Distribution, deformation and alteration of fault rocks along the GSJ core penetrating the Nojima Fault, Awaji Island, Southwest Japan.

秀実, 田中; 孝幸, 樋口; 直人, 冨田; 光一郎, 藤本; 具幸, 大谷; 久男, 伊藤

doi:10.5575/geosoc.105.72

Cited by 18 publications

(32 citation statements)

References 4 publications

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“…Here, the terminology by Chester et al (1993) or Tanaka et al (1999) is employed for fault structure as a rule. In the case of the Nojima fault, Ito et al (1996) and Tanaka et al respectively have pointed out from the geophysical and the geological data that there are damaged rocks of a few tens meter width a side along the fault core axis.…”

Section: Definition Of Damaged Zonementioning

confidence: 99%

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

2014

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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.

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Section: Definition Of Damaged Zonementioning

confidence: 99%

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

2014

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“…(b) Distribution of fault rocks and sample localities of the Geological Survey of Japan 750 m borehole. Rock type after Tanaka et al [1999].…”

Section: Geological Outline and Samplesmentioning

confidence: 99%

Ancient thermal anomaly of an active fault system: Zircon fission‐track evidence from Nojima GSJ 750 m borehole samples

Murakami

Tagami

Hasebe

2002

Geophysical Research Letters

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To understand better heat generation and transfer along faults, fission track (FT) thermochronologic analysis was performed on zircon separates from Cretaceous granitic rocks around the Nojima fault, activated during the 1995 Kobe earthquake. Samples were collected from the Geological Survey of Japan (GSJ) 750 m borehole and nearby outcrop. Zircon FTs showed reduced mean lengths and ages for samples less than ∼10 m away from the fault at depth. This is interpreted as representing an ancient thermal anomaly by heat transfer or dispersion via fluids at the deep crustal interior because the degree of FT annealing is correlated to that of deformation/alteration of borehole rocks. From extrapolation of data in the mean FT age‐length plot as well as from model age calculation, it is suggested that last cooling took place at ∼30–40 Ma.

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“…The characteristics of deformation and alteration mentioned earlier are not always recognized simultaneously. For example, the content of brown feldspar varies among samples and it tends to increase towards the veins (Tanaka et al 1999).…”

Section: Weakly Deformed and Altered Granodioritementioning

confidence: 99%

“…We report the characteristics of the fault rocks based on their textural analysis and the fault zone structures of the Nojima Fault along the Hirabayashi GSJ borehole. The description of fault rocks from depths between 553.4 m and 718.2 m along the Hirabayashi GSJ borehole is given in Tanaka et al (1999), and the mineralogy and the rock chemistry in the fault zone is discussed in the paper by Fujimoto et al (2001). In the present paper, the terminology of the fault rocks is based on that of Sibson (1977).…”

Section: Introductionmentioning

confidence: 99%

Internal structure of the Nojima Fault zone from the Hirabayashi GSJ drill core

Ohtani¹,

Tanaka²,

Fujimoto³

et al. 2001

Isl Arc

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The internal structures of the Nojima Fault, south-west Japan, are examined from mesoscopic observations of continuous core samples from the Hirabayashi Geological Survey of Japan (GSJ) drilling. The drilling penetrated the central part of the Nojima Fault, which ruptured during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake) (M7.2). It intersected a 0.3 m-thick layer of fault gouge, which is presumed to constitute the fault core (defined as a narrow zone of extremely concentrated deformation) of the Nojima Fault Zone. The rocks obtained from the Hirabayashi GSJ drilling were divided into five types based on the intensities of deformation and alteration: host rock, weakly deformed and altered granodiorite, fault breccia, cataclasite, and fault gouge. Weakly deformed and altered granodiorite is distributed widely in the fault zone. Fault breccia appears mostly just above the fault core. Cataclasite is distributed mainly in a narrow (ª1 m wide) zone in between the fault core and a smaller gouge zone encountered lower down from the drilling. Fault gouge in the fault core is divided into three types based on their color and textures. From their cross-cutting relationships and vein development, the lowest fault gouge in the fault core is judged to be newer than the other two. The fault zone characterized by the deformation and alteration is assumed to be deeper than 426.2 m and its net thickness is > 46.5 m. The fault rocks in the hanging wall (above the fault core) are deformed and altered more intensely than those in the footwall (below the fault core). Furthermore, the intensities of deformation and alteration increase progressively towards the fault core in the hanging wall, but not in the footwall. The difference in the fault rock distribution between the hanging wall and the footwall might be related to the offset of the Nojima Fault and/or the asymmetrical ground motion during earthquakes.

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Distribution, deformation and alteration of fault rocks along the GSJ core penetrating the Nojima Fault, Awaji Island, Southwest Japan.

Cited by 18 publications

References 4 publications

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

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

Ancient thermal anomaly of an active fault system: Zircon fission‐track evidence from Nojima GSJ 750 m borehole samples

Internal structure of the Nojima Fault zone from the Hirabayashi GSJ drill core

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