Scientists scour 921 quake zone

Since the devastating 921 earthquake that claimed 2,297 lives in 1999, scientists in Taiwan and from overseas have pondered several questions related to the tremor. Why did the earthquake hit central Taiwan? How did the fault slip, causing the earthquake? Why did the quake cause such serious damage?

To answer these questions, scientists have already begun a series of experiments sponsored by the National Science Council (NSC) and conducted along the Chelungpu Fault (車籠埔斷層). The results were released at the first-year mid-term symposium of the Program for Earthquake and Active-fault Research (PEAR) last Friday and Saturday. The symposium was held at National Chungcheng University.

Wang Chien-ying (王乾盈), an earth science professor at National Central University, said that one interesting discovery has been that evidence collected thus far supports a widely known theory called the Thin-skin Model of regional tectonics. The theory up to this point has lacked much concrete evidence, until the 921 earthquake.

The Thin-skin Model was presented by seismologists in the 1960s, and argues that so-called shallow detachment surfaces buried underground will cause the earth to thrust upwards during an earthquake.

The theory accurately explains the phenomena seen during the 921 quake, such as why the displacement of the east side of the Chelungpu Fault was greater than on its west side.

Drilling

Most of the evidence so far has been obtained from drilling exploration and a deep-sounding seismic-reflection experiment which helps scientists picture the structure of the Chelungpu Fault.

Wang and his team have worked on obtaining underground samples of the Chelungpu Fault at two drilling sites. A shallow drilling project with two holes, both about 300m deep, has been sponsored by both Taiwan and Japan.

On Nov. 20 last year, scientists drilled the first hole in Fengyuan (豐原), Taichung County, which is along the northern part of the fault. On Dec. 29, they drilled another hole in Mingchien (名間) township, Nantou County, along the southern portion of the fault.

"By observing samples from the fault we obtained from the drilling project, we can understand the mechanism responsible for the 921 earthquake," Wang said at a press conference held last Thursday at Chienchiou Elementary School (千秋國小) in Nantou County, the site of the southern drilling project.

Japanese scientists and US students have joined Wang's team since November last year to study geophysical characters of the fault.

"We are interested in what is the world's biggest displacement created by the 921 Earthquake in Fengyuan (豐原), which is 10m high," Hidemi Tanaka, an earth science assistant professor at Ehime University in Japan, told the Taipei Times.

Tanaka said that several Japanese scientists have worked with Taiwanese scientists in order to learn more about the earthquake by comparing Taiwan's 921 earthquake with Japan's Kobe earthquake in 1995.

"Some of us [Japanese scientists] have found that activities occurring in the southern part of the Chelungpu Fault are similar to those we observed during the Kobe earthquake," Tanaka said.

Two US graduate students from the Department of Geology at Utah State University, Matt Pachell and Richard Heermance, have worked with Taiwanese geologists in Nantou for months to produce their master's degree theses.