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 ~260 to 900 m in nine drill holes that intersected the Nojima Fault (NF), on which the 1995 Mw 6.9 Kobe (Japan) earthquake occurred. Logging data and an analysis of mesostructures and microstructures in drill cores show that (i) a ~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. Plain language Summary Drilling investigations and structural analyses of drill cores reveal that a ~60‐m wide fault damage zone containing a 10‐ to 30‐cm‐thick fault gouge zone developed along the Nojima Fault (NF), on which the 1995 Mw 6.9 Kobe (Japan) earthquake occurred. The fault gouge zone was observed at depths of ~260 to 900 m in nine drill holes that intersected the NF. Our findings show that (i) the fault gouge zone observed at different depths in the nine drill cores is the principal fault slip zone of the NF, and (ii) the individual colored layers of the fault gouge zone provide a record of repeated seismic slip events within the NF and may therefore represent earthquake fossils, as do pseudotachylyte veins and fault scarps that record the systematic offset of valleys and terrace risers along the NF during the late Pleistocene‐Holocene. Key Points Drilling investigations were carried out on the Nojima Fault that triggered the 1995 Mw 6.9 Kobe (Japan) earthquake A 10‐ to 30‐cm‐thick fault gouge zone was observed in nine drill holes at depths of 260‐900 m, containing 11‐20 thin layers of different colors 11‐20 thin layers of the gouge zone probably records more than 11‐20 seismic faulting events during the late Pleistocene‐Holocene