Effect of underground water on gravity observation at Matsushiro, Japan

The short-term effect of underground water on superconducting gravimeter observation at Matsushiro, Japan is investigated. Analysis of rainfall data as well as gravity data reveals that observed gravity decreases immediately after rainfall by the amount which can be explained by a Bouguer model. This suggests that Newtonian attraction by the mass of water above the gravimeter supplied by rainfall is the dominant part of the effect of underground water. Gravity excursion due to rainfall decays linearly with time, suggesting the existence of a physical mechanism involving percolation, evaporation or drainage of water at a steady rate. A simple numerical model is introduced to calculate the gravity effect based on rainfall data. The model is shown to reproduce the observed gravity changes well.