VELOCITY-FIELD OF GPS STATIONS IN THE TAIWAN AREA

The 131 stations of the 'Taiwan GPS Network' were surveyed 4-6 times f rom 1990 to 1995 with dual-frequency geodetic receivers. The standard deviation of an observed baseline length with its linear trend removed is in the range of 6-10 mm for a 3-120 km long baseline. The average rates of length change for all baselines of the network and those from nine continuously monitoring permanent stations are used in a least s quares adjustment to estimate the velocities of the GPS stations relat ive to Paisha, Penghu, situated at the Chinese continental margin. To the south of Fengping, in the a northern Coastal Range, the velocity v ectors of stations in Lanhsu, Lutao, and the Coastal Range trend in th e directions of 306 degrees-322 degrees with rates of 56-82 mm/yr. In contrast, there is a dramatic decrease in the rates to the north of Fe ngping. This may be caused by the motion along the NE-SW-trending thru sts which obliquely cut the northern Coastal Range. A discontinuity of about 30 mm/yr in the rates along with a remarkable change in the dir ections of station velocity is observed across the Longitudinal Valley , then the moving directions gradually shift to the west for the stati ons in the Western Foothills. In the Kaohsiung-Pingtung coastal area, the station velocities are even directed toward the southwest. To the north of the Peikang High, the velocity vectors of the stations change direction from the west gradually to the north and finally to the eas t and southeast. Significant NW-SE extensional deformation is found in the nan Plain and northern Taiwan. In general, the pattern of the vel ocity field for GPS stations in the Taiwan area is quite consistent wi th the directions of present-day tectonic stress.