Geometry of the Nojima Fault at Nojima-Hirabayashi, Japan – II. Microstructures and their Implications for Permeability and Strength

Abstract

Samples of damage-zone granodiorite and fault core from two drillholes into the active, strikeslip Nojima fault zone display microstructures and alteration features that explain their measured present-day strengths and permeabilities and provide insight on the evolution of these properties in the fault zone. The least deformed damage-zone rocks contain two sets of nearly perpendicular (60-90°angles), roughly vertical fractures that are concentrated in quartz-rich areas, with one set typically dominating over the other. With increasing intensity of deformation, which corresponds generally to increasing proximity to the core, zones of heavily fragmented rock, termed microbreccia zones, develop between prominent fractures of both sets. Granodiorite adjoining intersecting microbreccia zones in the active fault strands has been repeatedly fractured and locally brecciated, accompanied by the generation of millimeter-scale voids that are partly filled with secondary minerals. Minor shear bands overprint some of the heavily deformed areas, and small-scale shear zones form from the pairing of closely spaced shear bands. Strength and permeability measurements were made on core collected from the fault within a year after a major (Kobe) earthquake. Measured strengths of the samples decrease regularly with increasing fracturing and fragmentation, such that the gouge of the fault core and completely brecciated samples from the damage zone are the weakest. Permeability increases with increasing disruption, generally reaching a peak in heavily fractured but still more or less cohesive rock at the scale of the laboratory samples. Complete loss of cohesion, as in the gouge or the interiors of large microbreccia zones, is accompanied by a reduction of permeability by 1-2 orders of magnitude below the peak values. The core samples show abundant evidence of hydrothermal alteration and mineral precipitation. Permeability is thus expected to decrease and strength to increase somewhat in active fault strands between earthquakes, as mineral deposits progressively seal fractures and fill pore spaces.

Geometry of the Nojima Fault at Nojima-Hirabayashi, Japan – I. A Simple Damage Structure Inferred from Borehole Core Permeability

“…These three zones are referred to in this paper as the 1140, 1312, and 1800 m NIED shear zones. Strength and permeability measurements reported here, as well as petrographic observations presented by (MOORE et al, 2008), all indicate that unlike the other shear zone crossings, the 1800 m NIED shear zone was not activated by the 1995 Kobe earthquake. TANAKA et al (2007) concluded that the main slip of the Kobe earthquake occurred at the 1140 m shear zone.…”

“…2). Fault core material at this depth contained smectite and other weak clay but also significant amounts of fine-grained quartz, feldspar, calcite and relatively strong alteration minerals (MIZOGUCHI et al, 2008;MOORE et al, 2008;OHTANI et al, 2000;TANAKA et al, 2001TANAKA et al, , 2007. The NIED hole was started 302 m to the SE of the fault trace.…”

“…In fact, the permeability and strength measurements are in excellent agreement with the idealized fault structural model described above. Additional data regarding petrography and microcrack characteristics are provided in a companion paper (MOORE et al, 2008). Vol.…”

Simulating seismic chimney structures as potential vertical migration pathways for CO2 in the Snøhvit area, SW Barents Sea: model challenges and outcomes

“…Faults were interpreted and mapped throughout the extent of the 3D seismic space and also in depth throughout the overburden, caprock and reservoir. Typical permeabilities are in the range of (0.00001 -0.0001 mD) for low permeable fault zones and (0.001 -10 mD) for highly permeable damage zones (Mizoguchi et al 2008;Moore et al 2009). Due to a lack of fault permeability and thickness data for the Snøhvit area, a fault permeability, based on the work of Mizoguchi, Hirose et al 2008 andMoore, Lockner et al 2009, ranging over (0,0001, 1, 50, 100 and 300 mD) (Figure 3a) was used for the simulations concerning the Snøhvit case.…”

Fault Permeability and Strength Evolution Related to Fracturing and Healing Episodic Processes (Years to Millennia): the Role of Pressure Solution

“…Recent works on the cored Nojima and Chelungpu active faults show relatively weak fine-grained gouges surrounded by more resistant damage zones of fractured rocks . The damage zones act as high permeability conduits for both vertical and horizontal flow whereas the fine-grained gouge acts as relatively impermeable barriers than prevent significant fluid flow across the fault (Doan et al, 2007;Moore et al, 2009). Figure 1 a) Schematic view of damaged zone deformation near active fault: earthquake-induced fracturing networks (black) that are progressively sealed by self-healing and sealing (white) associated with pressure solution creep processes (Fig.…”