Abstract
In order to detect the precursors of deep groundwater changes related to the anticipated large earthquakes off Miyagi Prefecture, Japan, we established an observation network of six boreholes with a real-time automatic data-acquisition system. Durning the observation period in 2008 and 2009 groundwater temperatures and levels in three deep boreholes responded to the off-Ibaraki Prefecture earthquake (MJ7.0, Japan Meteorological Agency earthquake magnitude scale) on 8 May 2008, the Iwate-Miyagi inland earthquake (MJ7.2) on 14 June 2008, the off-Fukushima Prefecture earthquake (MJ6.9) on 19 July 2008, and the northern Iwate prefecture coast earthquake (MJ6.8) on 24 July 2008. Examining the responses of our observation system to the earth tides, volumetric strain changes equivalent to water level changes were estimated at 2.27×10−9/mm and 0.5×10−9/mm for two of three boreholes. Earthquake-related changes in borehole groundwater levels, which were free from the artifacts, are consistent with the estimates from static volumetric strain changes calculated from dislocation models. The lower detection limit of our observation system to co- and post-seismic groundwater changes is approximated by the equation MJ=2.4 log r+1.0, where r represents hypocentral distance in kilometers. Similar equations can be derived not only from the attenuation relation for static strain changes but also from those for ground motion amplitude.
