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Temporal change in permeability of the Nojima fault zone by repeated water injection experiments
Tectonophysics 443 (2007) 183 – 192 www.elsevier.com/locate/tecto
Temporal change in permeability of the Nojima fault zone by repeated water injection experiments Yuichi Kitagawa a,⁎, Kunio Fujimori b , Naoji Koizumi a,1 a
b
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan Received 5 April 2004; accepted 29 January 2007 Available online 23 March 2007
Abstract Three boreholes were drilled near the Nojima fault, which the 1995 Hyogoken–Nanbu earthquake occurred on. In order to research the properties and the healing process of the fault, water injection experiments were conducted every 3 years. In this report, we researched the permeability of the fault as a measurement of crack density or porosity of the fault zone. Pore water pressure changes in rock due to the water injections at one borehole were observed as discharge changes or groundwater level changes at the other borehole. Using numerical calculations, the permeability of the fault fracture zone was estimated for each experiment. The permeability has been decreasing as time passed, which is thought to show the fault healing process of the Nojima fault after the 1995 Hyogoken–Nanbu earthquake. © 2007 Published by Elsevier B.V. Keywords: 1995 Hyogoken–Nanbu earthquake; Nojima fault; Permeability; Water injection experiment; Fault healing
1. Introduction It is presumed that rocks in and around a fault zone are fractured and/or deformed when an earthquake occurs, and thereafter recover from their damage. Detection of the recovery of a fault zone is linked to understanding of the postseismic process and the preparation process for the next earthquake. However, there are few reports that detected the damage and recovery of a fault zone after an earthquake. There are some methods for detection of a recovery. One is to measure a temporal change in seismic wave velocity of a ⁎ Corresponding author. Fax: +81 29 855 1298. E-mail addresses: [email protected] (Y. Kitagawa), [email protected] (K. Fujimori), [email protected] (N. Koizumi). 1 Fax: +81 29 855 1298. 0040-1951/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.tecto.2007.01.012
fault zone. Vidale and Li (2003) have been measuring seismic wave velocity of the fault zone of the 1992 Landers earthquake (M7.3) using repeated explosions. They concluded that seismic wave velocities recovered after the 1992 Landers earthquake and temporarily turned back at the 1999 Hector Mine earthquake (M7.1) occurrence. Another method is to measure a temporal change in permeability of a fault zone (Shimazaki et al., 1998). It is thought that more fractured rock has larger porosity and is more permeable. It is expected that permeability of rock is closely related to the degree of fracturing, which is induced by fracture and/or deformation during an earthquake. The Nojima Fault Zone Probe Project was designed to research the properties and the fault healing process of the Nojima fault, on which the 1995 Hyogoken–Nanbu earthquake, with a JMA magnitude of 7.3, occurred. The
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Fig. 1. The observation site is located near the southwestern end of the Nojima fault. The square shows the observation site. The star shows the epicenter of the 1995 Hyogoken–Nanbu earthquake.
Nojima fault exhibited mainly right-lateral strike slip movement with a slight thrust component. Three boreholes with depths of 500, 800, and 1800 m were drilled near the Nojima fault and the Nojima branch fault (Figs. 1 and 2). In the southwestern end of the Nojima fault system, the Nojima fault mainly slipped at the 1995
earthquake and its displacement was 1.0 to 1.5 m. Awata and Mizuno (1998) reported that the displacement of the Nojima branch fault was 0.1 to 0.6 m at the 1995 earthquake. The water injection experiments were conducted from the 1800-m borehole every 3 years. The series of experiments is probably the first attempt in the world for
Fig. 2. Layout of three boreholes. (a) Horizontal location of three boreholes. The broken lines are projections of the boreholes on the ground surface. (b) Vertical projection of three boreholes.
0.74 1.57 (±0.20) cm3/h/cm Ratio of 2000 to 2003
1.20 (± 0.20)
Unknown − 0.31/hpa − 0.42/hpa 1.19 (±0.20) 1.32 (±0.12) 0.84 (±0.03) cm3/h cm3/h cm Discharge in 1997 Discharge in 2000 Ground water level in 2003
1.46 (± 0.26) 1.57 (± 0.13) 1.30 (± 0.11)
Barometric response 4–10 days/cycle Tidal comp of O1 25.8 h/cycle Tidal comp of M2 12.4 h/cycle Unit
Fig. 4. The observation results at the 800-m borehole. The shaded zones show the periods of the water injection experiments.
The term
the active fault just after the occurrence of large earthquake. Repeated experiments aim to measure the characteristics of the fault zone directly and to detect its temporal changes. Kitagawa et al. (2002) concluded that the permeability of the damage zone of the Nojima branch fault in 2000 became lower than that in 1997. In this report, we estimated the permeability of the fault zone in
Table 1 The tidal and barometric responses and the long-term trends of the discharge and ground water level
Fig. 3. Outline of the observation methods at the 800-m borehole.
−1.80/day −0.36/day −0.56/day
185 Long-term trend
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Y. Kitagawa et al. / Tectonophysics 443 (2007) 183–192 Fig. 5. (a) The left-side figures show the injection pressures and flow rates. Bold lines mean actual injection pressures. Thin lines mean actual injection flow rates. Dotted lines mean the values of injection flow rate used in the calculation. The right-side figures show the comparison between the corrected observed discharges and calculations. The corrected observed discharges are equal to the raw data minus the tidal and barometric responses and the long-term trends. Thin lines mean the corrected observed discharges. Bold line in upper graph means 25-hour moving average of the corrected observed discharge. In the upper graph, dotted line means 25-hour moving average of the calculation using the parameters D = 2.0 and Ss = 1.7 × 10− 6. In the middle graph, dotted line means the calculation using the parameters D = 0.6 and Ss = 3.2 × 10− 6. In the lower graph, dotted line means the calculation using the parameters D = 0.6 and Ss = 3.0 × 10− 6. (b)The left-side figures show the injection pressures and flow rates. Bold lines mean actual injection pressures. Thin lines mean actual injection flow rates. Dotted lines mean the values of injection flow rate used in the calculation. The right-side figures show the comparison between the corrected observed groundwater levels and calculations. The corrected observed groundwater levels are equal to the raw data minus the tidal and barometric responses and the long-term trends. Thin lines mean the corrected observed groundwater levels. Dotted lines mean the calculations using the parameters D = 0.4 and Ss = 2.6 × 10− 6. 187
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all experiments (1997, 2000, and 2003) and discussed its temporal change. 2. Outline of water injection experiments and observations at the 800-m borehole Three boreholes were drilled from October 1995 to November 1996 near the Nojima fault and the Nojima branch fault in Awaji Island, Hyogo Prefecture, Japan (Figs. 1 and 2). Water injection experiments were conducted from the 1800-m borehole in 1997, 2000, and 2003, respectively. At the time of the water injection experiments, the discharge or groundwater level at the 800-m borehole was measured. The 1800-m borehole, is used as an injection well, was drilled from the east side of the Nojima branch fault, passed through the Nojima branch fault, and arrived in the fractured part of the Nojima fault (Fig. 2). From longterm temperature monitoring in the 1800-m borehole, it is found that injected water was leaked at the depth of 540 m, but not at greater depth (Yamano and Goto, 2005). Thus, we judged that water was injected to east area of the Nojima branch fault at the depth of 540 m. The 800-m borehole, which is used as a monitoring well, was drilled vertically into the east area of the Nojima branch fault. The open interval of the 800-m borehole was at a depth of 785–791 m and lay within granite. This borehole is an artesian well. Until August 2000, the top of the borehole was open and the discharge from the borehole was measured (Fig. 3). From January 1997 to June 1997, the discharge was measured by means of a rain gauge (resolution of 1 count = 5.7 cm3). From December 1997 to August 2000, the discharge was measured by water level change of a tank in which the discharge water was stored (resolution is 0.071 cm3). Since August 2000, the borehole has been sealed, and the water pressure in the borehole was measured using a pressure gauge (Fig. 3). The water pressure data is used as equivalent to groundwater level data. The discharge and water pressure were recorded every 1 min. The discharge data time series consists of 1-hour cumulative totals. 3. Observation results at the 800-m borehole Fig. 4 shows the discharge during 1997 and 2000 experiments and the groundwater level during 2003 experiments. The resolution and stability of the discharge and the groundwater level differed depending on the method. However, during all experiments, the discharge and the groundwater level increased during the injections, and decreased just after the injections were stopped. The patterns of the changes associated with
injections, such as amplitude and slope, are different for each experiment, which infers the difference of permeability of the rock. The discharge and the groundwater level also have tidal responses, barometric responses, and long-term trends. The discharge data cannot be treated in the same way as the groundwater level data. Assuming that there is a proportional relationship between the discharge and the groundwater level (e.g., Yuhara and Seno, 1969), the discharge is converted to the equivalent groundwater level using tidal and barometric responses. We estimated the amplitude of tidal responses and the barometric responses of the discharge and the groundwater level during each period (Table 1). Then amplitude ratios of the discharge to the groundwater level for tidal and barometric responses were calculated. The ratio is regarded as a coefficient of proportionality. The coefficients were estimated to be 0.74–1.57 cm3/h/cm. The stain and barometric responses of groundwater level is usually frequency-dependent (e.g. Rojstaczer, 1988). It is thought that the response of discharge rate is similar. In this paper, the results from the experiments that ran continuously for 4–10 days were used, and therefore we chose to use the coefficient (0.74 cm3/h/cm) estimated from the barometric responses. In 1997, the barometric response of the discharge was not estimated. The tidal response of the discharge in 1997 equals that in 2000 within the range of error. Consequently, it is assumed that the barometric response of the discharge in 1997 equals that in 2000. The gradient of the long-term trend for each observation period was estimated (Table 1). 4. Estimation of the permeability of the fault zone The structure around the fault is thought to consist of three classifications (Evans et al., 1997; Seront et al., 1998). One is the fault core, which includes the slip surface and usually has low permeability. The second is the damage zone, which includes rocks fractured by fault slip and has high permeability. The third is the protolith, which is the bedrock, and has low permeability. Structure of the Nojima branch fault is likely to consist of the above three classifications parallel to the slip surface of the fault (Mizoguchi et al., 2000). The fault core consists of the fault gouge zone and the fault breccia. The damage zone corresponds to the fractured granite. The protolith is the fresh granite. Mizoguchi et al. (2000) described the thickness of the fault gouge zone as less than 1 m, the thickness of the fault breccia as about 2 m, and the thickness of the fractured granite as several tens of meters. Based on the above information, it is considered that both the injection point of the 1800-m borehole and the open interval of the
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800-m borehole are located in the damage zone (Fig. 2). It is presumed that the injected water infiltrated mainly into the damage zone. In order to estimate the permeability of the rock, the injection-induced changes were analyzed as follows. Based on the injection-induced flow rate, we numerically calculated the pore pressure distribution in the rock by a finite difference method. Then the hydraulic parameters of the rock were obtained by comparing the observation results with the calculation. The structural model and the method for the calculation are same as Kitagawa et al. (2002). The structural model for the calculation is stated below. For finite difference model, we simplified the fault core and protolith as the impermeable layer and the fractured granite as the permeable layer (two dimensional layered model). We assumed that the permeable layer is a homogenous and isotropic vertical plane with the 5 km wide, the 5 km deep, and has a uniform thickness of 50 m. This zone is divided into 10,000 grid rectangles, having a size of 50 × 50 m2 with a thickness of 50 m. The water injection may create an additional confining pressure (stress) of the rock and change the pore water pressure; therefore it is possible to change the permeability of the rock before and after the injection experiments (e.g., Takahashi, 1993). It is assumed that the hydraulic characteristics of the rock are constant throughout each experiment. In all experiments, pore water pressure change is little except for the vicinity of the injection point and it is expected that the permeability of the rock is not changed by injection itself. Also, the discharge was much less than injection flow rate, and consequently it is assumed that the effect of the discharge is negligible. Crustal strain changes during the experiments were also observed (Fujimori et al., 2001). Therefore, we need to consider the effect of interaction between pore water pressure and strain based
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Fig. 6. The temporal change in permeability estimated by the experiments. This plot used the relative value for each experiment to median value for the third experiment in 1997.
on poroelasticity (e.g., Rice and Cleary, 1976). However, the effect of interaction is excluded for simplification. We used the finite difference form of the following 2D diffusion equation; 2 dH d H d2 H ¼D þ 2 þ F ðQÞ; ð1Þ dt dx2 dz where H [m] is a head which is equal to a pore water pressure plus an elevation, D [m2/s] (= K / Ss) is the hydraulic diffusivity of the permeable zone, K [m/s] is the permeability of the permeable zone, Ss [1/m] is the specific storage of the permeable zone, and F(Q) represents the injection and is as follows. FðQÞ ¼
QðtÞ at the injection point; L3 Ss
ð2Þ
and
Table 2 The parameters estimated by the calculation Diffusivity m2/s
Specific storage 10- 6/m
Permeability 10- 6 m/s
Third in 1997
1.5 2.0 3.0
1.9–2.5 1.4–2.0 1.0–1.4
2.9–3.8 2.8–4.0 3.0–4.2
Second and third in 2000
0.5 0.6 0.7
3.6 3.2 2.9
1.8 1.9 2.0
Forth in 2000
0.6 0.7
3.0 2.7
1.8 1.9
First in 2003
0.4 0.5
2.6 2.2
1.0 1.1
Second in 2003
0.4
2.6
1.0
FðQÞ ¼ O at the others: In Eq. (2), L [m] is the size of a grid rectangle for this model and Q(t) is the injection flow rate. For the injection point during the experiments, we used the approximated values of Q(t) given as the dotted lines in Fig. 5. The upper boundary is the ground surface with a constant head (H = 0) and the other boundaries are impermeable. The injection point is a grid rectangle with the center along the horizontal direction and at the depth of 540 m and the observation point is a grid rectangle with 70 m away from the injection point and at the depth of 790 m. This analysis provided the permeability of the rock between the injection point and discharge point at the depth of less than 1 km. The obtained
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Fig. 7. The results of geophysical logging of the 1800-m borehole. (a) Density logging. (b) P-wave velocity logging. (c) The relationship between density and P-wave velocity at the depth from 500 m to 1000 m. The thick line shows the Eq. (5).
permeability is assumed to be a macroscopic value for the damage zone of the Nojima branch fault. Table 2 lists the hydraulic parameters estimated by the above calculation using a trial and error approach (Fig. 5). The temporal change in the permeability is shown in Fig. 6. The permeability of the rock has been decreasing with time since 1997. We note that the estimated absolute values of the hydraulic parameters depend on the structural model. Therefore the estimation of the permeability leaves the door open to further research. But for a fixed
structural model, the fact remains that the relative value of the permeability decreased with time. Consequently, it is a good bet that the macroscopic permeability of the damage zone decreased as time passed. 5. Discussion of the temporal change in the permeability Sato et al. (2000) concluded that the hydraulic diffusivity of the aquifer in the whole northern Awaji Island
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Table 3 Estimation of the porosity, the density, and the P-wave velocity Experiment
Relative value of permeability
Porosity ϕ
Density ρ g/cm3
P-wave velocity Vp km/s
1997 2000 2003
1.0 (±0.2) 0.54 (±0.3) 0.30 (±0.014)
0.0765 (0.0731-0.0793) 0.0676 (0.0668-0.0683) 0.0601 (0.0595-0.0607)
2.570 (2.565-2.576) 2.585 (2.584-2.586) 2.598 (2.597-2.599)
4.845 (4.804-4.893) 4.973 (4.962-4.984) 5.081 (5.074-5.090)
increased after the 1995 Hyogoken–Nanbu earthquake. Tokunaga (1999) indicated the hydraulic conductivity increased at least 5 times over the pre-coseismic value in northern Awaji Island. Their researches showed that the overall aquifer in northern Awaji Island, including the Nojima fault zone, became more permeable at the earthquake occurrence. It is thought that the permeability of the Nojima fault zone also increased at the earthquake occurrence. Combined with our result, the permeability of the Nojima fault zone increased at the earthquake occurrence and has decreased since the earthquake. In order to compare our result with seismic velocity change by Vidale and Li (2003), a trial calculation of P-wave velocity change from the permeability change is done in the following way. We use three relationships: between permeability and porosity, between porosity and density, and between density and P-wave velocity. First, we use the relationship between permeability and porosity of granite given by Watanabe and Seki (1982). It is assumed that the thickness of the permeable layer did not change and the permeability of the permeable layer changed uniformly throughout the layer. The relationship for the granite by Watanabe and Seki (1982) is K ¼ 0:2/5 ;
ð3Þ
where ϕ is porosity. Second, assuming that the density of the solid part of the rock is 2.7 g/cm3 and the pores are perfectly saturated with water and density of water is 1.0 g/cm3, the relationship between porosity and density is defined as q ¼ 2:7−1:7/;
ð4Þ
where ρ [g/cm3] is density. Finally, the relationship between density and P-wave velocity is estimated from the results of geophysical logging of the 1800-m borehole (Fig. 7). The relationship is approximated by Vp ¼ 8:5q−17;
ð5Þ
where Vp [km/s] is P-wave velocity. As we mentioned, above absolute values of the permeability depend on the structural model. Therefore we used the ratio K/K0 for each experiment, which K0 is
the median value in the third experiment in 1997. The logging of the 1800-m borehole was conducted in the depth range from 500 m to 1250 m in April 1996 and in the depth range from 1250 m to 1736.85 m in November 1996. From the result of density logging, the average density in the depth range from 500 m to 1000 m was 2.57 g/cm3 and consequently the porosity was estimated to 0.0765 using Eq. (4). It is assumed that this value was the porosity ϕ0 at the third experiment in 1997. From Eq. (3), porosity at each succeeding experiment is / ¼ /0
K K0
0:2 :
ð6Þ
Substituting ϕ in Eq. (4), density at each experiment is q ¼ 2:7−1:7/0
K K0
0:2 :
ð7Þ
From Eq. (5), Vp at each experiment is Vp ¼ 5:95−14:45/0
K K0
0:2 :
ð8Þ
Using Eqs. (6)–(8) and the relative permeability value K/K0 for each experiment in Fig. 6, the values of porosity, density, and Vp of rock at each experiment were calculated (Table 3). The Vp values inferred using this method change by more than twice the temporal change rate of Vp found by Vidale and Li (2003). The permeability is supposed to be strongly related to the porosity (crack density). The expected result is that news cracks formed at the earthquake occurrence and thereafter closed or became clogged. The decrease of permeability is consistent with the fault healing process of the Nojima fault after the 1995 Hyogoken–Nanbu earthquake. References Awata Y., Mizuno K., 1998, Explanatory text of the strip map of the surface fault ruptures associated with the 1995 HyogoKen–Nanbu Earthquake, central Japan — the Nojima, Ogura and Nadagawa Earthquake Fault, scale 1:10,000 (in Japanese), Tectonic map series (12), pp.44–49, Geol. Surv. Japan, Tsukuba, Japan.
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Evans, J.P., Forster, C.B., Goddard, J.V., 1997. Permeability of faultrelated rocks, and implications for hydraulic structure of fault zone. J. Struct. Geol. 19, 1393–1404. Fujimori, K., Ishii, H., Mukai, A., Nakao, S., Matsumoto, S., Hirata, H., 2001. Strain and tilt changes measured during water injection experiment at the Nojima fault zone, Japan. Isl. Arc 10, 228–234. Kitagawa, Y., Fujimori, K., Koizumi, N., 2002. Temporal change in permeability of the rock estimated from repeated water injection experiments near the Nojima Fault in Awaji Island, Japan. Geophys. Res. Lett. 29, 121-1–121-4. Mizoguchi, K., Hirose, T., Shimamoto, T., 2000. Permeability structure of Nojima fault: analysis of Funaki outcrop in Hokudan, Tsuna-gun, Hyogo Prefecture. Earth Mon. Extra 31, 58–65 (in Japanese). Rice, J.R., Cleary, M.P., 1976. Some basic stress diffusion solutions for fluid-saturated elastic porous media with compressible constituents. Rev. Geophys. 14, 227–241. Rojstaczer, S., 1988. Determination of fluid flow properties from the response of water levels in wells to atmospheric loading. Water Resour. Res. 24, 1927–1938. Sato, T., Sakai, R., Furuya, K., Kodama, T., 2000. Coseismic spring flow changes associated with the 1995 Kobe Earthquake. Geophys. Res. Lett. 27, 1219–1222. Seront, S., Wong, T.-F., Caine, J.S., Forster, C.B., Bruhn, R.L., Fredrich, J.T., 1998. Laboratory characterization of hydromechan-
ical properties of a seismogenic normal fault system. J. Struct. Geol. 20, 865–881. Shimazaki, K., Ando, M., Nishigami, K., Oshiman, N., 1998. Water injection experiments at Ogura along the Nojima fault, Japan. Earth Mon. Extra 21, 33–37 (in Japanese). Takahashi, M., 1993. Effect of pore pressure on permeability of rocks. Earth Mon. 15, 621–626 (in Japanese). Tokunaga, T., 1999. Modeling of earthquake-induced hydrological changes and possible permeability enhancement due to the 17 January 1995 Kobe Earthquake, Japan. J. Hydrol. 223, 221–229. Vidale, J.E., Li, Y.-G., 2003. Damage to the shallow Landers fault from the nearby Hector Mine earthquake. Nature 421, 524–526. Watanabe, K., Seki, Y., 1982. Natures of under ground water flow in granite acting as an agent in weathering. J. Mineral. Petrol. Econ. Geol. 77, 37–46 (in Japanese). Yamano, M., Goto, S., 2005. Long-term monitoring of the temperature profile in a deep borehole: temperature variations associated with water injection experiments and natural groundwater discharge. Phys. Earth Planet. Inter. 152, 326–334. Yuhara, K., Seno, K., 1969. Onsengaku. Tijinshokan, Tokyo, Japan, pp. 120–123 (in Japanese).
Temporal change in permeability of the Nojima fault zone by repeated water injection experiments Yuichi Kitagawa a,⁎, Kunio Fujimori b , Naoji Koizumi a,1 a
b
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan Received 5 April 2004; accepted 29 January 2007 Available online 23 March 2007
Abstract Three boreholes were drilled near the Nojima fault, which the 1995 Hyogoken–Nanbu earthquake occurred on. In order to research the properties and the healing process of the fault, water injection experiments were conducted every 3 years. In this report, we researched the permeability of the fault as a measurement of crack density or porosity of the fault zone. Pore water pressure changes in rock due to the water injections at one borehole were observed as discharge changes or groundwater level changes at the other borehole. Using numerical calculations, the permeability of the fault fracture zone was estimated for each experiment. The permeability has been decreasing as time passed, which is thought to show the fault healing process of the Nojima fault after the 1995 Hyogoken–Nanbu earthquake. © 2007 Published by Elsevier B.V. Keywords: 1995 Hyogoken–Nanbu earthquake; Nojima fault; Permeability; Water injection experiment; Fault healing
1. Introduction It is presumed that rocks in and around a fault zone are fractured and/or deformed when an earthquake occurs, and thereafter recover from their damage. Detection of the recovery of a fault zone is linked to understanding of the postseismic process and the preparation process for the next earthquake. However, there are few reports that detected the damage and recovery of a fault zone after an earthquake. There are some methods for detection of a recovery. One is to measure a temporal change in seismic wave velocity of a ⁎ Corresponding author. Fax: +81 29 855 1298. E-mail addresses: [email protected] (Y. Kitagawa), [email protected] (K. Fujimori), [email protected] (N. Koizumi). 1 Fax: +81 29 855 1298. 0040-1951/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.tecto.2007.01.012
fault zone. Vidale and Li (2003) have been measuring seismic wave velocity of the fault zone of the 1992 Landers earthquake (M7.3) using repeated explosions. They concluded that seismic wave velocities recovered after the 1992 Landers earthquake and temporarily turned back at the 1999 Hector Mine earthquake (M7.1) occurrence. Another method is to measure a temporal change in permeability of a fault zone (Shimazaki et al., 1998). It is thought that more fractured rock has larger porosity and is more permeable. It is expected that permeability of rock is closely related to the degree of fracturing, which is induced by fracture and/or deformation during an earthquake. The Nojima Fault Zone Probe Project was designed to research the properties and the fault healing process of the Nojima fault, on which the 1995 Hyogoken–Nanbu earthquake, with a JMA magnitude of 7.3, occurred. The
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Fig. 1. The observation site is located near the southwestern end of the Nojima fault. The square shows the observation site. The star shows the epicenter of the 1995 Hyogoken–Nanbu earthquake.
Nojima fault exhibited mainly right-lateral strike slip movement with a slight thrust component. Three boreholes with depths of 500, 800, and 1800 m were drilled near the Nojima fault and the Nojima branch fault (Figs. 1 and 2). In the southwestern end of the Nojima fault system, the Nojima fault mainly slipped at the 1995
earthquake and its displacement was 1.0 to 1.5 m. Awata and Mizuno (1998) reported that the displacement of the Nojima branch fault was 0.1 to 0.6 m at the 1995 earthquake. The water injection experiments were conducted from the 1800-m borehole every 3 years. The series of experiments is probably the first attempt in the world for
Fig. 2. Layout of three boreholes. (a) Horizontal location of three boreholes. The broken lines are projections of the boreholes on the ground surface. (b) Vertical projection of three boreholes.
0.74 1.57 (±0.20) cm3/h/cm Ratio of 2000 to 2003
1.20 (± 0.20)
Unknown − 0.31/hpa − 0.42/hpa 1.19 (±0.20) 1.32 (±0.12) 0.84 (±0.03) cm3/h cm3/h cm Discharge in 1997 Discharge in 2000 Ground water level in 2003
1.46 (± 0.26) 1.57 (± 0.13) 1.30 (± 0.11)
Barometric response 4–10 days/cycle Tidal comp of O1 25.8 h/cycle Tidal comp of M2 12.4 h/cycle Unit
Fig. 4. The observation results at the 800-m borehole. The shaded zones show the periods of the water injection experiments.
The term
the active fault just after the occurrence of large earthquake. Repeated experiments aim to measure the characteristics of the fault zone directly and to detect its temporal changes. Kitagawa et al. (2002) concluded that the permeability of the damage zone of the Nojima branch fault in 2000 became lower than that in 1997. In this report, we estimated the permeability of the fault zone in
Table 1 The tidal and barometric responses and the long-term trends of the discharge and ground water level
Fig. 3. Outline of the observation methods at the 800-m borehole.
−1.80/day −0.36/day −0.56/day
185 Long-term trend
Y. Kitagawa et al. / Tectonophysics 443 (2007) 183–192
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Y. Kitagawa et al. / Tectonophysics 443 (2007) 183–192 Fig. 5. (a) The left-side figures show the injection pressures and flow rates. Bold lines mean actual injection pressures. Thin lines mean actual injection flow rates. Dotted lines mean the values of injection flow rate used in the calculation. The right-side figures show the comparison between the corrected observed discharges and calculations. The corrected observed discharges are equal to the raw data minus the tidal and barometric responses and the long-term trends. Thin lines mean the corrected observed discharges. Bold line in upper graph means 25-hour moving average of the corrected observed discharge. In the upper graph, dotted line means 25-hour moving average of the calculation using the parameters D = 2.0 and Ss = 1.7 × 10− 6. In the middle graph, dotted line means the calculation using the parameters D = 0.6 and Ss = 3.2 × 10− 6. In the lower graph, dotted line means the calculation using the parameters D = 0.6 and Ss = 3.0 × 10− 6. (b)The left-side figures show the injection pressures and flow rates. Bold lines mean actual injection pressures. Thin lines mean actual injection flow rates. Dotted lines mean the values of injection flow rate used in the calculation. The right-side figures show the comparison between the corrected observed groundwater levels and calculations. The corrected observed groundwater levels are equal to the raw data minus the tidal and barometric responses and the long-term trends. Thin lines mean the corrected observed groundwater levels. Dotted lines mean the calculations using the parameters D = 0.4 and Ss = 2.6 × 10− 6. 187
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all experiments (1997, 2000, and 2003) and discussed its temporal change. 2. Outline of water injection experiments and observations at the 800-m borehole Three boreholes were drilled from October 1995 to November 1996 near the Nojima fault and the Nojima branch fault in Awaji Island, Hyogo Prefecture, Japan (Figs. 1 and 2). Water injection experiments were conducted from the 1800-m borehole in 1997, 2000, and 2003, respectively. At the time of the water injection experiments, the discharge or groundwater level at the 800-m borehole was measured. The 1800-m borehole, is used as an injection well, was drilled from the east side of the Nojima branch fault, passed through the Nojima branch fault, and arrived in the fractured part of the Nojima fault (Fig. 2). From longterm temperature monitoring in the 1800-m borehole, it is found that injected water was leaked at the depth of 540 m, but not at greater depth (Yamano and Goto, 2005). Thus, we judged that water was injected to east area of the Nojima branch fault at the depth of 540 m. The 800-m borehole, which is used as a monitoring well, was drilled vertically into the east area of the Nojima branch fault. The open interval of the 800-m borehole was at a depth of 785–791 m and lay within granite. This borehole is an artesian well. Until August 2000, the top of the borehole was open and the discharge from the borehole was measured (Fig. 3). From January 1997 to June 1997, the discharge was measured by means of a rain gauge (resolution of 1 count = 5.7 cm3). From December 1997 to August 2000, the discharge was measured by water level change of a tank in which the discharge water was stored (resolution is 0.071 cm3). Since August 2000, the borehole has been sealed, and the water pressure in the borehole was measured using a pressure gauge (Fig. 3). The water pressure data is used as equivalent to groundwater level data. The discharge and water pressure were recorded every 1 min. The discharge data time series consists of 1-hour cumulative totals. 3. Observation results at the 800-m borehole Fig. 4 shows the discharge during 1997 and 2000 experiments and the groundwater level during 2003 experiments. The resolution and stability of the discharge and the groundwater level differed depending on the method. However, during all experiments, the discharge and the groundwater level increased during the injections, and decreased just after the injections were stopped. The patterns of the changes associated with
injections, such as amplitude and slope, are different for each experiment, which infers the difference of permeability of the rock. The discharge and the groundwater level also have tidal responses, barometric responses, and long-term trends. The discharge data cannot be treated in the same way as the groundwater level data. Assuming that there is a proportional relationship between the discharge and the groundwater level (e.g., Yuhara and Seno, 1969), the discharge is converted to the equivalent groundwater level using tidal and barometric responses. We estimated the amplitude of tidal responses and the barometric responses of the discharge and the groundwater level during each period (Table 1). Then amplitude ratios of the discharge to the groundwater level for tidal and barometric responses were calculated. The ratio is regarded as a coefficient of proportionality. The coefficients were estimated to be 0.74–1.57 cm3/h/cm. The stain and barometric responses of groundwater level is usually frequency-dependent (e.g. Rojstaczer, 1988). It is thought that the response of discharge rate is similar. In this paper, the results from the experiments that ran continuously for 4–10 days were used, and therefore we chose to use the coefficient (0.74 cm3/h/cm) estimated from the barometric responses. In 1997, the barometric response of the discharge was not estimated. The tidal response of the discharge in 1997 equals that in 2000 within the range of error. Consequently, it is assumed that the barometric response of the discharge in 1997 equals that in 2000. The gradient of the long-term trend for each observation period was estimated (Table 1). 4. Estimation of the permeability of the fault zone The structure around the fault is thought to consist of three classifications (Evans et al., 1997; Seront et al., 1998). One is the fault core, which includes the slip surface and usually has low permeability. The second is the damage zone, which includes rocks fractured by fault slip and has high permeability. The third is the protolith, which is the bedrock, and has low permeability. Structure of the Nojima branch fault is likely to consist of the above three classifications parallel to the slip surface of the fault (Mizoguchi et al., 2000). The fault core consists of the fault gouge zone and the fault breccia. The damage zone corresponds to the fractured granite. The protolith is the fresh granite. Mizoguchi et al. (2000) described the thickness of the fault gouge zone as less than 1 m, the thickness of the fault breccia as about 2 m, and the thickness of the fractured granite as several tens of meters. Based on the above information, it is considered that both the injection point of the 1800-m borehole and the open interval of the
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800-m borehole are located in the damage zone (Fig. 2). It is presumed that the injected water infiltrated mainly into the damage zone. In order to estimate the permeability of the rock, the injection-induced changes were analyzed as follows. Based on the injection-induced flow rate, we numerically calculated the pore pressure distribution in the rock by a finite difference method. Then the hydraulic parameters of the rock were obtained by comparing the observation results with the calculation. The structural model and the method for the calculation are same as Kitagawa et al. (2002). The structural model for the calculation is stated below. For finite difference model, we simplified the fault core and protolith as the impermeable layer and the fractured granite as the permeable layer (two dimensional layered model). We assumed that the permeable layer is a homogenous and isotropic vertical plane with the 5 km wide, the 5 km deep, and has a uniform thickness of 50 m. This zone is divided into 10,000 grid rectangles, having a size of 50 × 50 m2 with a thickness of 50 m. The water injection may create an additional confining pressure (stress) of the rock and change the pore water pressure; therefore it is possible to change the permeability of the rock before and after the injection experiments (e.g., Takahashi, 1993). It is assumed that the hydraulic characteristics of the rock are constant throughout each experiment. In all experiments, pore water pressure change is little except for the vicinity of the injection point and it is expected that the permeability of the rock is not changed by injection itself. Also, the discharge was much less than injection flow rate, and consequently it is assumed that the effect of the discharge is negligible. Crustal strain changes during the experiments were also observed (Fujimori et al., 2001). Therefore, we need to consider the effect of interaction between pore water pressure and strain based
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Fig. 6. The temporal change in permeability estimated by the experiments. This plot used the relative value for each experiment to median value for the third experiment in 1997.
on poroelasticity (e.g., Rice and Cleary, 1976). However, the effect of interaction is excluded for simplification. We used the finite difference form of the following 2D diffusion equation; 2 dH d H d2 H ¼D þ 2 þ F ðQÞ; ð1Þ dt dx2 dz where H [m] is a head which is equal to a pore water pressure plus an elevation, D [m2/s] (= K / Ss) is the hydraulic diffusivity of the permeable zone, K [m/s] is the permeability of the permeable zone, Ss [1/m] is the specific storage of the permeable zone, and F(Q) represents the injection and is as follows. FðQÞ ¼
QðtÞ at the injection point; L3 Ss
ð2Þ
and
Table 2 The parameters estimated by the calculation Diffusivity m2/s
Specific storage 10- 6/m
Permeability 10- 6 m/s
Third in 1997
1.5 2.0 3.0
1.9–2.5 1.4–2.0 1.0–1.4
2.9–3.8 2.8–4.0 3.0–4.2
Second and third in 2000
0.5 0.6 0.7
3.6 3.2 2.9
1.8 1.9 2.0
Forth in 2000
0.6 0.7
3.0 2.7
1.8 1.9
First in 2003
0.4 0.5
2.6 2.2
1.0 1.1
Second in 2003
0.4
2.6
1.0
FðQÞ ¼ O at the others: In Eq. (2), L [m] is the size of a grid rectangle for this model and Q(t) is the injection flow rate. For the injection point during the experiments, we used the approximated values of Q(t) given as the dotted lines in Fig. 5. The upper boundary is the ground surface with a constant head (H = 0) and the other boundaries are impermeable. The injection point is a grid rectangle with the center along the horizontal direction and at the depth of 540 m and the observation point is a grid rectangle with 70 m away from the injection point and at the depth of 790 m. This analysis provided the permeability of the rock between the injection point and discharge point at the depth of less than 1 km. The obtained
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Fig. 7. The results of geophysical logging of the 1800-m borehole. (a) Density logging. (b) P-wave velocity logging. (c) The relationship between density and P-wave velocity at the depth from 500 m to 1000 m. The thick line shows the Eq. (5).
permeability is assumed to be a macroscopic value for the damage zone of the Nojima branch fault. Table 2 lists the hydraulic parameters estimated by the above calculation using a trial and error approach (Fig. 5). The temporal change in the permeability is shown in Fig. 6. The permeability of the rock has been decreasing with time since 1997. We note that the estimated absolute values of the hydraulic parameters depend on the structural model. Therefore the estimation of the permeability leaves the door open to further research. But for a fixed
structural model, the fact remains that the relative value of the permeability decreased with time. Consequently, it is a good bet that the macroscopic permeability of the damage zone decreased as time passed. 5. Discussion of the temporal change in the permeability Sato et al. (2000) concluded that the hydraulic diffusivity of the aquifer in the whole northern Awaji Island
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Table 3 Estimation of the porosity, the density, and the P-wave velocity Experiment
Relative value of permeability
Porosity ϕ
Density ρ g/cm3
P-wave velocity Vp km/s
1997 2000 2003
1.0 (±0.2) 0.54 (±0.3) 0.30 (±0.014)
0.0765 (0.0731-0.0793) 0.0676 (0.0668-0.0683) 0.0601 (0.0595-0.0607)
2.570 (2.565-2.576) 2.585 (2.584-2.586) 2.598 (2.597-2.599)
4.845 (4.804-4.893) 4.973 (4.962-4.984) 5.081 (5.074-5.090)
increased after the 1995 Hyogoken–Nanbu earthquake. Tokunaga (1999) indicated the hydraulic conductivity increased at least 5 times over the pre-coseismic value in northern Awaji Island. Their researches showed that the overall aquifer in northern Awaji Island, including the Nojima fault zone, became more permeable at the earthquake occurrence. It is thought that the permeability of the Nojima fault zone also increased at the earthquake occurrence. Combined with our result, the permeability of the Nojima fault zone increased at the earthquake occurrence and has decreased since the earthquake. In order to compare our result with seismic velocity change by Vidale and Li (2003), a trial calculation of P-wave velocity change from the permeability change is done in the following way. We use three relationships: between permeability and porosity, between porosity and density, and between density and P-wave velocity. First, we use the relationship between permeability and porosity of granite given by Watanabe and Seki (1982). It is assumed that the thickness of the permeable layer did not change and the permeability of the permeable layer changed uniformly throughout the layer. The relationship for the granite by Watanabe and Seki (1982) is K ¼ 0:2/5 ;
ð3Þ
where ϕ is porosity. Second, assuming that the density of the solid part of the rock is 2.7 g/cm3 and the pores are perfectly saturated with water and density of water is 1.0 g/cm3, the relationship between porosity and density is defined as q ¼ 2:7−1:7/;
ð4Þ
where ρ [g/cm3] is density. Finally, the relationship between density and P-wave velocity is estimated from the results of geophysical logging of the 1800-m borehole (Fig. 7). The relationship is approximated by Vp ¼ 8:5q−17;
ð5Þ
where Vp [km/s] is P-wave velocity. As we mentioned, above absolute values of the permeability depend on the structural model. Therefore we used the ratio K/K0 for each experiment, which K0 is
the median value in the third experiment in 1997. The logging of the 1800-m borehole was conducted in the depth range from 500 m to 1250 m in April 1996 and in the depth range from 1250 m to 1736.85 m in November 1996. From the result of density logging, the average density in the depth range from 500 m to 1000 m was 2.57 g/cm3 and consequently the porosity was estimated to 0.0765 using Eq. (4). It is assumed that this value was the porosity ϕ0 at the third experiment in 1997. From Eq. (3), porosity at each succeeding experiment is / ¼ /0
K K0
0:2 :
ð6Þ
Substituting ϕ in Eq. (4), density at each experiment is q ¼ 2:7−1:7/0
K K0
0:2 :
ð7Þ
From Eq. (5), Vp at each experiment is Vp ¼ 5:95−14:45/0
K K0
0:2 :
ð8Þ
Using Eqs. (6)–(8) and the relative permeability value K/K0 for each experiment in Fig. 6, the values of porosity, density, and Vp of rock at each experiment were calculated (Table 3). The Vp values inferred using this method change by more than twice the temporal change rate of Vp found by Vidale and Li (2003). The permeability is supposed to be strongly related to the porosity (crack density). The expected result is that news cracks formed at the earthquake occurrence and thereafter closed or became clogged. The decrease of permeability is consistent with the fault healing process of the Nojima fault after the 1995 Hyogoken–Nanbu earthquake. References Awata Y., Mizuno K., 1998, Explanatory text of the strip map of the surface fault ruptures associated with the 1995 HyogoKen–Nanbu Earthquake, central Japan — the Nojima, Ogura and Nadagawa Earthquake Fault, scale 1:10,000 (in Japanese), Tectonic map series (12), pp.44–49, Geol. Surv. Japan, Tsukuba, Japan.
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ical properties of a seismogenic normal fault system. J. Struct. Geol. 20, 865–881. Shimazaki, K., Ando, M., Nishigami, K., Oshiman, N., 1998. Water injection experiments at Ogura along the Nojima fault, Japan. Earth Mon. Extra 21, 33–37 (in Japanese). Takahashi, M., 1993. Effect of pore pressure on permeability of rocks. Earth Mon. 15, 621–626 (in Japanese). Tokunaga, T., 1999. Modeling of earthquake-induced hydrological changes and possible permeability enhancement due to the 17 January 1995 Kobe Earthquake, Japan. J. Hydrol. 223, 221–229. Vidale, J.E., Li, Y.-G., 2003. Damage to the shallow Landers fault from the nearby Hector Mine earthquake. Nature 421, 524–526. Watanabe, K., Seki, Y., 1982. Natures of under ground water flow in granite acting as an agent in weathering. J. Mineral. Petrol. Econ. Geol. 77, 37–46 (in Japanese). Yamano, M., Goto, S., 2005. Long-term monitoring of the temperature profile in a deep borehole: temperature variations associated with water injection experiments and natural groundwater discharge. Phys. Earth Planet. Inter. 152, 326–334. Yuhara, K., Seno, K., 1969. Onsengaku. Tijinshokan, Tokyo, Japan, pp. 120–123 (in Japanese).