921 Earthquake - Technical Data

Technical Data

The earthquake struck at 1:47:12 a.m. (Taiwan Time) on Tuesday, September 21, 1999 (i.e., 1999-09-21, hence "921"). The epicentre was at 23.77° N latitude, 120.98° E longitude, 9.2 km (5.7 mi) southwest of Sun Moon Lake, near the town of Jiji, Nantou. The tremor measured 7.6 on the Moment magnitude scale, 7.3 on the Richter scale, and the focal depth was 8.0 km (5.0 mi). The Central Weather Bureau recorded a total of 12,911 aftershocks in the month following the main tremor. The earthquake was in an unusual location for Taiwan, which experiences the majority of its earthquakes off the eastern coast, such quakes normally causing little damage. One of the aftershocks, on September 26, was a strong earthquake in its own right, measuring 6.8 on the Richter scale and causing some already weakened buildings to collapse, killing another three people.

At the time of the quake Taiwan had the most extensive network of sensors and monitoring stations in the world, resulting in "probably the best data set ever collected for an earthquake". At one station, a peak ground motion of 300 cm/s (3 m/s; 10 ft/s) was recorded, the highest ever measurement taken in an earthquake anywhere. Soil liquefaction was observed at Yuanlin and caused settlement of building foundations and filling in of water wells from sand boils. The earthquake occurred along the Chelongpu Fault (Chinese: 車籠埔斷層; pinyin: Chēlóngpǔ duàncéng) in the western part of the island of Taiwan. The fault stretches along the foothills of the Central Mountains in Nantou County and Taichung County (now part of Taichung City). Some sections of land near the fault were raised as much as 7 m (23 ft). Near Dongshih, near the northern end of the fault, a 7 m (23 ft) high waterfall was created by the earthquake.