Groundwater pressure changes in Central Japan induced by the 2011 off the Pacific coast of Tohoku Earthquake

Abstract

[1] In the 2011 off the Pacific coast of Tohoku Earthquake, groundwater pressure changes were observed in and around the Mizunami Underground Research Laboratory (MIU) in Central Japan, where two vertical shafts and horizontal research galleries are excavated in the granitic rock mass. Coseismic changes of groundwater pressure are believed to correspond to crustal dilation/contraction induced by earthquakes. In this study we calculated volumetric strain changes due to the Tohoku Earthquake based on previously reported fault slip models. The calculation indicates approximately 2 Â 10 À7 of dilational strain around the MIU. Based on the strain sensitivities calculated from tidal responses at the monitoring boreholes, the dilation corresponds to drawdowns of several tens of centimeters, and is almost the same as the drawdown observed in the boreholes at distances greater than 1 km from the MIU. In contrast, rapid elevation of groundwater pressures associated with the earthquake was observed in the boreholes within the 500 m vicinity of the MIU. The anomalous elevation is explained by a temporary recovery of the drawdown due to excavation of the shafts and a unique permeability increase induced by the coseismic dilation of heterogeneous local geological structures such as impervious faults controlling the hydrogeological environment.

Tidal response variation and recovery following the Wenchuan earthquake from water level data of multiple wells in the nearfield

“…; T.-P. Liu et al, 1989;Niwa et al, 2012;Roeloffs, 1996;Sil, 2006), springs and stream discharge (Manga, 2001;Manga and Rowland, 2009;Manga et al, 2003;Mohr et al, 2012;Montgomery and Manga, 2003;Wang et al, 2004). Among them, changes in well water levels are the most commonly reported phenomenon.…”

Ground Subsidence Following Groundwater Drawdown by Excavating of 500 m Deep Investigation Shafts in Granite Body in Mizunami, Central Japan in 2004-2012

“…Another boreholes close to the shafts monitored by MUI recorded the groundwater level rise [18]. In stank contrast, there were some boreholes shows the groundwater drawdown in some distances of 1 -10 km from the shafts (Niwa et al, 2012).…”

“…Groundwater level rise of 13 m was observed following the 2011 Tohoku earthquake at the borehole [17]. Same co-seismic groundwater level changes are reviewed in boreholes around MUI [18]. The changes are not only rise but also drawdowns, including with different time responses, it suggests that co-seismic groundwater level changes are more complex.…”

“…Around MUI, they occupy the many boreholes within 10 km distance and monitoring the groundwater level changes. Reference [18] already discussed the ground water level changes at the 2011 Tohoku earthquake.…”

“…Moreover, the patterns of rise and drawdown are also more complex. Some boreholes show a dilatory rise after co-seismic water level drawdown [18].…”