Estimation of subsidence using GPS measurements, and related hazard: the Pingtung Plain, southwestern Taiwan

Four years of GPS measurements were done since 1996 in southwestern Taiwan in order to investigate crustal deformation and land subsidence. The network of 48 stations revealed the velocity field in the Pingtung Plain. Horizontal velocities range from 31 to 54 mm yr(-1) towards azimuths 247 degrees to 273 degrees. They show clear anti-clockwise deviation in the coastal area, consistent with tectonic transtension related to lateral escape at the transition collision-subduction. The deformation resembles a particle flow towards a free boundary, and is not a rigid rotation. Vertical velocities range from +13 to -25 mm yr(-1). Twenty stations show fast subsidence in the coastal area, with rates averaging -16 mm/yr. The comparison with Holocene subsidence suggests that about 75% of the present-day subsidence result from decreasing groundwater level induced by over-pumping, adding significant short-term component to the natural risk resulting from long-term tectonic subsidence.