A study of site effects in Ilan, Taiwan, based on attenuation relationships of spectral acceleration

The purpose of this paper is to study the site effects for Ilan in northeastern Taiwan based on attenuation relationships of spectral acceleration (SA). Over 2660 accelerograms recorded from 92 shallow earthquakes with M (sub w) magnitude ranging from 4.0 to 7.7 are used to develop these regional attenuation relationships. Site effects are investigated using the site residuals obtained from the SA attenuation relationships. The results show the standard deviations of residuals decrease by 1.7%-16.4% in horizontal SA after incorporating the site-effect term V (sub S30) in the attenuation relationship of periods greater than 0.3 s. Clearly, the attenuation relationships are improved by including the site-effect term, especially for the longer periods. In addition, from the contour maps of intraevent site residuals relative to an attenuation equation of our model 2 phase 1 for horizontal SA, we find high amplification factors of short period (0.1-0.2 s), reaching 2.7 in Nanao with hard schist rocks. High amplification factors of intermediate periods (0.3-1.0 s) reach 1.9 and 2.0, respectively, in the city of Ilan and Zhuangwei. High amplification factors of long period (1.0-3.0 s) reach 2.1 in Zhuangwei, where there is the greatest sediment thickness. The results call for special attention to the seismic design of structures in these locations. Furthermore, the intraevent residuals of the four site classes converging the 0.15-0.2 s period range are in agreement with the findings by Aki (1988). Accordingly, site classes D and E have the largest ground-motion amplification factor in the 0.15-0.5 s and >0.5 s period ranges, respectively. Finally, the predominant-period contour maps deduced from V (sub S30) , V (sub S75) , and sediment depth in the Ilan Plain fall in the ranges of 0.4-1.0, 1.0-1.5, and 1.5-8.0 s, respectively. This suggests that construction of buildings taller than nine stories in the Zhuangwei area should pay special attention to avoid resonance of seismic excitation.