Pressure dependence of fluid transport properties of shallow fault systems in the Nankai subduction zone
© Tanikawa et al.; licensee Springer. 2014
Received: 25 February 2014
Accepted: 6 August 2014
Published: 13 August 2014
We measured fluid transport properties at an effective pressure of 40 MPa in core samples of sediments and fault rocks collected by the Integrated Ocean Drilling Program (IODP) NanTroSEIZE drilling project Expedition 316 from the megasplay fault system (site C0004) and the frontal thrust (site C0007) in the Nankai subduction zone. Permeability decreased with effective pressure as a power law function. Permeability values in the fault zones were 8 × 10−18 m2 at site C0004 and 9 × 10−18 m2 at site C0007. Stratigraphic variation in transport properties suggests that the megasplay fault zone may act as a barrier to fluid flow, but the frontal thrust fault zone might not. Depth variation in permeability at site C0007 is probably controlled by the mechanical compaction of sediment. Hydraulic diffusivity at shallow depths was approximately 1 × 10−6 m2 s−1 in both fault zones, which is small enough to lead to pore pressure generation that can cause dynamic fault weakening. However, absence of a very low permeable zone, which may have formed in the Japan Trench subduction zone, might prevent facilitation of huge shallow slips during Nankai subduction zone earthquakes. Porosity tests under dry conditions might have overestimated the porosity.
KeywordsPermeability Porosity Integrated Ocean Drilling Program Expedition 316 (NanTroSEIZE) Hydraulic diffusivity
Fluid transport properties of fault zones in the Nankai accretionary prism control static hydrologic behavior in the fault zone as well as dynamic fault processes. Historical pore pressure evolution at a depth in the accretionary prism is related to sediment consolidation and deformation. In addition, pore pressure distribution in fault zones influences fault strength, and a continuous increase in pore pressure can trigger dynamic slip motion (Sibson 1992). When a fault zone is impermeable, a sudden pore pressure rise due to frictional heating or shear compaction during a seismic slip increases fault instability (Lachenbruch 1980; Andrews 2002) and promotes large displacement (Tanikawa and Shimamoto 2009). A low permeable fault zone is also related to the generation of highly pressurized fluid in the megathrust induced by dehydration and fluid influx from oceanic crust (Yoshida and Kato 2011; Kimura et al. 2012; Mitsui et al. 2012). Fluid transport properties in a fault zone also play an important role in the generation of slow-slip events and low-frequency earthquakes (Suzuki and Yamashita 2009), which have been observed in the Nankai accretionary prism (Ito and Obara 2006).
The key parameter characterizing fluid flow and transport in media with high porosity is hydraulic diffusivity, which is determined from permeability and specific storage measurements. Permeability of sediments and fault rocks is often reported in the literature, but specific storage data are seldom reported. In this study, we measured permeability and specific storage under 40 MPa of confining pressure and less than 1 MPa of pore pressure in core samples from sites C0004 (holes C0004C and C0004D) and C0007 (holes C0007C and C0007D) drilled by IODP Expedition 316. We then estimated the horizontal depth distribution of fluid transport properties in two fault zones in the Nankai accretionary prism.
Samples and experimental settings
Fault zone characteristics
Locations and transport properties of the core samples used for the laboratory experiments
Core depth below seafloor (m)
In situ effective pressure (MPa)
Matrix density (g/cm3)
Shape of specimen
316-C0004D-10H-1, 114 to 116 cm
1.80 × 10−16 × (10−6 Pe)−1.58
316-C0004D-19R-1, 75 to 77 cm
1.24 × 10−17 × (10−6 Pe)−1.25
0.452 × exp(−3.30 × 10−9 Pe)
k = 497.51 × Φ57.4
316-C0004D-28R-2, 33 to 38 cm
8.40 × 10−18 − 3.89 × 10−25 Pe
0.454 to 0.00956 × log(Pe)
316-C0004D-39R-1, 71 to 73 cm
5.81 × 10−17 × (10−6 Pe)−1.26
0.312 × exp(−1.64 × 10−9 Pe)
k = 43,340 × Φ42.7
316-C0004D-54R-3, 68 to 70 cm
7.22 × 10−16 × (10−6 Pe)−1.56
0.334 × exp(−1.59 × 10−9 Pe)
k = 1.38 × 1013 × Φ60.9
316-C0007C-7X-5, 77 to 79 cm
9.17 × 10−17 × (10−6 Pe)−1.19
316-C0007D-18R-3, 80 to 83 cm
4.13 × 10−17 × exp(−0.290 × 10−6 Pe)
0.483 to 0.0158 × log(Pe)
k = 9.9 × 1013 × Φ85.4
316-C0007D-20R-2, 85 to 87 cm
9.87 × 10−17 × (10−6 Pe)−1.36
316-C0007D-23R-1, 80 to 84 cm
4.87 × 10−17 × (10−6 Pe)−1.15
0.397 to 0.00799 × log(Pe)
316-C0007D-25R-3, 110 to 112 cm
3.76 × 10−17 × (10−6 Pe)−1.16
0.393 to 0.00897 × log(Pe)
k = 2.6 × 1038 × Φ117
316-C0007D-28R-2, 111 to 114 cm
3.10 × 10−17 × (10−6 Pe)−1.17
0.493 to 0.00950 × log(Pe)
where Q is the volume of fluid measured per unit time, k w is the water permeability, A is the cross-sectional area of the sample, η is the dynamic viscosity of water, L is the sample length, and Pup and Pdown are pore fluid pressures at the upstream (the bottom of the specimen) and downstream ends (top) of the specimen, respectively. We used a pressure regulator to keep Pup constant (0.20 to 1.2 MPa in absolute pressure). The rate of water outflow per unit time from the downstream end of the samples was measured with a digital balance. We assumed a constant value of 0.1 MPa for Pdown because the water flowing out of the lower end of the specimen was released to atmospheric pressure.
where Vp is the pore volume, Pc is the confining pressure, and P is the pore pressure. Our experimental conditions mostly satisfied the conditions for which Equation 3 is valid because the pore pressure change was very small compared to the confining pressure change. Therefore, to determine specific storage, we calculated the drained pore compressibility with Equation 3 using the results of the porosity measurements (Wibberley and Shimamoto 2005). Fluid compressibility in Equation 2 was assumed to be constant at 4.3 × 10−10 Pa−1 (Fine and Millero 1973). Permeability and porosity were measured at each step as confining pressure was increased in steps from 1 to 40 MPa, and the duration before starting the permeability tests after changing the confining pressure was about 5 to 10 min.
The initial porosity of the specimens at the lowest effective pressure varied considerably between 31% and 45% (Figure 4b). Porosity declined by 2.5% to 3.7% at 40 MPa from the initial porosity. The largest reduction in porosity was found in the fault breccia in core sample C4D–1. Logarithmic (k w = a − b · log(Pe)) or exponential functions fitted the porosity-effective pressure curves well (Table 1).
Initial specific storage values were between 3 × 10−9 and 1 × 10−9 Pa−1, and these decreased with increasing effective pressure (Figure 4c). A rapid reduction of specific storage was observed by 10 MPa of effective pressure, after which it increased again in several samples.
where Φ0 is the initial porosity at 0 mbsf (z = 0) and α is the compaction constant.
Equation 5 fitted the porosity-depth data at site C0004 well, which were measured on board using discrete samples and a helium gas pycnometer (Figure 2d; Kimura et al. 2008) where the parameter values Φ0 = 65% and α = 0.00125 were chosen. We selected Φ0 = 50% and α = 0.0006 to reproduce the porosity-depth curve at site C0007 in an approximate manner (Figure 2h). We assumed constant ρr = 2,650 kg m−3 and constant ρf = 1,000 kg m−3 in Equation 4. In hole C0004D below 200 mbsf, the permeability seemed to increase a little with depth, and permeability in the fault zone was 7.6 × 10−18 m2 (Figure 2a). In situ specific storage was calculated from the porosity-effective pressure curve in our laboratory tests. Hydraulic diffusivity, which was calculated as k w /ηS s and equaled 9.3 × 10−7 m2 s−1 in the fault zone, displayed the lowest value in C0004D (Figure 2c). The relative change in permeability in C0004D is similar to the permeability trend reported by Ikari et al. (2009) using core samples, although our permeability values are two orders of magnitude larger than theirs. In hole C0007D, the permeability and hydraulic diffusivity slowly decreased with depth (Figure 2e,g). Permeability and hydraulic diffusivity were both lowest at 429.54 m in fault zone 3, which is very near to the depth of the slip localization layer.
Discussion and summary
Permeability and hydraulic diffusivity were lower in the shallow megasplay fault zone than in the frontal thrust fault zone 3 (8.7 × 10−18 m2), but the difference was very small. Hydraulic diffusivity in both fault zones was around 10−6 m2 s−1; this value might be small enough to cause thermal pressurization (Mizoguchi et al. 2007), which is a dynamic fault-weakening mechanism. However, transport properties will evolve by rapid shear deformation, which can drastically change the permeability of fault rocks (Tanikawa et al. 2010). Tanikawa et al. (2012) performed shear-induced permeability tests at a high-velocity condition on simulated fault gouge samples from core materials taken from the megathrust fault and the frontal thrust. Permeability values in both gouges were reduced by about an order of magnitude after the friction test. Therefore, shear-induced changes in transport properties must facilitate dynamic fault weakening induced by thermal pressurization. Ikari et al. (2009) measured the permeability of simulated gouges after shearing using a true triaxial testing machine. The gouge sample was made from the core blocks that were disaggregated and sieved to small grains. Therefore, the large difference between our permeability data and that of Ikari et al. (2009) can probably be explained by the shear-induced compaction of gouge samples and by the relative smaller permeability for disaggregated gouge samples than that of intact samples (Tanikawa et al. 2012).
We did not measure the permeability of fault rock at the Nankai Trough subduction zone because of the rarity of samples. As total clay contents and smectite content in the Nankai frontal thrust are lower than those in the Japan Trench plate boundary fault (Ujiie et al. 2013), we expect that the permeability of the Nankai Trough faults is larger than that of the Japan Trench. The lower friction coefficient in the Japan Trench décollement material than that in the material from the Nankai Trough (Ujiie et al. 2013) will be probably explained by the difference in permeability, as well as in the proportion of weak clay minerals.
Sample size probably influences the permeability as well. A number of reports show the permeability for very small samples with thicknesses from 0.5 to 20 mm (Lenormand and Fonta 2007; Lenormand et al. 2010), and we used similar small samples for the transport property measurements. However, our results suggest that permeability is independent of sample length and shape (see Additional file 1). Ma and Morrow (1996) reported that several effects of sample size on permeability will be avoided if the sample length is greater than 10 grain particle diameters; our sample size was large enough to satisfy this condition.
Hydraulic diffusivity was also lower in the Nankai megasplay fault zone than that in the surrounding host rock samples, which suggests that the fault zone can act as a barrier to fluid flow so that tectonic loading and fluid influx from the deep crust might cause the pore pressure to become relatively elevated within the fault zone. However, the difference in permeability between the fault zone and the host rocks of the frontal thrust was very small. We measured the transport properties of mudstone and fault breccia of mudstone origin, but the transport properties of sandstone and ash layers, which were observed throughout site C0007, were not measured. These rocks probably are permeable. In addition, the fracture permeability and its scaling effect on fluid transport properties have not been investigated yet. Therefore, we are not certain whether fluid flows preferentially within the fault zone in the frontal thrust region or whether it functions as a barrier. Variation in the fluid transport properties of a fault zone causes heterogeneity in the pore pressure distribution and fault strength, which influence future fault rupture paths in a fault system such as the Nankai branching fault system.
This research used core samples provided by the Integrated Ocean Drilling Program (IODP). The authors gratefully acknowledge support provided by the D/V Chikyu drilling crew and staff. This study was supported by two grants from the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Science Research program (Nos. 25800284 and 21107004). We also thank the guest and chief editors and two anonymous reviewers for improving our paper.
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