Gravity and microearthquake studies in the Nankan- Taoyuan area, northwestern Taiwan.

Gravity and microearthquake surveys had been carried out in the Nankan-Taoyuan area, NW Taiwan in the summer of 1984 and 1985. Bouguer anomaly data were used to determine the subsurface structures of this area. The results indicate that the Nankan fault is a normal fault. Its total vertical displacement is about 150 m. The fault plane of the Taoyuan fault is nearly vertical. The hinge character of this fault is recognized from the Bouguer map. The Hsinchuang fault strikes in the NE-SW direction with a dipping angle of about 35o SE. A temporary seismographic network with 9 stations was deployed to study the seismicity of the area. 49 events with magnitudes between 0.9 and 3.8 were located in the study area during the survey period of 43 days. Most of them were in depths shallower than 25 km and in the SE of Taoyuan city. Some earthquakes occurred very close to the Hsinchuang fault. It is noted that the Hsinchuang fault may be active. 427 earthquakes in the study area were collected by the Taiwan Telemetered Seismographic Network (TTSN) during the period of 1973-1985. Their distribution pattern is quite similar to that revealed by the temporary array. In general, the seismicity in this area is not very active. -Authors