Investigation on the 921 Ji-Ji Earthquake, Taiwan, of 21 September 1999

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This research was carried out by 15 researchers for the investigation into the damage and collection of information and data during early October to December 1999. The following became clear after the investigation.* The Philippine Sea plate subducts along the Ryukyu Trench beneath the Eurasian plate. The Eurasian plate is underthrusting beneath the Philippine Sea plate, this caused the earthquake. The maximum dislocation measured at ground surface was about 10 meters vertically, and several meters horizontally.* Before the earthquake, more than 700 strong motion observation stations have been distributed and maintained by the Central Weather Bureau, Ministry of Transportation and Communications, Taiwan. Near the epicenter, very strong acceleration, over 900 gals, was recorded. Furthermore, the velocity record from the integrated result of the acceleration record near the northern end of the fault was over 300 kine.* Many transportation facilities (including highway bridges, railway bridges and tunnels) suffered damage. It can be guessed that the most governmental factor of the damage is differential ground displacements in vertical and horizontal directions caused by the faulting.* The concrete gravity dam was damaged by the surface faulting. This will rise a new technical subject of the earthquake resistance of infrastructures against fault displacements.* The power supply system in Taiwan is concentrated on the one switching yard, and the earthquake hit it directly. Therefore the redundancy of the power system is necessary.