In the early morning of 21 September, 1999, experienced its most powerful inland earthquake (with Richter magnitude 7.3) of the century. The earthquake occurred near the small town of Chi-Chi in central Taiwan, in an area with a low recent background of seismicity although it is surrounded by belts of high seismicity. It caused an extensive surface ruptures totaling about 100km long and a tremendous number of aftershocks. Using seismological and morphological data, we investigate the tectonic environment in central Taiwan and the associated plate structure in the Taiwan region, with emphasis on the cause of the earthquake and its aftershock sequence.

Taiwan island is situated on the northwestern corner of the Philippine Sea plate, where its interaction with the Eurasian plate is complicated. The Philippine Sea plate moves northwestward with respect to the Eurasian plate. The former subducts northwestward, in general, along the Ryukyu trench. However, due to the impediment of its motion, it subducts northward and underthrusts westward in the Taiwan region, with the meridian line of 121.5°E being the present western boundary of the active subducting lithosphere. The western boundary is migrating westward, at a rate of about 6 cm/y, along with the northwestward movement of the Philippine Sea plate. The Philippine Sea plate, like a plow, pushes and uplifts Taiwan island situated on the Asian continental margin, as demonstrated by a seismic wedge to the east of 121°E. In central Taiwan, there exist two obvious linear belts of frequent earthquakes, almost perpendicular and individually trending NW and NE. These linear belts form the boundaries of a triangular area with low background seismicity in front of the seismic wedge, including the Peikang basement high, an uplifted pre-Miocene basement beneath the Western Coastal Plain and its offshore zone. The Chi-Chi earthquake occurred in a subarea of the triangular area near the vertex, confined by the seismic belts and the Peikang basement high. The northwestward movement of the subarea has been impeded by the Peikang basement high, and the associated crustal strain energy may have thus been stored, and then released to trigger the Chi-Chi earthquake, the ruptures and the aftershocks.