The 1999 Chi-Chi, Taiwan Earthquake: A subduction zone earthquake on land

Tetsuzo SENO1), Kenshiro OTSUKI2) and Chao-Nan YANG3)

1)Earthquake Research Institute, University of Tokyo
2)Graduate school of Earth Sciences, Tohoku University
3)Dept of Geology, National Taiwan University

Bull. Earthquake Res. Inst., 75, 57-77, 2000

The Chi-Chi earthquake occurred at the thrust-decollement in the accretionary prism of a young collision zone in central Taiwan. This event is thus not different from a subduction zone earthquake if Taiwan were covered by the sea water. The surface ruptures accompanied little damage except for collapses of the buildings which stood just across the surface faults. The slip directions of the surface faults are mostly NW, consistent with the earthquake slip vector, but there are also many W-SW and N directed slips. At the northwest corner of the earthquake fault, significant uplifts and multiple thrusts in the river bed occurred.
These might all be related to the fact that the shallow portion of the earthquake fault cut the weak accretionary prism and the sediment on it. The uplifts at the northwestern corner imply an abnormal tsunami if the area were under the sea water, thus suggesting a new factor for the mechanism of tsunami earthquakes: deformation of the sediment or weak accretionary prism at the lowest trench slope (Seno, 2000). The Chi-Chi earthquake might provide a unique chance to observe a subduction earthquake on land.