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

The Nojima Fault Zone, on which the 1995 M-w 6.9 Kobe (Japan) earthquake occurred, is characterized by pulverized cataclastic rocks with numerous fractures and subsidiary faults. The fractures and subsidiary faults were observed in trench walls and using borehole televiewer (BHTV) images from boreholes NFD-1 (similar to 1,000-m depth) and AFD-1 (similar to 700-m depth), which were drilled through the Nojima Fault and its branch, the Asano Fault, respectively. Measurements show that the orientations of fractures and subsidiary faults in the two boreholes are concentrated at N10-70 degrees E, averaging N40 degrees E. In contrast, the orientations of fractures and subsidiary faults in the walls of a trench across the main surface trace of the Asano Fault are constrained to N0-40 degrees E, averaging N20 degrees E, parallel to the general trend of the Asano Fault but different with that observed in the deep holes at the Ogura site. Structural analysis shows that the fractures and subsidiary faults are mainly concentrated in Riedel Y and R shears of the right-lateral strike-slip Nojima Fault and its branch Asano Fault. Our findings demonstrate that the fractures and subsidiary faults within the Nojima Fault Zone were formed by dextral strike-slip shearing under the current ENE-WSW to E-W compressive regional tectonic stress field, related to ongoing subduction of the Pacific and Philippine Sea plates beneath the Japanese Islands.