The contribution to tectonic subsidence by groundwater abstraction in the Pingtung area, southwestern Taiwan as determined by GPS measurements

The three sets of Global Positioning System (GPS) data collected from 1996 to 1999 are used to investigate the contribution to tectonic subsidence by groundwater abstraction in the Pingtung plain, southwestern Taiwan. The horizontal station velocities varied from 32 to 54 mm/yr for azimuths ranging from 247.2° to 272.6° with respect to the permanent station S01R located in the Penghu islands. In the central and western part of the study area, GPS stations move generally towards the west, whereas in the Kaohsiung–Pingtung coastal area, the displacement vectors demonstrate a clear counter-clockwise deviation towards the SW. The southern part of the offshore coastal area shows remarkable extension rates of 0.6–2.0 μstrain/yr along azimuth 015–020°. The significant southward increase of extensional strain rates is attributed to the lateral extrusion of blocks bounded by major discontinuities in the study area. For the vertical movement, the station velocities are from ∼13 to −25 mm/yr. Significant subsidence rates from ∼11 to ∼25 mm/yr have been observed. These results clearly demonstrate the existence of transtensional deformation and the southward increase of extensional deformation in the along-strike direction throughout the study area. The comparison with the pattern of Holocene subsidence rates and the isopach of fine-grained sediments suggest that about 75% of this subsidence may result from the decrease in groundwater levels induced by over-pumping. These localized anthropogenic activities contributed to the natural risk that results from tectonic subsidence associated with tectonic extrusion and lateral extrusion at the southern tip of the Taiwan collision belt.