Logging in a fault fracture zone -- A case study at Hirabayashi NIED borehole drilled through Nojima fault

摘要

The Hyogo-ken Nanbu earthquake (1995.1, M_(JMA) = 7.2) activated the Nojima fault in the northern part of Awaji Island, southwest Japan and a surface rupture appeared more than 10 km long. After the earthquake, the Hirabayashi NIED borehole was drilled penetrating through the fault zone to a depth of 1838 m from a point about 302 m SE from the surface trace of the Nojima fault. In the borehole, physical well logging was done from 251 m depth down to the bottom. Cores were recovered from 1000 m depth with an almost 100 percent recovery rate. In the cores, remarkable fractured zones containing cataclastic rocks were found at three depths, around 1140 m, 1300 m and 1800 m. Results of well logging show that, in the depth interval of host granitic rocks, normal resistivity is from several hundreds to several thousands ohm centre dot m, micro resistivity is several tens ohm centre dot m, P wave velocity is 5-6 km/sec, density is about 2.6 g/cm~3 and neutron porosity is several percent. On the other hand, in the depth interval of the fracture zone, those properties decrease down to several tens ohm centre dot m, several ohm centre dot m, 2-4 km/sec, 1.5-2.0 g/cm~3 and increase up to several tens percent, respectively. Investigating correlations between physical properties measured by well logging, characteristic features are indicated in the fracture zones In the fracture zones, neutron porosity is beyond 10 percent and P wave velocity is less than 5 km/sec, the density decrease more steeply and normal resistivity in logarithmic scale increases more gently with the increase of neutron porosity than in the depth interval of host rocks. The P wave velocity does not have a good correlation with the neutron porosity and the normal resistivity does not obey the Archie's relation in the fracture zones. The fracture zone at a depth of 1800 m indicates some different characters from the other two fracture zones; P wave velocity, density and neutron porosity appear to change gently contrasting to a remarkable decrease of normal resistivity. In the shallower fracture zones, all of those properties change remarkably. The difference between fracture zones may reflect their fracturing history during past earthquake cycles.