Strength and Permeability of Core Samples Taken From Drillholes Crossing the Nojima Fault of the 1995 Kobe Earthquake
Figure 1a. Map of Awaji Island with location of drillsites.
The 1995 Kobe earthquake in Japan, magnitude 7.2, ruptured along the Nojima fault. After this event, two scientific drillholes were completed near the epicenter on Awagi Island that crossed the fault zone at depth. These drillholes provide a unique opportunity to study the properties of an active fault immediately after a major rupture event. The shallower hole, drilled by the Geological Survey of Japan (GSJ), was started 75m to the SE of the surface trace of the Nojima fault and crossed the fault at a depth of 624m, along a narrow (4 cm wide) zone of clay-rich material. A deeper hole, drilled by the National Research Institute for Earth Science and Disaster Prevention (NIED) was started 302m to the SE of the fault and crossed three distinct fault strands below a depth of 1140m (Figure 1). The two shallower shear zones of this drillhole had clay-rich cores, while the deepest zone was more diffuse and possibly inactive in the 1995 earthquake. We have measured strength and matrix permeability of core samples taken from these two drillholes in order to characterize the physical properties of the materials within the fault zone. We found a strong correlation between permeability (the ease with which fluids flow through the rock) and proximity to the fault zone shear axes, with low permeability in the narrow shear zone, higher permeability on either side, and again low permeability still farther away in the unfractured country rock. The width of the high permeability zone (approximately 20 to 40m) is in good agreement with overall fault zone width inferred from trapped wave analysis and other evidence. In addition, shear strength of the core material gradually increased with distance from the shear axis. These permeability and strength observations are consistent with a fault zone model in which a highly localized core or shear zone of low permeability clay-rich material is surrounded by a damage zone of fractured rock. In this case, the damage zone will act as a high-permeability conduit for vertical and horizontal fluid flow in the plane of the fault. The clay core region, however, will impede fluid flow across the fault. These findings are pertinent to understanding the important role that fluids play in fault zone mechanics.
Figure 1b. Map of Japan. Figure 1c. Schematic diagram of the Nojima Fault near the GSJ and NIED drillholes.
Axial shortening tests were performed on cylindrical samples cut from the drillhole core rocks. Deformation tests were conducted at 50 MPa confining pressure. Typical stress-axial shortening plots are shown in Figure 2 for samples from the NIED drillhole. The intact rock (far from shear zones) is a very competent granodiorite, and this rock fractured during testing, resulting in large, sudden stress drops. The damage zone material, closer to the shear zone, is already naturally fractured and altered by the passage of fluids. Damage zone rock is weak, but stronger than the clay-rich shear zone core material. Small drops in strength occurred in all samples when deformation was stopped to measure permeability.
Figure 2. Differential stress versus displacement of core samples.
If we take the maximum strength from each plot (the highest point on each curve), we can construct a profile of strength versus location relative to the shear zone. Figure 3 shows a typical example from the GSJ hole. Peak strength for each sample is plotted and expressed as the ratio of shear strength to normal stress. This ratio is known as the coefficient of friction, µ, and is an important physical parameter that describes the relative strength of materials. The clay-rich core material has the lowest strength with a coefficient of friction, µ, of approximately 0.55. Farther from the shear zone the coefficient of friction of the rock exceeds 1.0.
Figure 3. Profile of peak shear strength of GSJ core samples with distance from the shear zone. Strength is lowest at the shear zone axis and recovers to the intact rock strength outside of the damage zone.
Permeability of each sample was measured repeatedly during deformation tests. Measurements were first conducted at 10, 30, and 50 MPa confining pressure. Then, additional measurements were made as each sample was deformed (at 50 MPa confining pressure) at axial shortening of up to 5 mm (Figure 4). In many cases (i.e., intact sample in Figure 2), the sample reached a peak strength and then failed by forming a through-going fracture. This generally would result in a reversal in stress path and an increase in permeability. Samples underwent a rapid drop in permeability with the application of confining pressure. Application of deviatoric stress and introduction of microcracks typically lead to an increase in permeability.
Figure 4. Permeability first under hydrostatic loading, then with diviatoric stress.
The fault zone core contains clay minerals with permeabilities of approximately 0.1 to 1 microdarcy at 50 MPa confining pressure. Within a few meters of the fault zone core, the rock is highly fractured but has sustained little net shear. Matrix permeability of this zone is approximately 100 microdarcy at 50 MPa confining pressure. Outside this damage zone, matrix permeability drops to sub-nanodarcy values.
Figure 5. Profile of matrix permeability for the GSJ hole measured at 50 MPa confining pressure. The low permeability shear zone is surrounded by a high permeability damage zone.
We can construct a permeability profile relative to distance from the shear zone (similar to that in Figure 3) for each of the shear zones in the two drillholes. Figure 5 shows a typical example for the GSJ hole. In most cases, the permeability profiles show a general trend of increasing permeability on the flanks of the shear zone with an abrupt drop in permeability of the clay-rich shear zone core material. The deepest shear zone in the NIED drillhole appeared to be partially healed/sealed and was probably not activated by the 1995 earthquake, consistent with the uniformly high strength of rocks around this part of the fault.
One important question not addressed by this study is how rapidly the enhanced fault zone permeability structure will be reduced by sealing and crack healing processes. If the sealing and restrengthening process can occur over a single earthquake cycle, it could have an important influence on repeat time, stress drop and rupture nucleation.
- Following the Kobe earthquake, the shallow fault zone is characterized by a localized clay-rich shear zone with relatively low permeability and frictional strength.
- The fault core is surrounded by a damage zone of intensely fractured rock characterized by low strength and high permeability.
- The damage zone appears to extend 15 to 30 meters on either side of the fault core.
- Permeability drops rapidly outside the damage zone.
- Thus, this fault structure provides a conduit for vertical and horizontal flow of fluid in the fault plane. However, the low permeability of the shear zone core will inhibit flow across the fault, an important consideration for understanding fault zone mechanics.