Tectonic environment of the 1999 Chi-Chi earthquake in central Taiwan and its aftershock sequence

In the early morning of 21 September, 1999, Taiwan experienced its most pow erful inland earthquake (with Richter magnitude 7.3) of the century. The ea rthquake occurred near the small town of Chi-Chi in central Taiwan, in an a rea with a low recent background of seismicity although it is surrounded by belts of high seismicity. It caused an extensive surface ruptures totaling about 100 km long and a tremendous number of aftershocks. Using seismologi cal and morphological data, we investigate the tectonic environment in cent ral Taiwan and the associated plate structure in the Taiwan region, with em phasis 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 Ph ilippine Sea plate moves northwestward with respect to the Eurasian plate. The former subducts northwestward, in general, along the Ryukyu trench. How ever, due to the impediment of its motion, it subducts northward and undert hrusts westward in the Taiwan region, with the meridian line of 121.5 degre es E being the present western boundary of the active subducting lithospher e. The western boundary is migrating westward, at a rate of about 6 cm/y, a long with the northwestward movement of the Philippine Sea plate. The Phili ppine Sea plate, like a plow, pushes and uplifts Taiwan island situated on the Asian continental margin, as demonstrated by a seismic wedge to the eas t of 121 degrees 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 l ow background seismicity in front of the seismic wedge, including the Peika ng basement high, an uplifted pre-Miocene basement beneath the Western Coas tal Plain and its offshore zone. The Chi-Chi earthquake occurred in a subar ea of the triangular area near the vertex, confined by the seismic belts an d the Peikang basement high. The northwestward movement of the subarea has been impeded by the Peikang basement high, and the associated crustal strai n energy may have thus been stored, and then released to trigger the Chi-Ch i earthquake, the ruptures and the aftershocks.