Spatial Variability and Attenuation of Arias Intensity during the 1999 Chi-Chi Mw 7.6 Earthquake, Taiwan

Arias intensity is a measure of earthquake intensity that captures the potential destructiveness of an earthquake. Using the strong ground motion records of the M-w 7.6 Chi-Chi earthquake, Taiwan, we analyze the spatial distribution of the vertical, horizontal, fault-parallel, and fault-normal components of the Arias intensity. We investigate the effects of the hanging wall and footwall on ground motions, and develop a relationship between Arias intensity and peak ground acceleration (PGA), incorporating the influence of local site conditions. We also evaluate the applicability of some widely used Arias intensity attenuation relationships by comparing the observed values with empirical estimates. Our results show the vertical Arias intensity is generally lower than the horizontal component, with nearly 90% of the vertical-to-horizontal ratios between 0.1 and 0.6. During this typical reverse mechanism earthquake, the fault-normal component of the Arias intensity is approximately 20% higher than the fault-parallel component for sites within 40 km of the fault. Arias intensities on the hanging wall are systematically higher than those on the footwall, indicating clear hanging-wall effects. Site conditions influence the correlation between Arias intensity and PGA. Arias intensity shows a good correlation with PGA and VS30. Arias intensity increases with increasing PGA and decreasing V-S30. The Arias intensities observed within 40 km of site class C locations, and within 80 km of site class D locations, are generally lower than the mean estimates from empirical predictions. In contrast, at distances of >80 km from site class D locations, the observed Arias intensities are much larger than the empirical predictions as a result of site amplification. Considering the variability of ground motion between earthquakes, the attenuation model of Travasarou et al. (2003), model 4 of Stafford et al. (2009), and the model of Lee et al. (2012) provide a good fit with observations from the Chi-Chi earthquake.