Abstract
In-situ permeability of the Median Tectonic Line (MTL) fault zone in Mie Prefecture, southwest Japan, was estimated using hydraulic tests and groundwater pressure observations in two boreholes. The screen depths in Holes 1 and 2 are located, respectively, in a major strand of the MTL fault zone within the Sambagawa metamorphic rocks and a branching fault developed in the hanging wall of the MTL within the Ryoke mylonite. The estimated permeability at Hole 1 ranges from 5.3 × 10−17 to 5.0 × 10−16 m2, and that at Hole 2 ranges from 4.4 × 10−16 to 1.5 × 10−15 m2. We also measured the permeability of the protolith close to the screened depth of Hole 1 (3.4 × 10−19 and 3.7 × 10−19 m2) and Hole 2 (3.1 × 10−19 and 6.2 × 10−19 m2). The permeability of the fault zone was found to be more than 100 and 700 times higher than the protolith permeability at Holes 1 and 2, respectively. The permeability data for Holes 1 and 2 are consistent with previously reported permeability data for samples from an MTL outcrop. The permeability observed in this study reflects the complex fault zone permeability structure of the MTL fault zone.
Highlights
The permeability structure around fault zones plays an important role in fault hydrogeology and movement
We studied the fault zone permeability in two of these boreholes (Fig. 1) using hydraulic tests and groundwater pressure observations and measured the permeability of drillcore samples
These estimations are approximate because Eq (2) is applicable when groundwater pressure/level suddenly decreases in the well (Bouwer and Rice 1976), whereas background water pressure self-recovered gradually by groundwater flowing into the aquifer
Summary
The permeability structure around fault zones plays an important role in fault hydrogeology and movement. A number of studies about laboratory measurements of permeability of natural and synthetic fault materials indicate low permeability within the fault core and variable permeability in the complex damage zone structure, which is governed by macroscale fracture networks and low-permeability deformation bands (e.g., Faulkner et al 2010; Carpenter et al 2014). Wibberley and Shimamoto (2003) reported laboratory-derived permeability data for the Median Tectonic Line (MTL) fault zone using samples from the Tsukide outcrop. Their permeability data show wide variation with fault rock microstructure, and central slip zone gouges have the lowest permeability
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