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

The mineralogy, fluid inclusions, and distribution of fault rocks of the No jima fault were examined in the core recovered from a borehole drilled by t he Geological Survey of Japan (GSJ) 12 months after the 1995 Kobe (Hyogo-ke n Nanbu) earthquake (M-JMA = 7.2) in southwest Japan. The borehole was dril led across a slipped portion of the fault to a depth of 746.7 m. Nearly con tinuous 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 co re 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 fa ult zone are 0.3 m and >46.5 m, respectively. Three types of hydrothermal a lteration are recorded by mineral assemblages and fluid inclusions. The fir st type is characterized by chloritization of mafic minerals at >200 degree s C and occurred prior to the fault activity during the intrusion and cooli ng of the granodiorite. The second type occurred during faulting and is rec orded by zeolite mineralization at <200 degrees C. The third type is record ed 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-precipit ated fault gouge. This may record repeated changes in fluid chemistry withi n the fault zone in connection with the seismic cycle. Although the Nojima fault slipped in the 1995 earthquake, ancient fault-related textures and mi neral alteration are well preserved in the fault rocks.