Porosity network of the Nojima fault zone in the Hirabayashi Hole (Japan)
Abstract
The GSJ Hirabayashi hole in Japan crosses an important fault zone at 625m depth. The Nojima fault zone is one of the major fault that implied the "Kobe" Earthquake (M= 7.2) in 1995. The host rock is highly fractured and altered and the lithology is mostly constituted by granodiorite. The structure of the fault corresponds to the model described by Caine et al., 1996, with a fault core, a damage zone and a protolith. The aim of this study was to characterise the structure of the porous media on cores to determine the evolution of porosity and the threshold repartition by mercury injection porosimetry and the mineralogical behaviour using STEM observations and X-ray diffractions analysis on clay fraction. The results showed a variation of porosity in a range of 0.2 to 5.7 %. The pore threshold (0.001µm and 5 µm) and the void shape varied with alteration and fracturing intensity. Secondary minerals occured within the matrix and along the fractures and cracks. We saw swelling clays in the matrix of the damage zone but Fukushi et al., 1999 observed the disappearance of these swelling clays in the fault core. Computered X-ray microtomography were performed on different types of material. We looked at the possible connectivity between the fracture network and the matrix porosity around these fractures. We assumed that porosity modifications in the damage zone were induced by secondary mineral formation due to fluid circulation in the fault during the deformation. These mineralogical modifications and particularly the swelling clays formation were used to emphase the fluid pressure variations. The increase of fluid pressure in the system could then reach the rupture. Interactions between mineralogical and porosity behaviour during deformation has to be taken into account to understand the mechanism of deformation in the presence of fluid. Each part of the fault zone acts as a particular pathway for fluid.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....12946S