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

Abstract

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 15 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 20show 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 25 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 30 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 Solid Earth Discuss., https://doi

High‐Angular Resolution Electron Backscatter Diffraction as a New Tool for Mapping Lattice Distortion in Geological Minerals

“…Interpretation of the geological processes recorded in residual stress fields has been hindered by the challenge of measuring elastic strains and residual stresses at the grain scale. Techniques that are currently employed include Raman spectroscopy (Kohn, 2014) and X-ray Laue microdiffraction (Chen et al, 2015(Chen et al, , 2016Boullier et al, 2017). Another technique that could potentially be employed is convergent beam electron diffraction in the transmission electron microscope (Champness, 1987).…”

High-angular resolution electron backscatter diffraction as a new tool for mapping lattice distortion in geological minerals

“…Interpretation of the geological processes recorded in residual stress fields has been hindered by the challenge of measuring elastic strains and residual stresses at the grain scale. Techniques that are currently employed include Raman spectroscopy (Kohn, 2014) and X-ray Laue microdiffraction (Boullier et al, 2017;Chen et al, 2015Chen et al, , 2016. Another technique that could potentially be employed is convergent beam electron diffraction in the transmission electron microscope (Champness, 1987).…”