Changes in magnetic and fractal properties of fractured granites near the Nojima Fault, Japan

Abstract

Anisotropy of magnetic susceptibility (AMS) has been used to infer finite strain fabrics in plastically deformed rocks, but there are few studies of magnetic properties in fractured fault rocks. Changes in magnetic and fractal properties of fractured granites from the Disaster Prevention Research Institute, Kyoto University (DPRI) 500 m drilling core towards the Nojima Fault and of the well-foliated fault gouge are described. Fractal analysis of fractured granites shows that the fractal dimension (D) increases linearly toward the gouge zone of the fault. In weakly fractured granites (D = 1.05-1.24), it was found that the degree of AMS correlates positively with the fractal dimension, suggesting a fracture-related magnetic fabric due to fracturing. In strongly fractured granites (D = 1.25-1.50), weaker, nearly isotropic AMS is found, suggesting erasure by the fragmentation of the magnetic minerals. Within the fault gouge zone, an isotropic AMS fabric was found, as well as twofold increases in magnetic intensity and susceptibility. These changes reflect the production of new magnetite grains, subsequently confirmed by hysteresis studies, which suggests that fault gouge might be regarded as the source of the regional geomagnetic field contrast along active faults. Thus, AMS is clearly a potentially useful tool for inferring the fracturing texture of magnetic minerals in fractured rocks and detecting active faults from the high susceptibility contrast of fault gouge.

ESR and ICP analyses of the DPRI 500 m drill core samples penetrating through the Nojima Fault, Japan

“…As the signal intensity of Fe 3+ is proportional to the number of unpaired electrons, its increase indicates increased magnetic susceptibility. Nakamura and Nagahama (2001) report that the fault gouge in the Nojima 500 m core samples has magnetic susceptibilities 30 times as high as the source rock, a finding that is consistent with the ESR data reported in the present study.…”

Quantitative modeling of the newly formed magnetic minerals in the fault gouge of 1999 Chi‐Chi earthquake (Mw 7.6), Taiwan

“…The euhedral shape of goethite is a clear indication of late growth within the gouge (supporting information, Figure S1 and Movie S1). A similar observation of neoformed goethite has been reported by Nakamura and Nagahama [2001] in the Nojima fault, Japan. Of particular interest, the goethite carries a large portion of the characteristic remanent magnetization (ChRM).…”

Fractal particle size distribution of pulverized fault rocks as a function of distance from the fault core

“…Equation (7) describes the relationship between the fractal dimension and distance from the fault core. A similar linear relationship has been found in the fractal analysis of natural and experimentally created fracture systems [Udagawa, 1999;Nakamura and Nagahama, 2001;Yamaki et al, 2013]. Equation (7) indicates a linear decrease in D s as a function of distance based on energy considerations, which is consistent with Figure 5.…”