Deep structure of the Nagamachi-Rifu fault deduced from small aperture seismic array observations

Imanishi, Kazutoshi; Ito, Hisao; Kuwahara, Yasuto; Mamada, Yutaka; Yokokura, Takanobu; Kano, Naomi; Yamaguchi, Kazuo; Tanaka, Akiko

doi:10.1186/bf03353297

Cited by 9 publications

(12 citation statements)

References 18 publications

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“…Many S-wave reflectors and scatterers, which are considered to be another aspect of fluid reservoirs, have been found beneath the study area (Imanishi et al, 2002;Umino et al, 2002b;Hori et al, 2004), as is shown in Fig. 8.…”

Section: Deep Structure Around the Nagamachi-rifu Faultmentioning

confidence: 94%

“…Focal mechanism of the main shock projected on the lower focal hemisphere (Umino et al, 2002a) Nakajima et al (2001a) from seismic array observations and proposed the possibility that the NRF has a low dip-angle in the mid crust as a detachment fault. Imanishi et al (2002) attempted to deduce the distribution of P-wave scatterers in the region from small aperture seismic array observations and found that the high scatter region spatially correlates with the distribution of microearthquakes, S-wave reflectors, and calderas. Ogawa et al (2004) conducted a MT survey around the fault and detected two low-resistivity bodies: one is located at depths of 10-30 km below the volcanic front, and the other is located in the mid-crust zone below the surface trace of the NRF.…”

Section: Introductionmentioning

confidence: 99%

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Crustal heterogeneity around the Nagamachi-Rifu fault, northeastern Japan, as inferred from travel-time tomography

et al. 2006

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An M=5 earthquake occurred on September 15, 1998, in the Nagamachi-Rifu fault (NRF), northeastern Japan. In the aftermath of this event, many seismograph stations were constructed temporarily around the fault, forming a dense network of stations with a spatial separation of 5 km. We report here our estimation of the threedimensional velocity structures of the P and S waves using arrival-time data recorded at these stations with the aim of understanding the heterogeneous structure around the NRF. Low-Velocity and high Poisson's ratio anomalies are imaged in the lower crust beneath the volcanic area, which are probably associated with the partially molten materials conveyed through the upwelling flow in the mantle wedge. A distinct low-velocity anomaly, which is explainable by the existence of H 2 O-filled pores, is observed in the mid crust at the deeper extension of the NRF. Two low-velocity anomalies that are probably associated with the remnants of magmatic activity that formed the Shirasawa caldera and with the existence of thick late-Cenozoic sedimentary layers are observed at depths shallower than 10 km in the hanging wall of the NRF. Our results successfully characterize the major features of the complex velocity structure around the NRF, with implications for the existence of fluid-rich regions in the mid to lower crust.

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Section: Deep Structure Around the Nagamachi-rifu Faultmentioning

confidence: 94%

Section: Introductionmentioning

confidence: 99%

Crustal heterogeneity around the Nagamachi-Rifu fault, northeastern Japan, as inferred from travel-time tomography

et al. 2006

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“…There has been much research into the mechanism of this earthquake because it occurred in the deepest part of an active fault (the Nagamachi-Rifu fault) underlying the city. The geometry of the fault and the geological structure beneath the fault have been investigated through the analysis of earthquake data from in and around the fault, and by observations of the responses to explosive and vibrator sources at a smallaperture seismic array (e.g., Imanishi et al, 2002). Hori et al (1999) revealed the existence of reflected P×P and S×S waves in the waveforms of aftershocks of the magnitude 5.0 event.…”

Section: Introductionmentioning

confidence: 99%

Spectral matrix analysis for detection of polarized wave arrivals and its application to seismic reflection studies using local earthquake data

Moriya

2009

Earth Planet Sp

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Local earthquakes observed at Sendai, Japan, were analyzed to confirm the validity of a method of polarization analysis using the spectral matrix of seismic wave and its application to seismic reflection studies of the crust using local earthquake data. Reflectors (Bright spots) are known below the Nagamachi-Rifu fault, which caused an M 5.0 class event in 1998. Polarization analysis was applied to earthquake data in and around the fault. Use of the Z -parameter, which is defined using the eigenvalues of the spectral matrix and a statistical value representing the confidence level for the detection of the arrival of polarized waves, allowed detection of linearly and elliptically polarized waves in coda waves. The Z -parameter was also used to image the reflectors by using a migration technique that assumes the P × P and S × S reflection waves travel through a multi-layered velocity structure. Distinct reflectors were detected at depths of around 10 km, 14 km, 17 km, 21-26 km, 35 km and 40 km, that is, from deeper than the fault and the Moho. This study demonstrated the feasibility of using the spectral matrix of three-component seismic signal to detect polarized waves and to image reflectors in the earth's crust and upper mantle.

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“…These have included, crustal movement observations with a dense GPS array (Nishimura et al, 2004), resistivity structure surveys using the magnetoteluric (MT) method (Ogawa et al, , 2003, gravity survey (Komazawa and Mishina, 2002), reflection and refraction seismic surveys (Sato et al, 2002;Umino et al, 2002a;Nakamura et al, 2002;Imanishi et al, 2002), seismic observations (Nakajima et al, 2004), and receiver function analyses (Yoshimoto et al, 2001). Utilizing these observational results, we constructed a first-order approximation model of the stress accumulation process of the Nagamachi-Rifu Fault Zone (NRFZ) with the finite element method.…”

Section: Introductionmentioning

confidence: 99%

A comprehensive model of the deformation process in the Nagamachi-Rifu Fault Zone

et al. 2014

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A two-dimensional finite element model was constructed along a cross section almost perpendicular to the Nagamachi-Rifu Fault Zone, in order to clarify the stress accumulation process on an intraplate earthquake fault. We explain the surface deformations observed by the dense GPS network and leveling surveys using models with heterogeneities in the crust. These heterogeneities are identified from various geophysical surveys in the region. We found that the observed surface deformations cannot be explained by a model having a weak zone in the upper crust, but can be explained by models having a weak zone in the lower crust. Models having an aseismic fault or fault zone in the lower crust can reproduce the spatial pattern of the observed deformations, but amplitudes predicted by these models are smaller than those observed. The weak zone in the lower crust probably plays an important role in the stress accumulation process on the Nagamachi-Rifu fault zone.

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Deep structure of the Nagamachi-Rifu fault deduced from small aperture seismic array observations

Cited by 9 publications

References 18 publications

Crustal heterogeneity around the Nagamachi-Rifu fault, northeastern Japan, as inferred from travel-time tomography

Crustal heterogeneity around the Nagamachi-Rifu fault, northeastern Japan, as inferred from travel-time tomography

Spectral matrix analysis for detection of polarized wave arrivals and its application to seismic reflection studies using local earthquake data

A comprehensive model of the deformation process in the Nagamachi-Rifu Fault Zone

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