Repeated Seismic Slipping Events Recorded in a Fault Gouge Zone: Evidence From the Nojima Fault Drill Holes, SW Japan

Drilling investigations and structural analysis of drill cores reveal that a fault gouge zone of 10-30 cm in width was observed at depths of similar to 260 to 900 m in nine drill holes that intersected the Nojima Fault (NF), on which the 1995 M-w 6.9 Kobe (Japan) earthquake occurred. Logging data and an analysis of mesostructures and microstructures in drill cores show that (i) a similar to 60-m wide fault damage zone containing a 10- to 30-cm-thick fault gouge zone developed in the NF, (ii) the fault gouge zone can be divided into 11-20 thin layers of different color, and (iii) the individually colored layers contain different color breccias of fault gouge that are offset and/or cut by cracks and crack-filled calcite and quartz veinlets. Our results reveal that the fault gouge zone probably records more than 11-20 paleoseismic faulting events along the NF during the late Pleistocene-Holocene.