Changes in permeability of the Nojima fault damage zone inferred from repeated water injection experiments
© The Author(s) 2016
Received: 28 July 2016
Accepted: 9 November 2016
Published: 21 November 2016
Keywords1995 Hyogoken-Nanbu earthquake Nojima fault Permeability Water injection experiment Healing
When a large earthquake occurs on a fault, stress acting on the fault is released and rocks surrounding the fault are fractured. Thereafter, the fractured rocks are thought to recover physical properties, and then, stress begins to accumulate again, until the next earthquake. The Nojima fault, Awaji Island, central Japan, ruptured during the 1995 Hyogoken-Nanbu earthquake, which had a Japan Meteorological Agency magnitude (M JMA) of 7.3. To investigate the recovery process of a fault zone just after a large earthquake, repeated water injection experiments have been conducted near the Nojima fault since 1997. The objective of the Nojima fault zone probe project is to study the properties and recovery processes of the fault zone (Ando 2001). Kitagawa et al. (2007) showed by groundwater observations that from 1997 to 2003 the permeability of the fractured zone near the fault decreased, and observations of crustal strain (Mukai and Fujimori 2007) and self-potential (Murakami et al. 2007) during the same period supported their findings.
Other studies have investigated temporal changes in fault zones just after a large earthquake. For example, Vidale and Li (2003) used active seismic techniques to measure seismic wave velocity around the Johnson Valley fault, California, which ruptured during the 1992 Landers earthquake (M 7.3), and found that P-wave and S-wave velocities <1 km from the fault trace increased after 1994 and that velocities in the fault zone decreased after the 1999 Hector Mine earthquake (M 7.1). Li et al. (2007) similarly observed the fault zone damage and healing associated with the 2004 M6 Parkfield earthquake. Xue et al. (2013) studied the tidal response of water levels in a borehole drilled through the Wenchuan earthquake fault zone in China and suggested that decreasing permeability in the fault zone might reflect the healing process.
Here, we report on the results of repeated water injection experiments conducted at the Nojima fault from 1997 to 2006, focusing on the temporal changes in the permeability of the fractured zone near the fault after 2003, since the relative change in the permeability is more evidently estimated than the absolute value of that.
Methods: water injection experiments and observations
We insert the geological information in Fig. 1b, c (Ando 2001; Murata et al. 2001). The geology at this site is composed of the Osaka Group, the Kobe Group, and the Cretaceous Ryoke granitic rock. The water injection experiments were conducted in the 1800-m borehole, which was drilled from a site east of the surface trace of the branch fault. This borehole penetrated the branch fault and reached the fractured zone of the Nojima fault. Several holes of the casing for the screen were made at 1596–1671 m depth by the perforation. A vertical array of seismographs was installed in the 1800-m injection borehole, and the vertical distribution of temperature was also observed every 1 m from the surface to 1460 m depth by use of an optical fiber cable (Yamano and Goto 2005). The 800-m borehole was drilled vertically from another site east of the branch fault. Crustal strain was observed with a strainmeter at the bottom of the 800-m observation borehole (Mukai and Fujimori 2007). We observed the discharge rate until August 2000 and pressure of the groundwater since August 2000 within the 785- to 791-m depth interval of the 800-m borehole, at which there is a casing in shallower than 785 m depth and a cementing inside the borehole for install of the borehole strainmeter in deeper than 791 m depth (Kitagawa et al. 1999). The 500-m borehole, which was drilled with an inclination to the vertical of 29° from a site between the surface traces of the two faults, passed through the Nojima fault. Several holes of the casing for the screen were made at 302.2–304.8 m depth by the perforation. In this borehole, groundwater levels were observed, and electromagnetic measurements were taken at the bottom of the borehole. Both seismic (Nishigami et al. 2008) and self-potential observations were made at the surface around the 1800-m injection borehole (Murakami et al. 2007) during the water injection experiments. Water injection experiments were conducted in 1997, 2000, 2003, 2004, 2006, 2009, and 2013. From 1997 to 2009, water was injected into the top of the 1800-m borehole. At first, the experiment scheme was to inject water from the screen at 1596 to 1671 m depth of the 1800-m borehole, though the vertical distribution of temperature in the borehole during these experiments showed that the injected water leaked out of the borehole at a depth of around 540 m, but not at greater depths (Yamano and Goto 2005). In the 2013 experiment, therefore, a packer was installed at 559 m depth, and water was injected into the 1800-m borehole through the screen at 1596–1671 m depth (Nishigami and Research Group of Water Injection Experiment at the Nojima Fault 2014). In this paper, however, we discuss only the results of the water injection experiments conducted under similar conditions (i.e., excluding the 2013 experiment). Water pressure in the 800-m borehole was not successfully measured in 2009 because of a problem with the measurement system. Therefore, for this study we recomputed the results of the experiments conducted from 1997 until 2003, which differ from the permeabilities reported in Kitagawa et al. (2007), and added the results obtained during the 2004 and 2006 experiments; then, using the combined results, we investigated temporal changes in permeability of the fractured zone near the Nojima branch fault.
The resolution and stability of the discharge rate and pressure data from the 800-m borehole (Fig. 2) differed depending on the measurement method, and high-quality data were obtained by the improvement/replacement of the measurement system of discharge rate and water pressure. In all experiments, the groundwater discharge rate and pressure increased during the injections and then decreased as soon as the injections were stopped. The changes in the groundwater discharge rate and pressure associated with the injections differed in each experiment, depending on the injection rate and rock permeability during the experiment. The groundwater discharge and pressure data also show tidal responses, barometric responses, and long-term trends.
Estimates of the response of the groundwater discharge and pressure to the barometric pressure in the 800-m borehole
Period of data
Frequency range of the response (cycles/day)
Response to the barometric response
Standard deviation of the response
Units of the response
Conversion factor [cm/(cm3/h)]
February 10, 2000
March 2, 2000
January 1, 2002
June 30, 2002
November 1, 2002
February 28, 2003
September 1, 2003
November 30, 2003
January 1, 2005
June 30, 2005
Results: estimation of the permeability of the fault zone
The structure around a fault has three components (Evans et al. 1997; Seront et al. 1998): the fault core, which includes the slip surface and usually has low permeability; the damage zone, which includes rocks fractured by fault slip and usually has high permeability; and the protolith, the undeformed bedrock (host rock), the permeability of which trends to be lower than that of the damage zone. All three components are likely present, parallel to the slip surface, along the Nojima branch fault (Mizoguchi et al. 2008); the fault core comprises fault gouge zone and fault breccia, the damage zone consists of fractured granite, and the protolith is undeformed granite. The fractured granite is 20 m thick (Mizoguchi et al. 2008) based on surface samples around this area, and the thickness of the permeable zone has been estimated to be 15–25 m (Lockner et al. 2009) based on core samples from a borehole drilled 4 km northeast of our study site. On the basis of these estimated thicknesses, we consider both the injection point in the 1800-m borehole and the open interval in the 800-m borehole to be located in or near the damage zone of the Nojima branch fault (Fig. 1); thus, it is presumed that the injected water infiltrated into the damage zone.
We also estimated the permeability of the rock by using the numerical model and method described by Kitagawa et al. (2002, 2007). For simplicity, we considered the fault core and protolith to be impermeable and used a two-dimensional (2D) model for the permeable layer in the fractured granite. We assumed that the permeable layer was a homogeneous and isotropic vertical plane 5 km wide and 5 km deep with a uniform thickness of 50 m. The layer is divided into 10,000 grid cells, each 50 m × 50 m × 50 m thick.
Macroscopic parameters of the fault fracture zone estimated by numerical calculations and fracture aperture widths calculated by assuming a simplified model with a single open fracture
Macroscopic parameters of the fault fracture zone
Single open fracture model
S s (1/m)
Lower limit of D (m2/s)
Upper limit of D (m2/s)
Central value of K (m/s)
Error of K (m/s)
Ratio to 1997—3rd
Error of ratio
Error of aperture (mm)
2000—2nd and 3rd
As discussed by Kitagawa et al. (2007), Sato et al. (2000) and Tokunaga (1999) estimated that the permeability of the shallow crust of northern Awaji Island increased when the earthquake occurred. The decrease in permeability during the 8 years following the earthquake suggests that the earthquake-induced enhanced permeability in the fault fracture zone was approximating the state before the earthquake during that time.
The permeability estimated in this study is an order of magnitude less than that determined in 1996 in a borehole about 4 km northeast of the 800-m borehole site, where the permeability of the Nojima fault zone was determined by hydrophone vertical seismic profiling (Kiguchi et al. 2001). Kiguchi et al. (2001) estimated the permeability of a 1-m-thick zone. Here, the permeable layer was modeled as a layer with a thickness of 50 m. Therefore, if the actual permeable layer of the fault fracture zone is only a few meters thick, then the permeability of the actual permeable layer at our study site is probably similar to the previously reported permeability.
The permeability estimated here is orders of magnitude greater than permeabilities estimated in core samples (Mizoguchi et al. 2008; Lockner et al. 2009). It is difficult to obtain core samples for laboratory experiments that include the fracture or fractures primarily controlling the permeability; therefore, it is not surprising that permeabilities estimated for core samples are considerably smaller than permeabilities estimated by in situ testing for a granitic rock (Brace 1980).
Li and Vidale (2001) reported that the P–S-wave traveltime ratio decreased after the 1992 Landers earthquake and suggested that cracks near the fault zone were initially partly filled with fluid and became more fluid saturated with time after the earthquake. In the case of the Nojima fault zone, both the 800-m and 1800-m boreholes are artesian wells such that cracks must be fully saturated. In addition, an increase in water saturation in cracks reflects an increase in permeability to water. Therefore, an increase in water saturation probably cannot account for the decreased permeability 8 years after the earthquake.
If it is assumed that L = 50 m, as described in “Results: estimation of the permeability of the fault zone” section, the hydraulic fracture aperture b can be calculated from the estimated macroscopic permeability (Table 2). Nakashima et al. (2010) measured fracture apertures in a granite sample under the barometric pressure in the laboratory and made a rough estimate that the fracture apertures were generally <1 mm wide, and the average width was 0.39 mm. Therefore, the aperture widths estimated here (Table 2) are consistent with the values reported by Nakashima et al. (2010). Given our assumption of a single open fracture with uniform aperture width, the decrease in permeability from 1997 to 2003 suggests that the fracture aperture decreased in width by 28%. Actually, however, a number of fractures probably exist in the fractured granite zone. Therefore, some fractures may have decreased in aperture, and others may have closed or become clogged. The fracture aperture width seems to be associated with the strength of the fault rock. We have inferred that the decrease in permeability in the fault zone reflects the healing of the Nojima fault after the earthquake.
We could not identify a cause for the slight increase in permeability after 2003. Xue et al. (2013) reported that permeability in the Wenchuan earthquake fault zone in China was enhanced by the occurrence of remote earthquakes, and Kinoshita et al. (2015) suggested that the 2011 Tohoku earthquake caused enhanced permeability in a mine in central Japan. In the Nojima fault, there is no convincing evidence that the permeability enhancement was due to notable earthquakes. In general, an increase in normal stress (confining stress) reduces both fracture aperture widths and permeability (Baghbanan and Jing 2008). On the other hand, an increase in shear stress might cause permeability to increase and thus might imply the beginning of stress accumulation on the Nojima fault.
In an 800-m borehole near the Nojima fault, water-injection-induced changes in the groundwater discharge rate and pressure were observed. On the basis of these changes, the macroscopic permeability of the fault fracture zone was estimated to range from 1 × 10−6 to 2 × 10−6 m/s. These permeabilities are less than values obtained by in situ testing at site about 4 km away, but we consider the difference to be small, given the simplified model assumed for the numerical calculation here. The macroscopic permeability of the fault fracture zone decreased from 1997 to 2003, after which it increased slightly. This decrease may correspond to a decrease in the width of fracture apertures in the fractured granite, or their closure. Permeability was enhanced when the earthquake occurred, and the subsequent decrease in permeability may reflect the recovery of the strength of the fractured granite in the fault zone after the earthquake.
Y. Kitagawa conducted the analysis and discussion of the data and prepared the manuscript. Y. Kano renewed the groundwater observation system and observed the groundwater at the 800-m borehole since 2006. Both authors read and approved the final manuscript.
We thank K. Fujimori and A. Mukai for provision of the groundwater data at the 800-m borehole from 2000 to 2006. We thank K. Nishigami for conduct of the water injection experiments at the Nojima fault. We are grateful to three anonymous reviewers for valuable comments.
Both authors declare that they have no competing interests.
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