Electric and magnetic anomalies at the Atotsugawa fault and their implications for fault activity.

Ohshiman, N.; Honkura, Yoshimori; Kuge, Keiko; Sakai, Hideo

doi:10.5636/jgg.39.143

“…In magnetic field studies (Won 1981), the highly magnetized dyke-like body is assumed to be a source of regional contrast of the geomagnetic field along some active fault Isikara et al 1985;Oshiman et al 1987Oshiman et al , 1991. Isikara et al (1985) and Oshiman et al (1991) used 1.73 ¥ 10 -2 (SI unit volume) as the model susceptibility value of its dyke-like body.…”

Section: Discussionmentioning

confidence: 99%

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

Nakamura¹,

Nagahama²

2001

Isl Arc

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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.

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“…In magnetic field studies (Won 1981), the highly magnetized dyke‐like body is assumed to be a source of regional contrast of the geomagnetic field along some active fault (Honkura et al 1985; Isikara et al 1985; Oshiman et al 1987, 1991). Isikara et al (1985) and Oshiman et al (1991) used 1.73 × 10 −2 (SI unit volume) as the model susceptibility value of its dyke‐like body.…”

Section: Discussionmentioning

confidence: 99%

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

Nakamura

¹

,

Nagahama²

2001

View full text Add to dashboard Cite

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.

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“…Meanwhile, research on the electromagnetic structure of the Nojima fault is just the beginning for us to reach our goal of whole understandings. Although information on the electromagnetic structure of the Nojima fault obtained so far is limited to the shallow portion of the earth's crust, it still gives us an important suggestion as to the existence of a low-resistivity zone, namely a fracture zone, along the fault trace, as revealed by previous electromagnetic research for active faults (e.g., Electromagnetic Research Group for the Active Fault, 1982;Ohshiman et al, 1987).…”

Section: Discussion and Concluding Remarksmentioning

confidence: 99%

Tectonoelectric Signal Related with the Occurrence of the 1995 Hyogo-ken Nanbu Earthquake (M 7.2) and Preliminary Results of Electromagnetic Observation around the Focal Area.

for¹

1997

J,Phys,Earth

16

0

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Tectonoelectric Signal Related with the Occurrence of the 1995 Hyogo-ken Nanbu Earthquake (M 7.2) and Preliminary Results of Electromagnetic Observation around the Focal Area Electromagnetic Research Group for the 1995 Hyogo-ken Nanbu Earthquake* In association with the 1995 Hyogo-ken Nanbu earthquake, we observed changes in the telluric field by about 10 mV/km about 10 s after the origin time with some long-span electrical dipole network located at about 70 km from the epicenter. After the occurrence of the earthquake, electric and magnetic measurements such as DC resistivity, VLF-MT, self-potential, and the geomagnetic total intensity across the Nojima fault were made at several places in Awaji Island. The low resistive zone beneath the western side of the fault was found in the derived resistivity structure of the shallower portion of the fault in Hirabayashi. On the other hand, however, no significant anomaly associated with the fault structure was found from the measurements of total intensity and self-potential. Prior to the occurrence of the main shock, no geomagnetic or electric continuous stations existed near the focal area. Approximately within 1 week after the occurrence of the main shock, we set up 10 continuous observation sites for the geomagnetic total intensity in the focal area in order to detect geomagnetic changes due to stress change caused by the aftershock activity. No changes in the total intensity in association with the individual aftershock activity have been observed so far. However, systematic temporal trends in the geomagnetic differences have been observed, especially at sites in the northern half of the observation area.

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Electric and magnetic anomalies at the Atotsugawa fault and their implications for fault activity.

Cited by 9 publications

References 11 publications

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

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

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

Tectonoelectric Signal Related with the Occurrence of the 1995 Hyogo-ken Nanbu Earthquake (M 7.2) and Preliminary Results of Electromagnetic Observation around the Focal Area.

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