The 21 September, 1999 Chi-Chi earthquake in Taiwan: Implications for Tsunami earthquakes

The 1999 Chi-Chi earthquake in Taiwan can be regarded as a subduction zone earthquake in a tectonic sense. It was associated with an abnormally uplift ed area in the northwestern corner of the earthquake fault, The area is in the river bed where the Ta-Chia River runs from east to west. The large hor izontal movement of the basement south of the river to the north could have produced the multiple thrusts and the abnormal uplifts amounting to 3-6 m due to the shortening of the accretionary prism. These inelastic uplifts would imply an abnormal tsunami if the area were un der the sea, thus suggesting a new factor for the mechanism of tsunami eart hquakes, which is an uplift of the sediment or weak accretionary prism caus ed by a sudden horizontal movement on the decollement beneath the lowermost inner trench slope like sand being pushed up by a bulldozer. This is consi stent with the features of tsunami earthquakes having the low dip angle thr ust extending to the trench.