Earthquake-related ground motion and groundwater pressure change at the Kamaishi Mine

Abstract

The Power Reactor and Nuclear Fuel Development Corporation (PNC) has been conducting studies on earthquake effects at the Kamaishi Mine. The aims of the studies are to observe the attenuation characteristics of ground motion with depth, and to understand the influences of earthquakes on the deep groundwater. Seven seismographs have been installed at four different levels of the mine with depths varying from 0 to 600 m. According to the observations since January 1991, maximum acceleration recorded deeper than 150 m has a tendency to decrease to so1 4 to 1 2 of that at the ground surface. We also monitored water pressure in boreholes, inflow rate and electric conductivity of groundwater from drift wall, and the water chemistry before and after earthquakes. Twenty cases of earthquake-related changes in water pressure have been observed during the period from November 1991 to December 1994. The range of groundwater pressure changes are generally less than 0.1 kgf/cm 2 with a maximum of 0.35 kgf/cm 2. Almost all these changes tend to recover slowly to the original state within about one week. In these twenty cases the static crust strain calculated after Dobrovolsky et al. (1979) (Estimation of the size of earthquake preparation zones. Pure Appl. Geophys., 117: 1025–1044) from the magnitudes and epicentral distances are larger than 10 −8. As interseismic variation, the annual groundwater pressure change is less than 1 kgf/cm 2, which corresponds well with the rainfall record.