Damage to structures and buildings from the Chi-Chi (Taiwan) earthquake

In the early morning hours of 21 September 1999, a devastating earthquake struck the central region of Taiwan. This earthquake became known as the 921 earthquake or the Chi-Chi (Taiwan) earthquake. The magnitude of the Chi-Chi (Taiwan) earthquake was Ms = 7·6 magnitude of surface wave (US Geological Survey (USGS)) or ML = 7·3 Richter magnitude (Central Weather Bureau (CWB) of Taiwan) and the main shock lasted for about 40 s. The epicentre of the earthquake was located at 120·82° east and 23·85° north near the town of Chi-Chi, Nantou County. The focal depth was 8·5 km. A surface rupture along the Chelungpu fault with a length of about 105 km was observed, with the largest measured vertical offset reaching more than 9 m. After the main shock there were over ten aftershocks with magnitudes greater than 6. The powerful thrust forces moved mountains, cut off rivers, caused liquefaction, damaged bridges, cracked highways as well as concrete dams, and toppled buildings. As a direct result of this earthquake, 2469 lives were lost, 10 002 people were injured, there were 17 484 totally/partially collapsed buildings, and an estimated property loss of US$ 10 billion. The post-quake investigations concluded that most of the collapsed buildings were designed in accordance with the Taiwan Building Codes but were constructed with serious deviations. The improper lap splices of rebars, column tie hooks without 135° bend, and poor concrete quality control at job sites have been cited as the main reasons for most of the damage to buildings.