceleration (Sa) at periods of 0.3, 1.0, and 2.0 s. The event-specific attenuation relationships were developed for each SGS site class (SGS-B, -C, -D, -E) and these relationships were used to define amplification (with respect to B rock sites). Figure 1 shows the distribution of the Chi-Chi mainshock data according to site category and distance (site classes based on Lee et al. 2001 BSSA). Figure 1 indicates that the majority of the strong motion data was recorded at distances greater than 10 km. In fact, there are no rock (site class SGS-B) recordings for distances less than 10 km and only one rock recording between 10 and 40 km (Figure 1). A similar issue was found for the aftershock data sets. Thus, the amplification values can only be derived for distances greater than 40 km, which represent low rock intensities (< 0.1 g).

The amplification ratios derived for sites classes C, D, and E for the mainshock and aftershocks are shown in Figure 2. For comparison, the amplification ratios recommended by IBC (2001) and Rodriguez-Marek et al. (B&R-M 2001) are also shown. In general, the mainshock and aftershock amplification ratios are in good agreement. The only exception is at long periods, where the aftershock data indicate significantly larger amplification. This may be a result of poor recording of long periods during the aftershocks. The derived amplification ratios are in good agreement with those proposed by others.

Figure 1 - Click on image to enlarge	Figure 2a (Site C)- Click on image to enlarge
Figure 2a (Site D)- Click on image to enlarge	Figure 2c (Site E)- Click on image to enlarge

PUBLICATIONS
Rathje, E.M., Kockar, M., Ozbey, M.C. 2005. "Observed Site Effects During the 1999 Chi-Chi Earthquake and its Aftershocks," Seismological Society of America Annual Meeting, Lake Tahoe, CA, 27-29 April, Abstract only.

Rathje, E.M. 2004. “Evaluation of Site Effects during the 1999 Chi-Chi Earthquake and its Aftershocks,” Final Report to United States Geological Survey, Earthquake Hazard Reduction Program, October.