Modeling Coseismic Groundwater Level Change with the Constraint of Gravity Data

The absolute gravity measurement by FG-5 gravimeter has been operated in three stations in and around the Tono Research Institute of Earthquake Science (TRIES) since 2003. During this period, two types of ground water level change was observed. One is the water level decrease more than 80 m in 12 years, which along the excavation of the 500 m deep shafts of the Mizunami Underground Research Laboratory (MIU). The other is the coseismic water level change. Generally, the gravity value increases if the groundwater level increases, and vice versa. Nevertheless, the gravity values in 3 stations show no decrease along with water level decrease in 12 years. The obvious coseismic gravity change was observed only in two cases. One is in the 2004 off Kii Peninsula Earthquake (Tanaka et al., 2006, G3), and the other is in the 2011 off the Pacific coast of Tohoku Earthquake. The distances of the gravity stations are within 2 km. The coseismic gravity decrease for the 2011 Tohoku earthquake were about 10 micro gals in all these stations. This suggest that the mass equivalently moved away from these three stations. Based on former researches of hydraulic geology which clarified the permeability structure, we suggest a model of the coseismic groundwater flow, which is explicable for both gravity and groundwater level data. The basic concept of our model is that “the water flows downward by earth’s gravity”.