Journal of Physics of the Earth

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Groundwater discharge, temperature and electrical conductivity of an artesian well were measured at Nishiawakura, Okayama Prefecture, southwest, Japan. The groundwater discharge showed tidal fluctuations of about 50 cm³/s induced by the tidal strain. Although the groundwater temperature also reflected the same tidal fluctuations as the discharge, the electrical conductivity did not manifest any tidal fluctuations. The discharge response to the tidal strain for the diurnal frequency constituents is different from that for the semi-diurnal frequency constituents. The response theory of a partially confined aquifer to the tidal strain, which has been used for explanation of the well water level fluctuations, can quantitatively explain these fluctuations of the discharge. It seems important that the theory could also be applied to the real groundwater flow fluctuations induced by the crustal strain because the strain-induced groundwater movement is considered to have close relation to geothermal and geochemical precursors to earthquakes.

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Global Positioning System (GPS) measurements with Trimble 4000SDs were started at five stations in the southern part of central Japan in September 1988. Baseline lengths in the network range from 22 to 172 km. Observations discussed in this paper have data acquisition times longer than 4 h and were performed during the period from March 1989 to March 1991. Baseline vectors were determined with the broadcast orbit elements of the satellites and the standard atmospheric model data. Strain rates obtained from the present data were (-2.3-1.7)×10^-7/year. The strain rates agree with the results obtained by conventional surveys which were carried out during the last 100 years in this region. It was found that GPS measurements for baselines shorter than 100 km are useful to discuss crustal movements with an accuracy of the order of 10^-7 even if they are obtained with broadcast ephemerides.

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