Recovery of Stress During the Interseismic Period Around the Seismogenic Fault of the 1995 M_w 6.9 Kobe Earthquake, Japan

Abstract

We analyzed the current stress state on the Nojima Fault, the source of the 1995 Mw 6.9 Kobe (Japan) earthquake, using breakouts in a borehole through the fault to a depth of ~1,000 m in 2017. The main fault was found at a depth of 529.3 m, with a damage zone ~60 m thick. Statistical analysis shows that the maximum horizontal stress (σHmax) in the depths shallower than 500 m rotates counterclockwise with depth toward the fault and reaches 138° at a depth of 500 m approximately perpendicular to the fault, coinciding with that measured in 1997. In the depths of ~650–1,000 m, the σHmax is oriented 241°, closely matching the regional tectonic stress direction. Our results reveal an accumulation of compressive stress, the driving force of faulting in the deep seismogenic zone since the 1995 earthquake, implying the fault zone has been healing continuously throughout the 22‐year postseismic period.

Fractures and Subsidiary Faults Developed in the Active Strike‐Slip Nojima Fault Zone, Japan, and Tectonic Implications

“…Considering the range in orientation of individual Riedel shear fabrics and the oblique angles to the main fault plane of the NFZ, we propose that the fractures and subsidiary faults from the deep drill holes mostly correspond to two Riedel shear structures: Y N and Y A shears parallel to the main NF and the branch AF trends, respectively, and R N and R A shears severally oblique to the main NF and the branch AF trends at an average clockwise angle of 10–30° (Figures c and d). These Riedel shear structures and striations observed on the main fault plane show that the NFZ accommodated dextral strike‐slip movement during ENE‐WSW to N‐W compressive stress, as shown by borehole logging data from the deep fault zone at >650 m (Figures b and c) (Nishiwaki et al, ; see below for details), the field observations in the Ogura site (Lin, ) and active fault orientation data (RGAFJ, 1991) (Figure b).…”

“…(c–e) Rose diagrams showing the relation between the Riedel shears and orientations of the fractures and subsidiary faults measured from the BHTV images of the NFD‐1 (c) and AFD‐1 (d) holes and observed at the Ogura trench (e). σ Hmax indicates the orientation of the maximum horizontal stress inferred from borehole breakouts in the Ogura trench (from Nishiwaki et al, ). Y N and Y A : Y shear in the NF and AF, respectively.…”

“…We used an ultrasonic BHTV to collect acoustic images from the vertical borehole walls to estimate maximum horizontal stress orientations (Nishiwaki et al, ) and to characterize the depth, direction of dip, dip angle, and strike of fractures and subsidiary faults. The BHTV images measure attributes of ultrasonic waves reflected at the borehole wall (Zemanek et al, ).…”

“…Typical BHTV images and related drill cores are shown in Figure . Structural analysis of fracture density from the borehole images as well as logging data (e.g., caliper, sonic, density, and electrical analyses of the hole wall) shows an anomalous variation in the NFD‐1 hole at depths between ~460 and ~650 m, in which the fracture density, P velocity, rock density, and electricity show lower values than shallower and deeper depths in the borehole (Lin & Nishiwaki, ; Nishiwaki et al, ). The NF main fault plane was found in the NFD‐1 hole at depth 529.3 m (Figure d) with a ~20‐cm‐thick fault gouge zone and a ~60‐m‐wide damage zone (Lin & Nishiwaki, ).…”

In Situ Stress Orientation From 3 km Borehole Image Logs in the Koyna Seismogenic Zone, Western India: Implications for Transitional Faulting Environment

“…However, this technique is unable to identify localized stress perturbations caused by small active faults. Localized perturbations in stress orientation could be determined from borehole breakouts, which therefore offer the most promising tool to constrain the subsurface stress regime (Chang et al, 2010;Ikeda et al, 2001;Lin et al, 2007;Lin et al, 2013Lin et al, , 2016Nishiwaki et al, 2018;Schmitt et al, 2012;Zoback et al, 1985). Breakouts are formed when the stress concentration around the borehole wall overcomes the compressive strength of the rock.…”