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

Abstract

The size distributions of particle in pulverized rocks from the San Andreas fault and the Arima‐Takatsuki Tectonic Line were measured. The rocks are characterized by the development of opening mode fractures with an apparent lack of shear. Fragments in the rocks in both fault zones show a fractal size distribution down to the micron scale. Fractal dimensions, dependent on mineral type, decrease from 2.92 to 1.97 with increasing distance normal to the fault core. The fractal dimensions of the rocks are higher than those of both natural and experimentally created fault gouges measured in previous studies. Moreover, the dimensions are higher than the theoretically estimated upper fractal limit under confined comminution. Dimensions close to 3.0 have been reported in impact loading experiments. The observed characteristics indicate that pulverization is likely to have occurred by a dynamic stress pulse with instantaneous volumetric expansion, possibly during seismic rupture propagation similar to impact loading.

The Effects of Earthquakes and Fluids on the Metamorphism of the Lower Continental Crust

“…The same applies to clinopyroxene grains (Petley‐Ragan et al, ). In this case the mean fragment size (for garnet) is 160 μm 2 , and the power law distribution for both garnet and pyroxene fragments has a slope of ~ −2 (Figure e), similar to the scaling behavior previously reported from pulverized wall rocks close to the fault core of shallow seismic faults (Muto et al, ). Inclusions in this garnet are concentrated along brittle fractures, some of which extend through a major fraction of the garnet crystal (Figures c and d).…”

“…These fragments fit a power law grain size (area) distribution with slope −2.0 over more than 4 orders of magnitude (Figure b). This is identical to the scaling behavior of the fragmented (“pulverized”) garnet from both the asymmetrical damage zone described in Figure c and from pulverized rocks around faults in the normal seismogenic regime (Muto et al, ). The garnet grain in the lower wall rock of Figure b has few internal misorientations but remains intact as a single grain (Figure c).…”

Microstructural evidence for the deep pulverization in a lower crustal meta‐anorthosite

“…Figure b shows the relationship between the cumulative number N ( d ) and the grain size d ; it can be fitted by the power‐law equation N ( d ) ∝ d ‐ D , and the resulting fractal dimension is D = 1.02 ± 0.02. The value of the fractal dimension is smaller than those ( D = 1.68–2.35) reported from the natural fault rocks (Muto, Nakatani, Nishikawa, & Nagahama, ) and similar to those ( D = 0.9–1.1) for small grains with the particle seize of <~2 μm from the deformation experiments (Keulen, Heilbronner, Stünitz, Boullier, & Ito, ). The low value of the fractal dimension may be caused by metamorphic recrystallization or grain growth via absorption of small grains and growth of large grains.…”

Variation of Hydraulic Properties Due to Dynamic Fracture Damage: Implications for Fault Zones

“…Separating the coseismic dynamic fracture contribution from the quasi‐static one is challenging, except when pulverized damage zone rock is present. The key characteristic of pulverized damage zone rock is their lack of shear or rotation of fragments—leading to preservation of its original texture (Dor et al, ; Mitchell et al, ), while they are fragmented down to the micron scale (Dor et al, ; Muto et al, ) and they disintegrate upon light touch (Dor et al, ; Mitchell et al, ). High strain rate loading experiments in compression recreated pulverization textures above a strain rate threshold of about 200 s −1 , whereby the lack of shear is explained by a short loading duration (Doan & Gary, ).…”