Fault rocks and past to recent fluid characteristics from the borehole survey of the Nojima fault ruptured in the 1995 Kobe earthquake, southwest Japan

Abstract

The mineralogy, fluid inclusions, and distribution of fault rocks of the Nojima fault were examined in the core recovered from a borehole drilled by the Geological Survey of Japan (GSJ) 12 months after the 1995 Kobe (Hyogo‐ken Nanbu) earthquake (MJMA = 7.2) in southwest Japan. The borehole was drilled across a slipped portion of the fault to a depth of 746.7 m. Nearly continuous coring between 152.2 and 746.7 m recovered granodiorite protolith, porphyry dikes, and fault‐related rocks. The fault zone was intersected at 426.2 m and is characterized by a greater intensity of brittle deformation and/or hydrothermal alteration than typical host granodiorite. The fault core consists of three types of fault gouge and occurs at the depth range of 623.1 to 625.3 m. The fault‐normal thicknesses of the fault core and the fault zone are 0.3 m and >46.5 m, respectively. Three types of hydrothermal alteration are recorded by mineral assemblages and fluid inclusions. The first type is characterized by chloritization of mafic minerals at >200°C and occurred prior to the fault activity during the intrusion and cooling of the granodiorite. The second type occurred during faulting and is recorded by zeolite mineralization at <200°C. The third type is recorded by carbonate mineralization related to recent fluid flow. Although most of the second type of alteration occurred prior to the third type, repeated mineralization is recorded by mutually crosscutting relationships between zeolite and carbonate veins and between zeolite vein and carbonate‐precipitated fault gouge. This may record repeated changes in fluid chemistry within the fault zone in connection with the seismic cycle. Although the Nojima fault slipped in the 1995 earthquake, ancient fault‐related textures and mineral alteration are well preserved in the fault rocks.