G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Variations of absolute gravity accompanying earthquakeinduced changes in subsurface pore water pressure at the Mizunami Underground Research Institute construction site, central Japan T. Tanaka Tono Research Institute of Earthquake Science, Association for the Development of Earthquake Prediction, 1-63 Yamanouchi, Akeyo-cho, Mizunami , Japan (tanaka@tries.jp) W. Salden Tono Geoscience Center, Japan Atomic Energy Agency, 1-64 Yamanouchi, Akeyo-cho, Mizunami , Japan A. J. Martin Quintessa K. K., Minatomirai, Nishi-ku, Yokohama , Japan Article Volume 7, Number 3 31 March 2006 Q03017, doi: /2005gc ISSN: H. Saegusa Tono Geoscience Center, Japan Atomic Energy Agency, 1-64 Yamanouchi, Akeyo-cho, Mizunami , Japan Y. Asai Tono Research Institute of Earthquake Science, Association for the Development of Earthquake Prediction, 1-63 Yamanouchi, Akeyo-cho, Mizunami , Japan Y. Fujita Tono Geoscience Center, Japan Atomic Energy Agency, 1-64 Yamanouchi, Akeyo-cho, Mizunami , Japan H. Aoki Tono Research Institute of Earthquake Science, Association for the Development of Earthquake Prediction, 1-63 Yamanouchi, Akeyo-cho, Mizunami , Japan [1] The Tono Research Institute of Earthquake Science has been measuring gravity using an FG5 absolute gravimeter located at the Mizunami Geoscience Academy (MGA) in central Japan since January Measured gravity decreased immediately following the 2004 earthquake off the Kii peninsula (M JMA 7.4) by about 6 mgal. Here, we investigate the empirical relationship between pore water pressure change in a borehole near the MGA and gravity change measured at the MGA. We reveal that (1) gravity change correlates inversely with pore water pressure change at 81 m below the surface at a particular borehole and (2) several different sets of conversion coefficients from pressure head to gravity can be used to explain 60 70% of gravity variations with less than 2 mgal uncertainty. These newly identified relationships may suggest that an absolute gravimeter alone could be used to observe the change of groundwater quantity. Components: 4272 words, 7 figures, 2 tables. Keywords: absolute gravity; gravity change; groundwater; pore water pressure. Index Terms: 1244 Geodesy and Gravity: Standards and absolute measurements; 1217 Geodesy and Gravity: Time variable gravity (7223, 7230); 1895 Hydrology: Instruments and techniques: monitoring. Received 22 August 2005; Revised 22 November 2005; Accepted 27 January 2006; Published 31 March Copyright 2006 by the American Geophysical Union 1 of 10

2 Tanaka, T., W. Salden, A. J. Martin, H. Saegusa, Y. Asai, Y. Fujita, and H. Aoki (2006), Variations of absolute gravity accompanying earthquake-induced changes in subsurface pore water pressure at the Mizunami Underground Research Institute construction site, central Japan, Geochem. Geophys. Geosyst., 7, Q03017, doi: /2005gc Introduction [2] The Tono Research Institute of Earthquake Science (TRIES) installed an FG5 absolute gravimeter at the Mizunami Geoscience Academy (MGA) on 14 January 2004, primarily to monitor the effects of groundwater disturbance caused by the construction of the Mizunami Underground Research Laboratory (MIU) of the Japan Nuclear Cycle Development Institute (JNC) [JNC, 2003a], which is the predecessor of JAEA, and gravity changes accompanying tectonic activity in the Tokai district (Figure 1). [3] Though it is well known that groundwater affects absolute gravity values, precise calculations are often difficult due to limited knowledge of aquifer structure and heterogeneity. However, the underground structure around the MIU site has been surveyed in detail and the pore water pressure has been continuously monitored at different depths in several boreholes (Figures 1b, 1c, and 2). The MSB-3 borehole, for example, which is the nearest to the MGA, has seven pore pressure sensors and intersects a NNW Fault. The pressure behavior can be classified into three groups; namely above, adjacent, and below the NNW Fault [JNC, 2004]. Typically, the shallowest pore water pressure fluctuates widely. Under the fault zone, obvious co-seismic water pressure changes have been observed [JNC, 2004]. This study will focus on a 2004 earthquake off the Kii peninsula (14:57 Universal Time, 5 September 2004) that produced a significant pore water pressure change in the deeper (>80 m) parts of the MSB-3 borehole (Figures 3c 3g). In addition, some pore water pressure values corresponded to each other between the two absolute gravity measurements. Therefore we can attempt to quantify the relationship between pore water pressure and gravity change by using these data. The mechanism of co-seismic groundwater level change in a similar geological environment, was studied by King et al. [1999, 2000] at the Tono mine, located about 2 km WNW of MGA. They considered waterlevel change from the point of view of earthquake prediction and emphasized that wells were subjected to local hydrogeological conditions. In this report, we do not touch on the mechanism of pore water pressure change due to the complexity (Figure 2). This is a problem to be solved in future. 2. Absolute Gravity Measurement [4] The FG5 is a commonly used global-standard absolute gravimeter made by Micro-g Solutions Inc. (USA) [Niebauer et al., 1995; Okubo et al., 1997]. TRIES installed Serial Number 225, which featured the auto-leveler mounted on the super spring tripod, in January It was installed by staff from Micro-g Solutions Inc., who carried out the two initial measurements ( and in Table 1 and Figure 4). Subsequent measurements taken from February to April of 2004 are considered unreliable due to improper adjustment and other technical problems and are omitted from the analysis here. [5] The data processing was performed using the g ver.4 software, which included tidal (solid earth, ocean loading, and polar) and atmospheric pressure change corrections. Figure 4 and Table 1 (A, B, K-N) show results of the gravity measurements. The absolute gravity values given in Table 1 are referenced to the measured plate surface, which has been corrected for instrument height (almost 130 cm) using mgal/cm vertical gradient determined by the Scintrex CG3-M gravimeter. 3. Pore Pressure Observation [6] Figure 1c shows a simplified geological profile in the MIU construction site. Cretaceous Toki granite is unconformably overlain by generally flat-lying Miocene Mizunami sedimentary formations varying in thickness from 100 to 200 m. On the basis of seismic studies the site is believed to be located at the edge of a paleo-channel on the granite erosion surface [JNC, 2003a, 2003b]. Four boreholes, MSB-1 to -4, were drilled into the fresh granite. Of these, only MSB-1 and MSB-3 have continuous pressure sensors installed. Early site mapping revealed a distinctive sub-vertical fault trending NNW through the construction site area. Preliminary results suggest this fault acts as 2of10

3 Geosystems G 3 tanaka et al.: variations of absolute gravity /2005GC Figure 1. Location map of Mizunami Underground Research Institute (MIU). (a) The epicenter of the earthquake (M JMA 7.4) that occurred off the Kii peninsula on 5 September 2004 is shown as a star. Toki GPS and Ena GPS (solid squares) are GPS stations used for estimating elevation change at the MGA. (b) Simplified map of the Mizunami Underground Laboratory construction site. The MSB-1 to -4, MIZ-1, and DH-2 boreholes and two shafts under construction are shown. The MSB-3 and MIZ-1 boreholes are inclined. (c) The geological section of x-x 0 shown in Figure 1b. The Multiple Piezometer (MP) system has been installed in the MSB-1 and -3 and DH-2 boreholes and is shown schematically here. Question marks show the possibility of MSB-1 borehole existing on the opposite side of the NNW fault. a local flow barrier, effectively dividing the site into northeast and southwest regions [JNC, 2004]. The MSB-3 borehole was drilled at an angle of 70 degrees to intersect this fault. As a result, the shallow part of MSB-3 is located in the northeast region, while the deeper is in the southwest region. In the case of the MSB-1 borehole, it is not clear which side of the NNW fault it lies [JNC, 2005] (Figures 1b and 1c). The AN-3 borehole is about 1.3 km in distance, in a WWN direction, from the MSB-3 borehole. It is obvious that local hydrogeological structure is quite different at a distance of only 100 m to 1 km from the MSB-3 borehole. Moreover, the MSB-3 borehole is more sensitive to pressure changes than other boreholes (Figure 2). Accordingly, we mainly focus on the MSB-3 borehole for examining the relationship of groundwater and gravity. [7] The MP System (Westbay Instruments, Canada) was installed in these three boreholes to provide continuous observations of pore water pressure from multiple zones in each borehole. Borehole isolation is provided by inflatable packers. The main purpose of these systems was to monitor changes to the hydrological environment caused by MIU construction activities. Single and multiple (interference) borehole hydrological tests have also been performed to investigate the local scale hydrogeology [JNC, 2004]. [8] The observed pore water pressure changes are a sum of artificial phenomena (hydrological test, excavation construction, borehole maintenance, etc.) and natural phenomenon (atmospheric pressure, rainfall, earthquake, earth tide, etc.). The precise quantification of individual components is in reality impossible although filtering allows for the identification and removal of many influences. However, no obvious impact is observed in pore water pressure caused by excavation work in the two shafts until June As shown in Figure 1c, the MSB-3 borehole has seven pressure probes, labeled PRB-1 to PRB-7. The behavior of these probes can be classified into three general groups: 3of10

4 Figure 2. Coseismic pore water pressure changes caused by the 2004 earthquake off the Kii peninsula. Three different boreholes responded differently. Lithological characteristics around each probe are described, and installation depth from ground level (meters) is shown. (a and b) Both PRB-1 (MSB-3 and MSB-1) are located in the Akeyo Formation of the Mizunami Group. However, the former appears to be confined aquifer, and the latter appears to be unconfined aquifer. (c) The AN-3 borehole is located about 1.3 km in a WNW direction from the MSB-3 borehole. PRB-0 of the AN-3 borehole shows atmospheric pressure. The AN-3 borehole has probes only in basement rock. Precipitation at the roof of MGA (see Figure 3 or 4) obviously affects PRB-1 of MSB-3, but the data of 30 October 2004 are missing. unconfined aquifer, insensitive aquifer, and confined aquifer. PRB-1 clearly responds to precipitation since it is the shallowest pressure probe and is situated in the main part of the Akeyo Formation. PRB-2 in the basal conglomerate of the Akeyo Formation has a almost linear trend with tidal response. PRB-3 to -7 are generally stable with tidal response, but have some trend caused by large earthquakes. PRB-3 is in the NNW fault zone. PRB-4 and -5 are situated in the Toki lignite-bearing Formation and PRB-6 to -7 are situated in the Toki Granite. Figure 3 shows these records while absolute gravity measurements were carried out. The pressure decline recorded by probes PRB-3 to -7 in January and February represents recovery following a water pressure rise caused by the 2003 Tokachi-Oki earthquake described by JNC [2004]. Starting on 5 September a rapid increase in pore water pressure resulting from the 2004 earthquake off the Kii peninsula can be observed. Therefore we should only take account of pore water pressure of probes PRB-1 and -7 for comparison with gravity change. Quite a similar behavior at probes PRB-3 to - 4of10

5 Geosystems 3 G tanaka et al.: variations of absolute gravity /2005GC Figure 3. PRB-1 to -7 pore water pressure records from the MSB-3 borehole measured from January 2004 to March Lithological characteristics around each probe are described, and installation depth from ground level (meters) is shown. A step error occurred in February 2004 in PRB-3. PRB-1 appears to be an unconfined aquifer affected by precipitation (the lower right). PRB-2 is stable with a slightly linear trend. PRB-3 to -7 behave similarly though the detailed characteristic feature of each PRB is barely different. 7 indicates that pore water pressure in this zone is controlled by some water-conducting network. [9] Figure 4 shows relative pressure head change at MSB-3 at probes PRB-1 and -7 after simply subtracting atmospheric pressure; (PPRB-1,-7 PPRB-0)/rg which plots as a straight line, where P is pressure, r is water density (1000 kg/m3), and g is gravity acceleration (9.8 m/s2). The data were obtained during a short (5 minutes) sampling interval and have a 0.01 KPa resolution. The origin of the vertical axis is observed values at 15:30 (JST), 14 January Limitation of Hydrogeological Theory [10] Figure 4 seems to show a correlation between the pore water pressure from PRB-1 and measured gravity over a short term (about one month). This fact seems a matter of course, because the porosity of the Akeyo Formation and the storativity of the Toki Granite are about and lower than 0.01, respectively [Kumazaki et al., 2003]. However, the gravity value measured on 11 September 2004, six days following the earthquake, shows a decrease in spite of the sudden rise of pore water pressure in the PRB-7. [11] In the MSB-1 borehole, the pore water pressure in the Akeyo Formation completely differs from that of the MSB-3 borehole in both amplitude and phase (Figures 2a and 2b). Thus we can see that it is difficult to quantify the heterogeneity of the hydrological structure and then to estimate gravitational effect due to groundwater. As a simplified step, we neglect the spatial hydrological heterogeneity and focus on the pore water pressure change of the MSB-3 borehole as this borehole seems to respond more to the total mass of regional groundwater than any other borehole. This way of thinking between gravity and pore water pressure (groundwater level) is analogous to that between Bouguer gravity anomaly and a single rock density. 5 of 10

6 Table 1 (Representative Sample). Summary of the Absolute Gravity Measurements and Gravity Effects From Both Pore Water Pressure and Vertical Crustal Deformation a [The full Table 1 is available in the HTML version of this article at Gravity Difference Smoothed Relative PH(PRB-1) Smoothed Relative PH(PRB-7) Effect of Upper Groundwater Effect of Lower Groundwater Total Effect of Groundwater Item Symbol (B) (C) (D) (E) (F) (G) (H) Calculation Procedure Date (Project Name) (Observed Gravity) min. to Day (Cubic Spline Fit) 5 min. to Day (Cubic Spline Fit) Cunco * (C) Cconf * (D) (E) + (F) Smoothed Relative Vertical Deformation Day to Day (Weighted LS Fit) Unit (A) mgal m m mgal mgal mgal cm a PH in the Item row means Pressure head. b in the Calculation procedure row means the free-air gradient ( 3 mgal/cm in this sheet). Cunco and Cconf are conversion coefficients of the upper part and lower part of the MSB-3 borehole, respectively. In this sheet, 1.26 and 6.70 are adopted, respectively. 6of10

7 Figure 4. (top) Solid circles show relative gravity change from the first measurement, on 14 January For clarity, individual error bars are not shown but are typically of the order of ±1 mgal. Two lines show relative pressure head change of PRB-1 and -7 of the MSB-3 borehole from the measurement on 14 January The two kinds of symbols are regressive daily value smoothed by cubic spline fitting. (bottom) Daily precipitation at the MGA. Then, we try to associate pore water pressure change with gravity change using the reasoning described below empirically. 5. Correction for Seasonal Height Change of Gravity Observation [12] Gravity variations can also be caused by elevation changes at the measurement point. Continuous elevation changes are observed at two GPS measurement stations located near the MGA; Toki (Station No ) and Ena (Station No ) (Figure 1a). These are maintained by the Geographical Survey Institute of Japan (GSI) and incorporate a new pillar design to minimize daily sunlight induced deformation (or thermal expansion). Daily solution coordinates from all the GPS stations are available from the GSI homepage ( The vertical components (namely, ellipsoidal heights) of both GPS stations fluctuate widely and show common seasonal variation (Figure 5 (top)). In this study, we assume that the smoothed ellipsoidal height change calculated by weighted least squares method [Chambers et al., 1983] of the Toki GPS station is equal to the elevation change at the MGA (Figure 5 (bottom), solid line, Table 1 (H)) and compute a correction to the measured gravity by multiplying the elevation change at Toki GPS station by a free-air gradient ( 3 mgal/cm) (Figure 5 (bottom), diamond, Table 1 (I)). We neglect the effects of soil consolidation and elastic thickening/thinning of the confined aquifers because the Mizunami Group is a consolidated Tertiary formation. 6. Estimation of Gravity Change Caused by Pore Water Pressure Change [13] The pore water pressure change is smoothed and decimated to daily values using the cubic 7of10

8 Figure 5. (top) Ellipsoidal height component of the daily coordinate solutions at the Toki and Ena GPS stations by Geographical Survey Institute of Japan. Bold gray lines are raw data, and thin black lines are smoothed data by weighted least squares method. (bottom) Solid line is smoothed relative ellipsoidal height change of the Toki station using the 14 January 2004 standard. Diamonds show gravity effect of elevation change multiplied by a free-air gradient of 3 mgal/cm to smoothed relative ellipsoidal height change for the days where gravity measurements exist. spline fitting method [Press et al., 1988] (two symbols in Figure 4 (top), Table 1 (C, D)). Pore water measurements at PRB-1 in the MSB-3 borehole that were taken during gravity readings of 17 June 2004 and 13 October 2004 differ by only 0.05 m in pressure head (Figure 4 (top), Table 1 (C)). A pressure head difference of this size under a condition of 0.5 effective porosity (or storativity) would alter gravity measurements by no more than 1 mgal. This error is almost coincident with the accuracy of absolute gravity measurement. Effective porosity or storativity of most geological layers are typically are by far less than 0.5. The 0.1 (= ) cm elevation change, Dh, between the two measurement dates would be expected to have increased the gravity by 0.3 mgal because of free-air gradient. We assume that the observed gravity change, Dg, 2.2 ( ) mgal between these two dates is attributable to 0.1 cm elevation change and to 2.10 (=1.65 ( 0.45)) m increase in the pressure head, W l, in the lower part of the borehole, (probe PRB-7). The relationship among them is expressed as follows: Dg ¼ bdh þ C conf W l ; where b is the free-air gradient, C conf is the conversion coefficient from pressure head to gravity value at PRB-7 of the MSB-3 borehole. From this equation, we can calculate C conf = 1.26 [mgal/m]. Its negative sign implies an inverse correlation between gravity change and the pore 8of10

9 by GPS) lying on the same line parallel to the vertical axis. 7. Discussion Figure 6. Comparison of three predicted values that explain the observed gravity change. Error bars of gravity change adopt set scatter shown in Table 1. Gray zone covers unreliable measurements which are mainly caused by peak voltage table of the Iodine stabilized He/Ne laser failing to renew. water pressure of the PRB-7. We should interpret that predominant behavior in the Toki lignitebearing Formation and the Toki Granite around the MGA is the same sense as the behavior in borehole AN-3. [14] Thus, in the general case, the change of the pressure head of PRB-1, W u and the conversion coefficient from pressure head to gravity at PRB-1, C unco can also be derived using the following: Dg ¼ bdh þ C unco W u þ C conf W l : Between any two gravity readings, C unco can be determined by this equation. For the average conversion coefficient whole the observation period, C unco = 3.77 (Figure 6). [15] We also determined two sets of conversion coefficients from two other periods in same way. Table 2 summarizes these parameters. Figure 6 shows the relationship between observed values and an estimated one. Figure 7 shows each component of estimated gravity change and the relation of observed and estimated gravity changes. Each filled circle equals the sum of three triangles (gravity effect of unconfined aquifer, gravity effect of confined aquifer, and elevation change derived [16] As shown in Figure 6, similar values of conversion coefficients are obtained from periods (a) or (b) in Table 2, both of which are within gravity observation error levels. However, these two sets of conversion coefficients do not account for observed gravity values in December 2004 at the MGA. On the other hand, the conversion coefficients obtained from period (c) in Table 2 can explain more observation values but cannot explain one during October One conceivable reason is that the co-seismic rise of pore water pressure in borehole MSB-3 caused by the 2004 earthquake off the Kii peninsula is very local, so that conversion coefficients estimated from period (c) in Table 2 overestimates the decrease of gravity change (Figure 7b, very low values of facing right triangles). Nawa et al. [2005] reported that 2 mgal gravity change at Inuyama (30 km west from Mizunami) was caused by the seismic dislocation of the 2004 earthquake off the Kii peninsula. However, this fact does not affect our results significantly because it is almost same level of uncertainty. We must consider a more complicated mechanism if (a) and/or (b) in Table 2 are more suitable. [17] The mechanism of how earthquake vibration causes different pore water pressure changes at a borehole is very site specific and affected by the local geology and hydrogeological environment [King et al., 1999]. For example, the sensitivity of the MSB-3 borehole could well be affected by the depression structure of the Toki Granite and the NNW fault which acts as barrier to groundwater flow [JNC, 2003b]. Hence, if we wish to reconstruct the 3-D hydrogeological structure over a wider area, it would be necessary to drill a very large number of boreholes and take measurements. Since this is not feasible, some formal statistical Table 2. Best Estimate of Combined Conversion Coefficients a Period Cunco Cconf (a) (b) (c) 17 June 2004 to 13 October October 2004 to 23 March December 2004 to 31 December (2) 1.26 (1) 3.77 (2) 2.51 (1) 1.26 (1) 6.70 (2) a The number in parentheses indicates turn of decision. 9of10

10 Figure 7. Calculated gravity change and its components. (a) Conversion coefficients, Cconf and Cunco, are 1.26 and 3.77, respectively. (b) Cconf and Cunco are 6.70 and 1.26, respectively. The sum of the three different triangles is shown by the solid circles (vertical axis), and the horizontal axis is observed gravity change. A good linear relation in calculated, and observed values can be seen in Figure 7b. method (e.g., Bayesian or kriging, etc.) needs to be developed in order to select a few points for drilling (or selecting existing boreholes) that best sample the regional 3-D hydrogeological structure. In other words, this study may imply that a gravimeter alone could be used to monitor the total mass of groundwater. Acknowledgments [18] T. Tanaka is supported by the joint research system of Mizusawa Astrogeodynamic Observatory, National Astronomical Observatory of Japan, and Special project for earthquake disaster mitigation in urban areas, Ministry of Education, Sports, Science and Technology, Japan. We are thankful to two anonymous referees and the Associate Editor, John Beavan, for their helpful comments, which led us to improve the manuscript. References Chambers, J. M., W. S. Cleveland, B. Kleiner, and P. A. Tukey (1983), Graphical Methods for Data Analysis, Duxbury, Pacific Grove, Calif. Japan Nuclear Cycle Development Institute (JNC) (2003a), Master plan of the Mizunami Underground Research Laboratory Project, JNC TN , Ibaraki, Japan. (Available at publications.html) JNC (2003b), Annual report of research and development on HLW geological disposal in 2002 (in Japanese), JNC Tech Rep. JNC TN , Ibaraki, Japan. (Available at JNC (2004), Annual report of research and development on HLW geological disposal in 2003 (in Japanese), JNC Tech Rep. JNC TN , Ibaraki, Japan. (Available at main.html) JNC (2005), H17: Development and management of the technical knowledge base for the geological disposal of HLW (in Japanese), Supporting Report 1: Geoscience study, JNC TN , Ibaraki, Japan. (Available at King, C.-Y., S. Azuma, G. Igarashi, M. Ohno, H. Saito, and H. Wakita (1999), Earthquake-related water-level changes at 16 closely clustered wells in Tono, central Japan, J. Geophys. Res., 104, 13,073 13,082. King, C.-Y., S. Azuma, M. Ohno, Y. Asai, P. He, Y. Kitagawa, G. Igarashi, and H. Wakita (2000), In search of earthquake precursors in the water-level data of 16 closely clustered wells at Tono, Japan, Geophys. J. Int., 143, Kumazaki, N., K. Ikeda, J. Goto, K. Mukai, T. Iwatsuki, and R. Furue (2003), Synthesis of the shallow borehole investigations at the MIU construction site, JNC TN , Ibaraki, Japan. (Available at ztounou/miu_e/publ/publications.html) Nawa, K., N. Suda, I. Yamada, and R. Miyajima (2005), Seismological and geodetic analyses of superconducting gravimeter records at Inuyama Station, paper presented at 2005 Japan Earth and Planetary Science Joint Meeting, Jpn. Geosci. Union, Chiba-city, Japan. (Available at www-jm.eps.s.u-tokyo.ac.jp/2005cd-rom/pdf/s043/s043p- 006_e.pdf) Niebauer, T. M., G. S. Sasagawa, J. E. Faller, R. Hilt, and F. Klopping (1995), A new generation of absolute gravimeters, Metrologia, 32, Okubo, S., S. Yoshida, T. Sato, Y. Tamura, and Y. Imanishi (1997), Verifying the precision of a new generation absolute gravimeter FG5: Comparison with superconducting gravimeters and detection of oceanic loading tide, Geophys. Res. Lett., 24, Press,W.H.,B.P.Flanney,S.A.Teukolsky,andW.T. Vetterling (1988), Numerical Recipes in C, Cambridge Univ. Press, New York. 10 of 10