HR: 1330h
AN: G52C-0049    [PDF]
TI: Groundwater-Driven Vertical Movement in Tsukuba Detected by GPS
AU: * Munekane, H
EM: munekane@gsi.go.jp
AF: Geographical Survey Institute, 1, Kitasato, Tsukuba, 305-0811 Japan
AU: Tobita, M
EM: tobita@gsi.go.jp
AF: Geographical Survey Institute, 1, Kitasato, Tsukuba, 305-0811 Japan
AU: Takasima, K
EM: takasima@gsi.go.jp
AF: Geographical Survey Institute, 1, Kitasato, Tsukuba, 305-0811 Japan
AU: Matsuzaka, S
EM: shigeru@gsi.go.jp
AF: Geographical Survey Institute, 1, Kitasato, Tsukuba, 305-0811 Japan
AU: Kuroishi, Y
EM: yuki@gsi.go.jp
AF: Geographical Survey Institute, 1, Kitasato, Tsukuba, 305-0811 Japan
AU: Masaki, Y
EM: ymasaki@gsi.go.jp
AF: Geographical Survey Institute, 1, Kitasato, Tsukuba, 305-0811 Japan
AB: Seasonal variations observed in the vertical coordinate timeseries of an IGS site TSKB (in Tsukuba, Japan) are investigated. First, we compared the GPS timeseries with the vertical displacements measured by a subsidence meter installed in a 190m-well near the site. It is found that the coherence between the two timeseries is as high as 0.7 and admittance is close to 1. Since the subsidence meter measures displacements occurred in the depth shallower than 190m, it suggests that the seasonal variations in the GPS timeseries are also caused in the corresponding depth. Here we focused on the ground-water as a possible cause of the displacements. A simple model is employed where strains caused by pore pressure changes in water tables are responsible for the vertical coordinate changes. Assuming the static pressure caused by the water column in wells are in equilibrium with the pore pressure in the corresponding water table, we simulate the vertical displacements using the water-level changes observed in the nearby wells, and estimate the volume compressibility of the rocks in the water tables by a least-square method. It is found that water tables between 40m and 190m are responsible for the displacements, and the model successfully explains 65% of the variances of the GPS timeseries. The volume compressibility of the rocks in the water tables is estimated to be 3$\times10^{-9}N/m^2$, which is a reasonable value. Our hypothesis that the water tables deeper than 40m are responsible is consistent with the fact that TSKB station and the Tsukuba VLBI station, which has a basis on the top of the water table just below 40m, move coherently.
DE: 1243 Space geodetic surveys
SC: Geodesy [G]
MN: 2003 Fall Meeting