抄録
We built an observational system in the Hosokura underground mine in northeast Japan. An underground laboratory for setting up equipment is located ∼1.5 km from the main gallery entrance and ∼70 m below the surface. To detect electric signals related to earthquakes, volcanic eruptions, and/or land slides, the room contains sensors for seismic waves, electric potentials, electric fields, air ion concentrations, air temperature, and air humidity. Long-term observations with a GPS-synchronized clock have identified daily and seasonal changes in the background levels of the quasi-static electric field and the air ion concentration, probably related to air movement in and out of the mine through the shaft near our laboratory. The background electric field may be caused by a bias distribution of negative and positive air ions.
