Seismologists to study the evidence, warn of aftershocks

Amid efforts to assess the geological and seismic factors behind the worst ever earthquake in Taiwan early yesterday morning, seismologists in both academia and the government's Central Weather Bureau (CWB) cautioned people yesterday to be prepared for aftershocks, particularly in the next two weeks.

The strongest-ever earthquake in Taiwan was measured as 7.3 on the Richter scale by the CWB while the United States Geological Survey, National Earthquake Information Center recorded it as 7.6.

Earthquake experts said yesterday that people should be cautious about aftershocks, as some might measured as high as five on the Richter scale.

The earthquake caused substantial damage and heavy casualties on the island, especially in central Taiwan. According to official data released by the bureau, the preliminary epicenter for the earthquake was located at latitude 23.81 degrees north and longitude 120.9 degrees east, 12.5km from Sun Moon Lake (日月潭), a famous scenic spot in Nantou County(南投縣), or about 150 kilometers south-southwest of Taipei.

CWB officials said yesterday that as the epicenter was only 1.1km beneath the surface, which is quite shallow, everyone on the island had experienced the internal wave of energy as it forced its way up from the earth. The energy released yesterday, officials said, was 40 times greater than that of the earthquake in Juili (瑞里), Chiayi County, which occurred in July last year. However, the cause of this worst-ever quake in Taiwan is still not known. CWB officials suspected first it was caused by movements of the Chelungpu Fault (車籠埔斷層), which is more than 100km in length and extends across Hsinchu, Taichung and Nantou Counties. Later, CBW officials said it might also have been caused by movements of another fault, the Damaopu-Shuangtung Fault (大茅埔-雙冬斷層), east of the Chelungpu Fault.

A fault is a line of fracture along which one body of rock or section of the earth's crust has been displaced relative to another. When movement along a fault is sudden and abrupt, it may generate a severe earthquake, and even rupture the earth's surface creating a topographic feature called a fault scarp. This dislocation may occur be in a vertical or horizontal direction, or a combination of the two.

"Whatever the cause, the accident was totally unexpected. In past decades, we've focused on the region in Chianan Plain (嘉南平原), south of Nantou County, where earthquakes occurred in the past," said Hsin Tsai-chin (辛在勤), the deputy chief of the bureau.

Earthquake experts at the Institutes of Seismology and Applied Geophysics of National Chung Cheng University (中正大學地震及應用地球物理研究所) in Chiayi conducted an investigation yesterday near the Chelungpu Fault, and found a three-meter vertical and horizontal displacement.

Institute director Shieh Chiou-fen (謝秋雰) said that the more likely cause of the quake was the Damaopu-Shuangtung Fault.

Shieh's team, composed of nine earth science professors and students, are planning to study the Damaopu-Shuangtung Fault in two days time, if roads are clear by then.

Shieh said that Taiwan is located in an area of tectonic activity, where earthquakes and volcanic eruptions tend to occur. It is known that the earth's crust is formed by 13 semirigid floating plates. In the Taiwan area, 51 active fault lines have been identified, generated by the crush of the Philippine Plate and the Euro-Asia Plate.