Mirror-Image Structural Relationship Between Western Taiwan and Nankai Trough Region, Southwest Japan, and its Significance for Earthquake Forecast

Western Taiwan is characterized by west-facing imbricate structure. The westernmost sheet in Taichung area contains a depositional basin (Taichung Basin), structural high (Pleistocene uplands) and frontal thrust (e.g., Changhua Fault) from east to west. Another older sheet underlain by the Chelungpu Fault, a causative fault of the 1999 Chi-Chi Earthquake, has nearly the same tectonic components. The northern tip of each underlying thrust curves eastwards and branches out into several faults. These geologic structures are a mirror image of the structure characterizing the source areas of great earthquakes along the Nankai Trough. This fact indicates that the two regions are controlled by the same tectonics, low-angle oblique subduction, and their mirror-image relationship reflects a left oblique subduction (western Taiwan) and a right oblique one (Nankai Trough). Based on this correlation, eastward bending structure of the northernmost Chelungpu Fault near Houli can be regarded as a segment boundary. In western Taiwan, there are three other eastward bending structures correlative to the segment boundary. They are distributed near Hsihchu, near Minhsiung, and on the north of Tainan. Western Taiwan is divided into five segments 60 to 80 km long by the eastward bending structures. This segment size is nearly a half of the Nankai Trough region (120 to 150 km long) and may regulate the size of future earthquakes resulting from ruptures of these segments. In the Nankai Trough region, faults at the segment boundaries are not likely to rupture independently of reactivation of the main thrusts. In western Taiwan, however, faults at the segment boundaries may have ruptured independently of the faulting of main thrusts during the 1906 Minhsiung, 1935 Hsihchu-Taichung and 1946 Hsinhua earthquakes. This difference between the two regions remains a problem to be solved.