Absolute gravity measurements in the Taiwan orogen

M. Mouyen*(1), F. Masson(1), C. Hwang(2), C-.C. Cheng(2), N. le Moigne(3), C-.W. Lee(4), R. Kao(4), W.-C. Hsieh(4)
*maxime.mouyen@eost.u-strasbg.fr
(1) IPGS – UMR7516 – CNRS , 5 rue René Descartes F67084 Strasbourg Cedex, France
(2) Department of Civil Engineering, National Chiao-Tung University, 1001 University road, Hsinchu, Taiwan 300, R.O.C.
(3) Géosciences Montpellier, Université Montpellier 2 – CNRS, Place E. Bataillon, 34095 Montpellier cedex 5, France
(4) Center for Measurements Standards, Industrial Technology Research Institute, Sec.2 Kuang Fu Rd., Hsinchu, Taiwan 30011, R.O.C.

Time-lapse microgravity is an efficient tool to monitor mass transfers and ground vertical movements. Hydrological, volcanologic or tectonic investigations take advantage of this method, which last improvements allow a measurement accuracy about 1 microgal (1 microgal=10^-8 m·s^-2). We present here results of yearly repeated absolute gravity surveys performed in Taiwan since 2006, as part of the Absolute Gravity in the Taiwan Orogen (AGTO) project, which focus on tectonic processes in this active area. The survey contains ten sites distributed along a globally east-west transect, in the south of the island, between Taitung and Tainan cities. This transect thus crosses the fives main geologic features of Taiwan, from the Coastal Range in the east to the Coastal Plain in the west. As g change results of any mass transfer around the measurements site, its straightforward tectonic interpretation cannot be performed. In particular, water effects are likely to account for some microgals. We try here to separate both effects. Concerning tectonic effects, vertical movements can be redundantly monitored by permanent GPS or precise levelling but deep mass transfers are theoretically accessible only using gravimetry so we need a modelling approach to clarify what could be expected. Concerning hydrology, we estimate its effects using global hydrological models and local rainfall data. Modelled deep mass transfers are small (lower than 1 microgal/a) and cannot be detected in our still short time series (4 years). But the gravity changes can be first explained considering vertical movements measured by GPS and erosion, active in Taiwan. It then appears that the remaining gravity signal can be correlated to rainfall amount. We try to understand this correlation by studying hydrological processes occurring near our gravity sites.