Hydrological response to earthquakes in the Haibara well, central Japan: possible mechanism inferred from time-varying hydraulic properties
Abstract
Twenty-eight coseismic groundwater-level decreases have been observed at the Haibara well, Shizuoka prefecture, central Japan, from 1981 to 1997. These groundwater-level changes cannot be explained as the poroelastic response to coseismic static strain. We use the well's atmospheric pressure and tidal responses, rock properties measured on core samples from the same formation, and pumping test results to characterize the hydraulic and mechanical properties of the aquifer. Differing pumping test results in 1979 and 1993 show that the wellbore skin effect has varied with time in this well. The Haibara well's responses to the M_2 earth tide constituent and to atmospheric pressure have varied over time. In particular, increasing amplitude and decreasing phase lags were observed after the 1993 pumping test, as well as after earthquakes that caused coseismic water-level changes. The atmospheric pressure responses in 1981, 1990 and 1993 match theoretical responses calculated using hydraulic properties inferred from the tidal responses in the 1982, 1990 and 1993, respectively, except for a transmissivity that is lower by a factor of three. The tidal response, together with the surface load efficiency derived from the atmospheric pressure response, is used to estimate the mechanical properties of the aquifer. The largest amplitude of the M_2 constituent, 2.2 mm, is small enough to imply that pore fluid in this system is about twice as compressible as water, possibly due to the presence of a small amount of exsolved gas. Diffusion of a coseismic pressure drop near the well could account for the observed time histories of the water-level changes. Because of their long duration, the water-level drops cannot be explained by earthquake-induced disturbances in the immediate vicinity of the wellbore wall. The time histories of the water-level drops are well matched by the decay of a coseismic pressure drop at least 80 meters away from the well. Removal of a small amount of gas from the formation in that location might in turn explain the coseismic pressure drops.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....10769M