[S-SS06] Fault Rheology and Earthquake Physics

[SSS06-13] Elucidation of the influence of the surface roughness of the host rock on the structure and mechanics of faults

*Momoyo FUKUDA¹, Kiyokazu Oohashi¹, Takehiro Hirose² (1.Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 2.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology)

Keywords:fault

It is known that the structure and mechanics of faulting are inseparable (Faulkner et al., 2010), and the relationship between mechanical data and fault structure has been discussed from comparable friction experiments. In particular, when focusing on the deformation concentration process and fault strength, it has been revealed that the weak minerals in the fault gouge form a connected structure, and when the deformation is localized, the fault strength is weakened (Collettini et al., 2009; Oohashi et al., 2013).It is known that natural fault zones have widths regardless of scale (Choi et al., 2016). Childs et al. (2009) revealed a positive correlation between cumulative displacement of various faults and width of fault zones, and that there is up to a three-digit difference in the same displacement for fault zone widths. With this, a hypothesis can be proposed that low-intensity faults, which have difficulty in widening, and high-intensity faults, which easily widen, lead to the diversity of fault zone widths. However, the process of formation including the mechanical data has not been elucidated with respect to the wide fault zone. Hence, the main purpose of this research is to clarify the factors which determine the ease and difficulty of the deformation concentration zone's spread and the influence of the width and formation process of concentrated deformation zone on the strength of the fault, and after conducting experiments on the quartz-graphite mixture powder and gabbro or sandstone cylinders, the surface roughness of the cylinders was measured using a confocal microscope.
As a result of experiments using sandstone cylinders, the friction coefficient was significantly higher than that of gabbro and a thick deformation zone composed of fine quartz was formed. Experiments in which the surface roughness of the sandstone cylinders was changed before the experiment revealed a tendency for the width of the deformation concentration zone to increase as the surface roughness of the pre-experiment becomes rougher. The results from the surface roughness measurement of the cylinder showed that the RMS of the sandstone cylinder was approximately two times higher than that of the gabbro cylinder throughout the experiment. This is due to the fact that the quartz contained in the sandstone has a high wear resistance, forming convex surfaces, while the relatively low wear resistance of the feldspar causes it to be worn away and form concave surfaces. SEM observation of the cylinder surface showed that graphite was stretched over the colored minerals in gabbro, forming a smearing that was identified as a weak sliding surface, which could be the cause of weakening and inhibiting the formation of force chains inside the gouge, thus explaining the weak deformation inside the gouge. On the other hand, the sandstone cylinder had rough surface during experiments, suggesting that the force chains were effectively formed inside the gouge. By this, the crushing of gouge proceeded and a deformation concentration zone was formed, however, no smearing of graphite or concentration on the slip plane occurred, which is thought to be the cause of maintaining a high friction coefficient. From these results, in the case of natural faults, if the surface roughness of the parent rock (i.e. the unevenness of the boundary between the fault zone and the protolith) is rough, the strength of the fault is high and it is likely that the width of deformation concentration zones such as gouge zones and cataclastic zones will be wide.