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Spatial Distribution and Age Dependence of Human-Fatality Rates from the Chi-Chi, Taiwan, Earthquake of 21 September 1999

Institute of Geophysics
National Central University
Chung-Li, Taiwan

Manuscript received 20 January 2001.

The 1999 Chi-Chi, Taiwan, earthquake caused many fatalities and much economic loss for people living in the central part of the island. It also provided many valuable lessons for mitigating future earthquake loss. In this article we have analyzed the human-fatality data from the earthquake in terms of spatial distribution and age dependence to reach the following conclusions. First, rupturing of the Chelungpu fault definitively influenced the spatial distribution of fatalities. Ground ruptures caused by unusually large thrust and left-lateral displacement of the east-dipping fault resulted in almost total destruction of structures on the hanging-wall block along the 100-km-long fault zone. Patterns of damaging ground motion were highly asymmetric about the fault trace. High ground accelerations above 400 gal resulted in high fatality rates up to 1.112% in the sparsely populated rural areas east of the fault. Fortunately, the densely populated urban areas west of the fault suffered substantially lower fatality rates below 0.002% due to low ground accelerations significantly below 400 gal. Secondly, clear age dependence of the human-fatality rate was found from demographic data of the two hardest-hit Nantou and Taichung counties. Results for both counties define almost identical functions that shows people older than age 40 are increasingly more vulnerable with increasing age to life loss in earthquakes. These two conclusions can be applied to make a reliable estimation of the total human fatalities in areas of high seismic intensity either before a large earthquake by performing scenario studies, or shortly after a real earthquake by a system of rapid intensity mapping. Finally, empirical time functions of the cumulative numbers of people found killed, injured, and missing during the first hours following the Kobe, Japan, and Chi-Chi, Taiwan, earthquakes both show that search and rescue operations were critical in the first 48 hr.