The Chi-Chi earthquake (ML7.3) occurred in Taiwan on September 21, 1999 at 1:47:16 a.m. The epicenter, with a focal depth of 8 km, was located 12.5 km west of Sun-Moon Lake. This earthquake was the strongest earthquake recorded during the 20th Century in Taiwan, and caused large scale multiple landslides in mountain areas and soil liquefaction in the plains. According to statistics, the damage caused by the earthquake included more than 2,000 deaths and missing persons, approximately 12,000 injured, with about 50,000 buildings completely destroyed, 50,000 buildings severely damaged, and more than 100,000 people without homes. The earthquake occurred along the Chelungpu fault, stretching more than 100 km. The resulting surface rupture, co-seismic surface displacement and deformation instantly caused buildings to collapse, heavy causalities, and several large scale landslides in the Tsao-Ling and Chiufenershan areas, including the small and large scale landslide damage along the Central Cross-Island Highway. Due to the Chi-Chi earthquake, the soil and rock formations in the central mountain areas have loosened and become unstable, which will subsequently lead to even more damage after the scouring of torrential rainfall by future typhoons.

References: 1. A geological survey report for the 1999 Chi-Chi earthquake (1999); 2. An overall damage survey for the 1999 Chi-Chi earthquake: a brief report, National Center for Research on Earthquake Engineering.