Internal structure and permeability of the Nojima fault, southwest Japan

Abstract

We conducted permeability measurements on representative fault rocks and surrounding country rocks taken from the Nojima fault zone, which was activated during the 1995 Kobe earthquake, under isotropic confining pressures up to 180 MPa. The results show that the Nojima fault zone consists of a low-permeability fault gouge zone (10 −20–10 −19 m 2 at 180 MPa) within a high-permeability damaged zone of fault breccia and fractured host rock (10 −18–10 −14 m 2 at 180 MPa). The fault gouge zone acts as a barrier to fluid flow across the fault, whereas the surrounding damage zone acts as a fluid conduit. The nature of this proposed permeability structure is consistent with the results of tests conducted on drillcore samples collected from the Nojima fault at depths of 0.6 and 1.8 km. We therefore propose that the permeability of a fault measured from fault rocks exposed at the surface can be used as a representative value for the fault to depths of up to 2 km. We also examined the possibility that thermal pressurization occurred upon the Nojima fault during the Kobe earthquake, based on the obtained permeability data. We found that frictional heating during the Kobe earthquake would have led to an increase in pore pressure at depths below 4 km, thereby resulting in a marked reduction in frictional resistance upon the fault.