High stresses stored in fault zones: example of the Nojima fault (Japan)

Abstract. During the last decade pulverized rocks have been described on outcrops along large active faults and attributed to damage related to a propagating seismic rupture front. Questions remain on the maximal lateral distance from the fault plane and maximal depth for dynamic damage to be imprinted in rocks. In order to document these questions, a core sample of granodiorite located at 51.3 m from the Nojima fault (Japan) that was drilled after the Hyogo-ken Nanbu (Kobe) earthquake is studied by using Electron Back-Scattered Diffraction (EBSD) and high resolution X-ray Laue microdiffraction. Although located outside of the Nojima damage fault zone and macroscopically undeformed, the sample shows pervasive microfractures and local fragmentation that are attributed to the first stage of seismic activity along the Nojima fault characterized by laumontite as the main sealing mineral. EBSD mapping was used in order to characterize the crystallographic orientation and deformation microstructures in the sample, and X-ray microdiffraction to measure elastic strain and residual stresses in a quartz grain. Both methods give consistent results on the crystallographic orientation and show small and short wave-length misorientations associated with laumontite-sealed microfractures and alignments of tiny fluid inclusions. Deformation microstructures in quartz are symptomatic of the semi-brittle faulting regime, in which low temperature brittle, plastic deformation and stress-driven dissolution-deposition processes occur conjointly. Residual stresses are calculated from elastic strain measured by X-ray Laue microdiffraction and stress peaks at 100 MPa (mean 141 MPa). Such stress values are comparable to the peak strength of a damaged granodiorite from the damage zone of the Nojima fault indicating that, although apparently macroscopically undeformed, the sample is actually highly damaged. The homogeneously distributed microfracturing of quartz is the microscopically visible imprint of this damage and suggests that high stresses were stored in the whole sample and not only concentrated on some crystal defects. It is proposed that the high residual stresses are the sum of the stress fields associated with individual dislocations, dislocation microstructures and originated from the dynamic damage related to the propagation of rupture fronts or seismic waves associated to M6 to M7 earthquakes during Paleocene on the Nojima fault at a 3.7–11.1 km depth (laumontite stability domain) where confining pressure prevented pulverization. The high residual stresses and the deformation microstructures would have contributed to the widening of the damaged fault zone with additional large earthquakes occurring on the Nojima fault.