| Abstract: | High-quality seismicity data and focal mechanism solutions obtained during 1973–1983 by the permanent Taiwan Telemetered Seismographic Network and several temporary local seismographic networks are used for a detailed study of the seismotectonics of the Taiwan area. Seismicity distribution in southern Taiwan clearly reveals an east-dipping Benioff zone which has a thickness of about 30 km and begins to deepen along 121°E at a dip angle of 55°–60°. The leading edge of this Benioff zone reaches a depth of about 180 km between 21°N and 22°N, but tapers off to a shallower depth of about 100 km from 22°N to 23°N. The presence of this seismic zone implies that subduction of the South China Sea plate under the Philippine Sea plate extends from Luzon northward to about 23°N. The position of the northern boundary of the South China Sea plate, as tentatively defined according to the seismicity distribution, passes through southern Taiwan from the offshore area in the Taiwan Strait west of Kaohsiung in an east-northeast direction to the Taitung area where a triple junction probably lies. Seismicity is found to disappear abruptly below a certain depth in many parts of Taiwan. This phenomenon may be attributed to the frictional to quasiplastic transition in the crust or upper mantle. Comparison of shallow seismicity with surface faults and fractures shows that all areas of active shallow seismicity are marked by densely-developed faults and fractures. However, the converse is not necessarily true. This may be partly due to the relatively short duration of seismicity data and partly due to excessive weakening of some of the severely faulted and fractured areas. Finally, focal mechanism solutions for west central Taiwan and the Kuangfu-Fuli area in eastern Taiwan predominantly show a maximum horizontal compression in the SE-NW direction which can be related to collision between the Eurasian and Philippine Sea plates. However, focal mechanism solutions for both the Hualien area in eastern Taiwan and the Tainan area in southwestern Taiwan show remarkable irregularities which may result from local tectonic complexities. |
