|Chi-chi (Taiwan) Earthquake||JAPANESE|
CHICHI (TAIWAN) EARTHQUAKE
A large and damaging earthquake occurred in the northwest region of Taiwan at 17:47 GMT on September 20, 1999 (01:47 AM on September 21 Taiwan Time). Preliminary location is in Nantou County The epicenter was near the small country town of Chichi. (23.87 N, 120.75 E) The shallow earthquake occurred 150 km (90 miles) south southwest of Taipei. Some of the towns that had significant damage and casualties include Chung Liao, Meishan, Taichung, Fongyen, and Dongsu. Casualties, collapsed buildings, and other damage were reported in the densely populated capital city of Taipei. Taiwanese television is reporting 2100 deaths with 8000 people injured. Preliminary magnitude is given by the U.S. Geological Survey National Earthquake Information Center (NEIC) is M7.6. This earthquake is more powerful than the event that killed more than 14,000 people in Turkey last month. The power has generally been restored across the country but there are still daily scheduled blackouts to save power.
Geologists from the National Central univesrsity have been in the field and report scarps with vertical offsets of 2 to 3 meters along a 60 km north-south stretch of the Chelongpu fault. At several locations in the northern part of the faulted region there are estimated displacements of 8 meters.
There has been a strong aftershock sequence. The Central Weather Bureau reported 5 events with magnitudes M6.0 or greater. There was a large aftershock (M6.8) on Wednesday morning that appears to be on a separate fault to the east. The large aftershock of September 26 (M6.8) caused deaths and damage in the Meishan. The focal mechanism of the mainshock indicates a thrust fault with a north northeast trend (USGS | Harvard | ERI).
Quaternary Faulting in Taiwan and Recent Activity
The island of Taiwan is located at the corner of the Manila Trench and Ryukyu Trench in the South China Sea. Several large thrust faults extend across the length of the island of Taiwan. Faults on the western side of the island tend to dip toward the east and seem to be an extension of the Manila Trench. Faults along the eastern seaboard tend to dip toward the west. Along the east coast there are up to 8 elevated Holocene shorelines which are thought to be due to earthquakes with recurrence intervals of greater than 1000 years (Liew et al., 1993)
There are large historical events on the island of Taiwan in 1935 (M7.1), 1941(M7.1), and 1964 (M7.0) (Bonilla, 1977)
Bonilla, M.G., Summary of Quaternary faulting and elevation changes in Taiwan. Geol. Soc. China Mem. 2, 43-55, 1977 Liew, P.M., Pirazzoli, P.A., Hsieh, M.L., Arnold, M. Barusseau, J.P., Fontugne, M. and Giresse, P, Holocene tectonic uplift deduced from elevated shorelins, eastern Coastal Range of Taiwan, Tectonophysics, 222, 55-68, 1993. Davis, D., J. Suppe, and F.A. Dahlen, Mechanics of Fold-and-Thrust Belts and Accretionary Wedges, J. Geophys. Res., 88, 1153-1172, 1983