Elastic property of damaged zone inferred from in-situ stresses and its role on the shear strength of faults

Abstract

The Nojima fault in Hyogo prefecture, Japan, ruptured during the 1995 Hyogo-ken Nanbu earthquake (M JMA = 7.3). The stress measurements at sites close to this fault have revealed that the direction of the largest horizontal stress is almost perpendicular to the strike of this sub-vertical fault and that, in the zone within about 100 m from the fault core axis, the ratio of the largest shear stress to the normal stress is significantly small compared with that of the outside. It is thus the logical consequence that the principal stress outside the zone tends to direct perpendicularly to the fault plane. A model called the fracture process model is introduced for the relationship between fracture strength and elastic property of rocks. Making use of this model on the assumption that the observed shear stress equilibrates to the shear strength of damaged zone, it is found that the elastic wave velocities estimated from the stress well explain the observed velocities of damaged zone. This model suggests further that the friction coefficient of fault can be smaller than 0.15 due to the characteristic deformation of damaged zone and that the pressurized fluid is not essential for the formation of weak faults.

Stress rotations and the long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment

“…With those slip vectors, the high angle of S H max imposes μ ≤ 0.09 along the Median Tectonic Line and μ ≤ 0.15 along the Nojima fault (Appendix A). Regarding the Nojima fault, our results are in striking agreement with the long‐term friction coefficient ( μ ≤ 0.15) calculated by Yamamoto et al [] from a compilation of stress memory analyses in core samples from three boreholes in the vicinity of the Rokko‐Awaji Segment.…”

“…Deformation structures include small‐scale striated faults (<2 m in length) or extension fractures (<10 cm) occasionally filled with syndeformation minerals (calcite ± quartz ± oxides), dikes, fold axes, and tilted beddings. For the Nojima area of the Rokko‐Awaji Segment, the data set also includes published long‐term stress orientation measurements from the analysis of microcracks [ Takeshita and Yagi , ] and stress memory in core samples [ Yamamoto and Yabe , ; Yamamoto et al, ] but does not include the distribution of fractures deduced from borehole logging [ Ito and Kiguchi , ] because the kinematics of these fractures is not known. Microstructures and mesostructures were measured at various distances of the faults from almost intact rocks (hereafter called the far field), to strongly deformed and altered rocks interpreted as the damaged zones, and even to the pseudotachylite bearing core zone of the Nojima fault [ Lin et al, ; Ohtani et al, ; Otsuki et al, ; Tanaka et al, 2001 , 2007].…”

Fluid pressure waves trigger earthquakes

“…Indeed, the issue of a low static friction coefficient at the onset of earthquake slip is per se a matter of high interest, and is the object of a separate study, currently in preparation. Suffice here to say that our assumption of a static friction coefficient of the order of 0.3 (or less) is corroborated by very consistent field evidence (Zoback et al 1987;Raesenberg & Simpson 1992;Iio 1997;Yamamoto & Yabe 2001;Yamamoto et al 2002;Kubo & Fukuyama 2004).…”

Focal mechanisms and stress field in the Atotsugawa fault area, central Honshu, Japan

“…Hardebeck and Michael (2004) proposed an extreme model in which all major active faults are weak. Yamamoto et al (2002) made stress measurements using borehole cores sampled at sites close to the Nojima fault that ruptured during the 1995 Hyogo-ken Nanbu earthquake (M JMA = 7.3). Since the direction of the largest horizontal stress is almost perpendicular to the strike of fault, they interpreted Nojima fault as weak fault.…”