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

Authors

  • Masakazu Niwa,

    1. Tono Geoscientific Research Unit, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, 959–31, Jorinji, Izumi, Toki, Gifu 509–5102, Japan
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  • Ryuji Takeuchi,

    1. Tono Geoscientific Research Unit, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, 959–31, Jorinji, Izumi, Toki, Gifu 509–5102, Japan
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  • Hironori Onoe,

    1. Tono Geoscientific Research Unit, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, 959–31, Jorinji, Izumi, Toki, Gifu 509–5102, Japan
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  • Koji Tsuyuguchi,

    1. Tono Geoscientific Research Unit, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, 959–31, Jorinji, Izumi, Toki, Gifu 509–5102, Japan
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  • Koichi Asamori,

    1. Tono Geoscientific Research Unit, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, 959–31, Jorinji, Izumi, Toki, Gifu 509–5102, Japan
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  • Koji Umeda,

    1. Tono Geoscientific Research Unit, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, 959–31, Jorinji, Izumi, Toki, Gifu 509–5102, Japan
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  • Kozo Sugihara

    1. Tono Geoscientific Research Unit, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, 959–31, Jorinji, Izumi, Toki, Gifu 509–5102, Japan
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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 × 10−7 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.

1. Introduction

[2] Hydrogeologic responses to earthquakes have been known for decades [e.g., Vorhis, 1966]. Earthquakes have two main types of effects on groundwater levels: oscillations [Cooper et al., 1965; Liu et al., 1989] and step-like offsets [Matsumoto, 1992; Roeloffs, 1998]. Responses of water levels and pressures with earthquakes are influenced by factors such as the magnitude and depth of the earthquake, distance from the epicenter, and the hydrogeologic environment, which may be consolidated rock or unconsolidated sediment or both [Sneed et al., 2003]. Step-like offsets are more likely to occur at distances up to ∼1 ruptured fault length, while oscillations are observed even at distances many times greater than the ruptured fault length [Roeloffs, 1998; Wang and Manga, 2010].

[3] The 2011 off the Pacific coast of Tohoku Earthquake (hereafter referred to as the Tohoku Earthquake), occurred on 11 March 2011 with Mw 9.0. The earthquake resulted from thrust faulting on or near the subduction zone between the Pacific and North America plates. This magnitude ranks as the fourth largest earthquake in the world since 1900 and the largest in Japan since instrument recordings began 130 years ago, according to the USGS News Releases on 14 March 2011 (http://www.usgs.gov/newsroom/). Changes in water level and pressure, and/or quality of groundwater and hot spring water related to the Tohoku Earthquake have been reported in many regions of Japan [Asai et al., 2011; Kawabe and Nakano, 2011; Kitagawa and Koizumi, 2011].

[4] The mechanisms for coseismic changes of groundwater pressure are considered that static volumetric strain changes [Wang, 1997; Ge and Stover, 2000; Hamiel et al., 2005], permeability enhancement induced by dynamic stresses generated by earthquakes [Elkhoury et al., 2006; Liu and Manga, 2009], and consolidation or liquefaction of near-surface unconsolidated materials [Manga, 2001; Montgomery et al., 2003]. In this paper, we first report on the groundwater pressure changes caused by the Tohoku Earthquake in the central part of Honshu, the main island of Japan, based on the observations in and around the Mizunami Underground Research Laboratory (MIU) where two vertical shafts and horizontal research galleries have been excavated in the granitic rock mass (Figure 1).

Figure 1.

Location of boreholes used for pore water pressure monitoring, showing single-day groundwater level change for the Tohoku Earthquake, calculated from the data between 14:30 local time on 11 and 12 March 2011. Each value is for the measurement interval having the maximum change in each borehole. For MIU-2, 3 and 4, numerator and denominator are the values in the hanging wall and footwall of the Tsukiyoshi Fault, respectively. Locations of the Mizunami Underground Research Laboratory (MIU), and the epicenters of the Tohoku Earthquake (large star) and the main shock of the 2004 Kii Peninsula earthquakes (2004Kii) are shown in the upper left of the figure. Base map in the center of the figure is from 1:25,000 topographic maps ‘Mitake’, ‘Takenami’, ‘Toki’ and ‘Mizunami’ published by the Geospatial Information Authority of Japan. Detailed base maps of the Shobasama and MIU sites are from 1:1,000 topographic map built by JAEA.

[5] Multidisciplinary research is being conducted in the MIU for improving the scientific basis for geological disposal of high-level radioactive waste. In this type of research, monitoring of hydrogeological conditions is standard research procedure. Therefore, groundwater pressures and levels have been continuously recorded in 15 boreholes drilled from ground surface in and around the MIU (Figure 1) during and prior to 2011, as part of the research to develop an understanding of regional groundwater flow, an important factor in the assessment of contaminant migration in deep underground and changes in the groundwater environment caused by excavation of shafts and galleries.

[6] Water pressure responses with the Tohoku Earthquake are observed in all the 15 boreholes. We discuss possible mechanisms for the groundwater pressure changes based on the calculations of volumetric strain changes due to the Tohoku Earthquake and of tidal response in each borehole, and subsequently assess the effects of shaft excavation on these parameters.

2. Groundwater Pressure Changes Caused by the Tohoku Earthquake

[7] The studied area is located approximately 600 km west–southwest from the epicenter of the Tohoku Earthquake (Figure 1). All the boreholes for groundwater monitoring are drilled into the Toki Granite, a Late Cretaceous intrusion (Figures 2 and 3). Depth of these boreholes ranges approximately from 200 m to 1300 m [Karino et al., 2011a, 2011b]. All measurement intervals except shallower parts of DH-15, MSB-1 and 3 are in the granite, under in situ groundwater pressure conditions. Intervals No.1 to 3 of MSB-1 and No. 1 to 4 of MSB-3 are in the Miocene sedimentary rocks of the Mizunami Group, covering the Toki Granite. Intervals No.1 of DH-15, No.4 of MSB-1 and No.5 of MSB-3 are in the basal conglomerate lying on the unconformity between the Mizunami Group and Toki Granite.

Figure 2.

Columnar sections of DH-7, 9, 11, 13 and 15 showing locations of the groundwater monitoring intervals. Each value is single-day groundwater level change for the Tohoku Earthquake, calculated from the data between 14:30 local time on 11 and 12 March 2011. E.L.m: elevation (meter).

Figure 3.

2D cross sections of NNE–SSW trend in (a) the Shobasama site and (b) the MIU site, with locations of the groundwater monitoring intervals, Tsukiyoshi Fault and Main-shaft Fault. A value in each interval is single-day groundwater level change for the Tohoku Earthquake, calculated from the data between 14:30 local time on 11 and 12 March 2011 except for No.8 to 10 of MIZ-1. Cross-sections of the Main Shaft and Ventilation Shaft are omitted.

[8] MIU-2, 3 and 4 in the Shobasama site penetrate the Tsukiyoshi Fault (Figure 3a). The fault dips about 70° to the south in the Toki Granite, and has a highly fractured zone with a width of several tens of meters overlying the fault core composed of foliated cataclasite, which exhibits fragmentation and hydrothermal alteration over a width of several meters [Hama et al., 2002; Onishi and Shimizu, 2005]. Intervals No.12 of MIU-2, No.6 and 8 of MIU-3, and No.9 and 10 of MIU-4 are located in the footwall (north side) of the fault. Interval No.8 of MIU-4 is included in the highly fractured zone and the fault core.

[9] The Main-shaft Fault trending NNW–SSE with a subvertical dip lie in the MIU site (Figure 3b). The fault zone, composed mainly of cataclasite and fault gouge with intense fragmentation and alteration, is approximately 10 m wide in the Toki Granite and is intersected by the Main Shaft [Itoh et al., 2006; Saegusa and Matsuoka, 2011]. All intervals of DH-2, intervals No.6 and 7 of MSB-3 and intervals No.8 to 10 of MIZ-1 in the granite are located in the south side of the fault, while intervals No.1 to 3 of MIZ-1 and interval No.5 of MSB-1 are in the north side. All intervals of 05ME06 are in the fault zone.

[10] The MP (multiple piezometer) System was installed in all boreholes except DH-15 and MIZ-1, to provide continuous observations of pore water pressure from multiple zones in each borehole. Borehole isolation is provided by inflatable packers. A hydraulic pressure transducer is included in each interval. The SPMP (standpipe multipacker) System was installed for DH-15 and MIZ-1. In this system, a hydraulic pressure transducer is inserted in a standpipe connected to each measurement interval bounded by packers. Pressure is recorded in five minute intervals in all boreholes, except for DH-11 and MIU-4, for which the interval is every 30 min. Total head is calculated from groundwater pressure, measured by the hydraulic pressure transducers, and elevation head.

[11] Total head changed drastically on 11 March 2011 in all boreholes (Figures 4, 5 and 6). Several measurement intervals had strong, episodic fluctuations in total head virtually simultaneously with the Tohoku Earthquake, at 14:46 local time on 11 March 2011 (Figure 7). The fluctuations are considered to be seismic oscillations that are often manifested as earthquake-induced groundwater pressure changes [Cooper et al., 1965; Liu et al., 1989].

Figure 4.

Total heads from measurement intervals in the granite bounded by packers in DH-7, 9, 11 and 13 before and several months after the Tohoku Earthquake. Each arrow in the graphs shows the time of the Tohoku Earthquake.

Figure 5.

Total heads from measurement intervals in the granite bounded by packers in the boreholes in the Shobasama site before and several months after the Tohoku Earthquake.

Figure 6.

Total heads from measurement intervals in the granite bounded by packers in the boreholes in the vicinity of the shafts of the MIU site (within 500 m) before and several months after the Tohoku Earthquake.

Figure 7.

Detailed changes of total heads just before and immediately after the Tohoku Earthquake between 14:00 to 19:00 local time on 11 March 2011.

[12] Total heads decreased sharply soon after the Tohoku Earthquake (Figures 4 and 5) except for those boreholes in the vicinity of the MIU. Maximum drawdowns of about 5 m were observed in MIU-2, 3 and 4, and AN-1 and 3, situated in the hanging wall (south side) of the Tsukiyoshi Fault.

[13] In some boreholes total head began to recover several hours to several days after the earthquake, but in others it did not recover, even after several months. Specifically at the Shobasama site, all measurement intervals in the footwall of the Tsukiyoshi Fault increased several hours after the earthquake, while in all measurement intervals in the hanging wall of the fault, total head stayed low for several months (Figure 5).

[14] In contrast, total head increased soon after the Tohoku Earthquake (Figure 6) in the most intervals in the granite at the boreholes within 500 m of the MIU (DH-2 and 15, and four boreholes at the MIU site). Specifically in the measurement intervals in the south side of the Main-shaft Fault, total head increased significantly (Figure 3b). Maximum head increase of about 15 m was observed in DH-2. The elevated head lasted about a month or more. These total heads did not recovered to their pre-earthquake state after six months, though recovery began one to five months after the earthquake.

[15] Exceptionally, intervals No.8 to 10 of MIZ-1 and No.9 to 11 of 05ME06 experienced drawdowns ranging from several tens of centimeters to 1 m. The drawdown in intervals No.8 to 10 of MIZ-1 was sustained for only several days and then total head increased and stayed high for several months.

[16] Shallower intervals in MSB-1 and 3 in the Miocene sediments had no significant offsets (Figure 8). Even in the Miocene sediments, intervals located in the basal conglomerate lying directly on the granite (red charts of the Figure 8; No.1 of DH-15, No.4 of MSB-1 and No.5 of MSB-3) show total head increase similar to the intervals in the granite.

Figure 8.

Total heads from measurement intervals in the Miocene sediments bounded by packers in DH-15, MSB-1 and 3 before and several months after the Tohoku Earthquake. No.1 of DH-15, No.4 of MSB-1 and No.5 of MSB-3 (red charts) are intervals located in the basal conglomerate lying directly on the granite.

[17] We calculated the single-day change of groundwater level based on the water pressure and elevation head data between 14:30 local time on the 11th to 12th of March 2011 (Figures 2 and 3). Noise originating from resolution of monitoring equipment, earth tides and atmospheric pressure variations are removed from the single-day change. For the extraction of the tidal constituent, we used BAYTAP-G [Ishiguro, 1981; Tamura et al., 1991], a program analyzing crustal and tidal changes by Bayesian model. Atmospheric pressure measurements are taken at each orifice of the boreholes. For only No.8 to 10 of MIZ-1, high-precision calculation of the tidal constituent is difficult due to many missing data. All boreholes located more than 1 km distant from the MIU had drawdowns ranging from 0.06 to 1.55 m. At the boreholes localized in the vicinity of the MIU, intervals in the granite in the south side of the Main-shaft Fault (all intervals in DH-2 and No.6 and 7 of MSB-3) had larger elevated heads ranging from 2.33 to 4.79 m. Intervals in the north side of the fault and in the fault core had smaller elevated heads ranging from 0.05 to 1.27 m, except for intervals No.9 to 11 of 05ME06 experienced drawdowns ranging from 0.64 to 1.10 m.

3. Relationship Between Earthquake-Induced Changes of Groundwater Pressure and Volumetric Strain

[18] The rupture process (slip distribution on the fault plane) of the Tohoku Earthquake has been estimated by inversion analyses using teleseismic waveforms [Ide et al., 2011; Shao et al., 2011; Yagi and Fukahata, 2011], strong motion records [Suzuki et al., 2011], GPS [Yue and Lay, 2011] or tsunami data [Saito et al., 2011]. We calculated volumetric strain changes at a depth of 1 km in Eastern Japan due to the Tohoku Earthquake using a dislocation analysis based on the theory of Okada [1992]on a half-infinite homogenous elastic solid. We used the program Coulomb 3.1 [Lin and Stein, 2004; Toda et al., 2005] for the calculation and input fault slip models reconstructed from the previously reported rupture processes. In the calculation, physical values of the elastic solid were defined as follows: Poisson's ratio of 0.25, Young's modulus of 8 × 105 bar and friction coefficient of 0.4. The calculation outputs approximately 2 × 10−7 of dilation strain around the MIU (Figure 9).

Figure 9.

Volumetric strain changes in Eastern Japan after the Tohoku Earthquake described by Coulomb 3.1. Fault model is from Yagi and Fukahata [2011].

[19] Previous studies [Roeloffs, 1996; Sato et al., 2004] indicate that drawdown/elevation of groundwater pressure is affected by crustal dilation/contraction. Strain response sensitivity for water pressure changes of artesian groundwater was estimated based on the calculation from earth tide variations [e.g., Roeloffs, 1988; Kawabe, 1991; Kashima et al., 2011]. Here we calculated theoretical amount of groundwater level changes due to coseismic strain. First, amplitude and phase of M2 tidal constituent were extracted from groundwater pressures for 2 weeks in March 2004 by BAYTAP-G. There had been no heavy rains, no large or nearby earthquakes, no equipment troubles, no artificial hydrological tests and no deep excavations in the study area for this period. Second, strain sensitivities were calculated to divide the given amplitude of M2 tidal constituent by theoretical tidal strain at the boreholes. The theoretical tidal strain was calculated by GOTIC2 [Matsumoto et al., 2001], a program to compute the loading tide in Japanese region by using a combination of global and regional ocean tide models and fine-scale land-sea grids. Finally, theoretical groundwater level changes were determined by multiplying the estimated coseismic strain (2 × 10−7 of dilation around the MIU) by the strain sensitivities.

[20] Theoretical groundwater level changes calculated from the crustal dilation associated with the Tohoku Earthquake and the tidal response in each borehole indicate a range of several tens centimeters, consistent with single-day drawdowns for the earthquake observed in the boreholes more than 1 km distant from the MIU (Table 1). However, significant increase of total heads observed in the boreholes in the vicinity of the MIU (Figure 6) conflicts with the volumetric strain calculation.

Table 1. Comparison of the Theoretical Coseismic Groundwater Level Changes to the Observed Ones at the Boreholes More Than 1 km Distant From the MIU
BoreholeaIntervalGroundwater Level Change (cm)
TheoreticalObservedb
  • a

    In DH-9, there are no data for the calculation of tidal constituent in March 2004.

  • b

    Single-day groundwater level change calculated from the data between 14:30 local time on 11 and 12 March 2011.

DH-7No.14462
DH-7No.23346
DH-7No.35488
DH-7No.56535
DH-11No.16819
DH-11No.36533
DH-11No.48435
DH-11No.57839
DH-13No.17780
DH-13No.44877
DH-13No.55075
DH-13No.7686
AN-1No.13772
AN-1No.66149
AN-1No.105835
AN-1No.126445
AN-3No.14972
AN-3No.34269
AN-3No.46074
AN-3No.53469
MIU-2No.24918
MIU-2No.55119
MIU-2No.96426
MIU-2No.129654
MIU-3No.18115
MIU-3No.45046
MIU-3No.67346
MIU-3No.87357
MIU-4No.65736
MIU-4No.86429
MIU-4No.96825
MIU-4No.107737

4. Groundwater Pressure Increase in the Vicinity of the MIU: Effect of Presence of Shafts?

[21] During the Tohoku Earthquake, total heads increased up to 15 m in the boreholes in the vicinity of the MIU, which contrasts with expected water pressure response due to volumetric strain change estimated from the rupture process. Direct strain measurement in a borehole near the MIU showed areal strain extension of approximately 4 × 10−7 strain for the Tohoku Earthquake [Asai et al., 2011], which supports our volumetric strain calculation. For the deeper intervals in MIZ-1 (No.8 to 10) and 05ME06 (No.9 to 11), measured drawdowns immediately after the earthquake, though at the MIU site, are consistent with the volumetric strain change. Detailed changes of total heads just before and immediately after the Tohoku Earthquake in DH-2 and MSB-3 seem to be curved increases rather than step-like offsets (Figure 7). The local groundwater pressure rise in boreholes proximal to the MIU is not due to local anomalies of volumetric strain. In addition, earthquake-induced consolidation or liquefaction [Manga, 2001; Montgomery et al., 2003] is not applicable to the trigger of the total head increase because the shallower intervals in the Miocene sediments show no significant changes of groundwater pressure even in the Tohoku Earthquake (Figure 8). Instead, the unique groundwater response near the MIU can be explained by (1) effect of the presence of excavated shafts and (2) heterogeneity of local geological structure controlling the hydrogeological environment (Figure 10).

Figure 10.

Schematic illustration showing the unique groundwater response near the MIU (not to scale).

[22] The boreholes in the vicinity of the MIU had shown continuous drawdown since the shaft excavations started at the MIU (Compare MSB-1 and 3 present-day total heads inFigure 6 and the heads shown in Figure 11). Total heads have decreased approximately 35 m in MSB-1 and 87 m in MSB-3 for six years and a half after 2004. In contrast, voluminous groundwater inflow was continuous into the shafts. Soon after the Tohoku Earthquake, inflow volume of groundwater increased more than 10% (Figure 12).

Figure 11.

Total heads in several boreholes in 2004, including the times of the earthquakes that occurred off the Kii Peninsula, Japan, on 5 September 2004 (black triangles).

Figure 12.

Volume of groundwater inflow into the two shafts at the MIU. Black arrow in the graph shows the time of the Tohoku Earthquake.

[23] Irregular elevation of water level soon after an earthquake similar to the case for the MIU has been reported from other excavation sites where continuous drawdown has been observed [e.g., Kitano and Tamai, 2005]. Dilation caused by an earthquake generally induces drawdown. However, in the vicinity of the MIU where continuous drawdown had kept since the shaft excavations started, additional inflow of surrounding drawdown groundwater could occur. It could induce anomalous increase of the inflow volume of groundwater into the shafts and the elevation of the groundwater pressure within the MIU site, as a temporal recovery of the drawdown due to shaft excavation. Although total head increases ranging from several meters to 15 m after the Tohoku Earthquake are substantial, the amount is much smaller than cumulative drawdown due to the shaft excavation (more than 30 m in MSB-1 and more than 80 m in MSB-3 for six years).

[24] Past monitoring data in and around the MIU [Fujita, 2005] indicates groundwater pressure changes due to the earthquakes that occurred off the Kii Peninsula, Japan, on 5 September 2004 with Mw 7.5 [Park and Mori, 2005] during which the MIU site was under a dilational strain state of approximately 0.8 × 10−7strain. Absolute gravity also decreased following the earthquake-induced dilation [Tanaka et al., 2006]. Drawdown was observed in the boreholes more than 1 km distant from the MIU, consistent with the volumetric strain change. However, groundwater pressure increased in No.6 and 7 of MSB-3 in the south side of the Main-shaft Fault at the MIU site, which is the same situation as for the Tohoku Earthquake (Figure 11). No.5 of MSB-1, in the north side of the fault, showed small drawdown. When the 2004 earthquakes occurred, the shafts were excavated to only 50 m depth, within the Miocene sediments covering the Toki Granite. Thus a temporal recovery of the drawdown due to shaft excavation have a low potential for a trigger of the groundwater pressure increase in MSB-3 right after the 2004 Kii Peninsula earthquakes.

[25] To explain the earthquake-induced groundwater pressure changes, ignoring volumetric strain changes, several studies suggest changes of permeability in a local geological structure closely related to the hydrogeological environment. For example, fluid pressure drops can be caused by escape of small amounts of exolved gas from pore space [Matsumoto and Roeloffs, 2003], or permeability can be changed due to unclogging of a fracture by flow induced by seismic waves [Brodsky et al., 2003; Liu and Manga, 2009]. Monitoring tests at the MIU site [Saegusa and Matsuoka, 2011] showed that responses were more sensitive in the south side of the Main-shaft Fault than in the north side.Asai [2006]suggests, based on long-term groundwater observations, that earthquake-triggered cracking in the Main-shaft Fault, which impedes groundwater flow under normal conditions, induced local permeability increases and the subsequent elevation of groundwater pressure in the south side of the fault. An amount of the elevation after the Tohoku Earthquake was larger in the south side of the Main-shaft Fault than in the north side and in the fault core (Figure 3b). Groundwater pressure responses were distinctly different between in the north and south sides of the fault for the 2004 Kii Peninsula earthquakes (Figure 11).

[26] Long-term groundwater pressure responses after an earthquake are also different on opposite sides of the Tsukiyoshi Fault, for both the Tohoku Earthquake and the 2004 Kii Peninsula earthquakes, based on data from the Shobasama site. Measurement intervals in the footwall of the fault increased several hours or days after the earthquake, while in the intervals in the hanging wall of the fault, total head stayed low for several months (Figures 5 and 11). In several boreholes in the Tono Mine (Figure 1) where groundwater level changes caused by past major earthquakes were studied [King et al., 1999, 2000], drawdown immediately after earthquakes and subsequent short-term recovery in the footwall of the fault had also been observed. These facts lead to the inference that heterogeneous geological structures such the Main-shaft Fault and the Tsukiyoshi Fault, which act as conduit or barrier to fluid flow, can affect the groundwater pressure changes induced by earthquakes and postseismic permeability recoveries.

[27] Past earthquake-induced groundwater pressure and level changes lasted approximately several days to a year [Japan Nuclear Cycle Development Institute, 2005]. We will continue to closely monitor long-term hydrogeologic conditions and response to the Tohoku Earthquake, and attempt to further identify the mechanism of the response by sensitivity analyses and additional hydrogeological tests considering the heterogeneity of the hydrogeological structure in and around the MIU.

5. Conclusion

[28] We identified the groundwater pressure changes induced by the Tohoku Earthquake in 15 boreholes in and around the MIU, and ten percent increase of inflow volume of groundwater in the two shafts. At the boreholes located more than 1 km from the MIU, total heads decreased sharply soon after the earthquake. We estimated strain response sensitivity for water pressure changes of artesian groundwater based on the calculation from earth tide variations. Meanwhile, we calculated volumetric strain changes based on the fault slip models reconstructed from the previously reported rupture processes of the Tohoku Earthquake. The calculation outputs approximately 2 × 10−7of dilation strain around the MIU. Theoretical groundwater level change in each borehole calculated from the crustal dilation associated with the Tohoku Earthquake and the strain response sensitivity for water pressure changes indicates drawdown of several tens centimeters. It is consistent with the single-day drawdown around the time of the Tohoku Earthquake observed in the boreholes more than 1 km distant from the MIU. Thus coseismic changes of groundwater pressure at the boreholes located more than 1 km from the MIU are considered to static volumetric strain changes induced by earthquakes.

[29] In contrast, total head increased soon after the Tohoku Earthquake in the most intervals at the boreholes within 500 m of the MIU. The amount of the increase was larger in the south side than the north side of the Main-shaft Fault. The unique groundwater response near the MIU and the anomalous increase of inflow volume of groundwater in the shafts can be explained by the temporal recovery of the ongoing drawdown due to shaft excavation and by the earthquake-induced permeability enhancement in the Main-shaft Fault closely related to the heterogeneity of hydrogeological structure. Different groundwater pressure responses in days to months scale after the earthquake on opposite sides of the Tsukiyoshi Fault can also be affected by the earthquake-induced unclogging in the fault.

Acknowledgments

[30] We wish to thank G. F. McCrank and M. Ishibashi for constructive comments for the manuscript. We are also grateful to the Senior Editor T. W. Becker and two reviewers for providing valuable comments.

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