3:30 PM - 4:30 PM
[J06-P-19] Drilling into Active Faults - In-situ Investigations on the Mechanics and Structure of Faults in Central Japan
The drilling is of great advantage to study on the mechanics and structure of fault. We did integrated investigations on active faults in central Japan by drilling into the faults, in-situ downhole measurements and core observations. Those faults have different elapsed time since the last earthquakes; Nojima fault, the 1995 Hyogo-ken Nanbu Earthquake, M7.2 (Kobe Earthquake); the Neodani fault, 1891 Nobi earthquake (M=8.0); the Atera fault, 1586 Tensho earthquake (M=7.9); the Atotsugawa fault, 1858 Hida earthquake (M=7.0); the Gofukuji Fault that is said to have activated more than a thousand years ago.
In-situ stress measurements indicate the maximum horizontal compressive stress was nearly perpendicular to the fault strikes as for Nojima Fault case, but for other fault cases, the orientation of the maximum horizontal compressive stress was oblique to the fault strike. These results support the idea that the differential stress is small at narrow zone adjoining fracture zone and fault is quite weak after the earthquake. On the other hand, the frictional strength is still high outside the narrow fractured zone. The shear stress is as the same level as the frictional strength of host rock of the fault. Downhole logging and macro- and micro-scopic observation of recovered cores indicate complicated and internally hierarchical fault zone structure. Sets of narrow hardly fractured zones with adjacent broad weakly fractured zones are distributed within the fracture zone. The narrow hardly fractured zones are characterized as low electrical resistance, low density, low P-wave velocity, and high porosity. Fault seems not to have slipped on a specific plane, but on several fault planes at different earthquakes in the past. The complicated fault zone structure should have formed progressively during repeating earthquake. The stress state and structure of different faults seem to reflect different stage in earthquake preparing process in the earthquake recurrence cycle.
In-situ stress measurements indicate the maximum horizontal compressive stress was nearly perpendicular to the fault strikes as for Nojima Fault case, but for other fault cases, the orientation of the maximum horizontal compressive stress was oblique to the fault strike. These results support the idea that the differential stress is small at narrow zone adjoining fracture zone and fault is quite weak after the earthquake. On the other hand, the frictional strength is still high outside the narrow fractured zone. The shear stress is as the same level as the frictional strength of host rock of the fault. Downhole logging and macro- and micro-scopic observation of recovered cores indicate complicated and internally hierarchical fault zone structure. Sets of narrow hardly fractured zones with adjacent broad weakly fractured zones are distributed within the fracture zone. The narrow hardly fractured zones are characterized as low electrical resistance, low density, low P-wave velocity, and high porosity. Fault seems not to have slipped on a specific plane, but on several fault planes at different earthquakes in the past. The complicated fault zone structure should have formed progressively during repeating earthquake. The stress state and structure of different faults seem to reflect different stage in earthquake preparing process in the earthquake recurrence cycle.