Variations in Groundwater Flow Rate at the KSM Well, a Most Sensitive Well to Earthquake Occurrences

Abstract

Precise measurement of the flow rate of groundwater was started in August 1991 at an observation well (KSM) in the eastern part of Fukushima Prefecture, Northeast Japan. This well, located right on the Futaba fault, has shown systematically earthquake-related changes in the radon concentration of the groundwater. Although anomalous changes in the flow rate associated with earthquakes have not been observed, the flow rate showed fluctuations responding to Earth tides and atmospheric loading. The phase of the response to the theoretical tidal strain advanced and the tidal strain sensitivity to the semidiurnal constituents are larger than that to the diurnal constituents. These responses can be explained by groundwater leakage from the aquifer, which is consistent with the fractured crustal structure around the strainer position.