Abstract
On April 11, 2011, a M7 shallow earthquake occurred in Iwaki City, Fukushima Prefecture, Japan. Here, I present the results of temporary observations of groundwater level changes in two deep boreholes (SMK and HLN) in the aftershock area. Comparison of the tidal components of water level changes with theoretical volumetric strain changes due to the earth tides indicates that 1 mm water level change correlates to 2.94 × 10−10 strain in the SMK borehole, and to 2.42 × 10−10 strain in the HLN borehole. Extrapolation of 50days observation data up to the occurrence of the main shock indicates an 8 m coseismic drop in the SMK borehole, and a 10 m rise in the HLN borehole, which is situated only 6.2 km to the east of SMK. Water level changes were also detected for as many as 27 M3.3 to M5.9 aftershocks. Focal mechanism solutions are available for 24 of these aftershocks, with 21 of these aftershocks having volumetric strain changes calculated by dislocation models that have good linear correlations with water level changes (R2 = 0.96), suggesting that the latter may be caused by the former. About 30% of the 27 aftershocks were associated with small water level changes between 20 minutes to 3 hours before coseismic water level changes. These are interesting signals, but cannot be identified as definitive seismic precursors.
