Shear-wave splitting analysis of the upper mantle at the Niigata-Kobe Tectonic Zone with the data of the Joint Seismic Observations at NKTZ

Iidaka, Takashi; Hiramatsu, Yuji

doi:10.1186/bf03352903

Cited by 7 publications

(3 citation statements)

References 56 publications

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“…The Japanese University Group of the Joint Seismic Observations at NKTZ started seismic observations around the Atotsugawa fault zone, north central Japan, from 2004 (The Japanese University Group of the Joint Seismic Observations at NKTZ, 2005). Iidaka et al (2009) confirmed strong anisotropy in the mantle wedge beneath the NKTZ from many shear wave splitting data obtained by this dense network.…”

Section: Introductionsupporting

confidence: 55%

Spatial variation in shear wave splitting of the upper crust in the zone of inland high strain rate, central Japan

et al. 2010

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We investigate a detailed spatial variation in shear wave splitting in the zone of inland high strain rate, called the Niigata-Kobe Tectonic Zone (NKTZ), central Japan. Most observations show stress induced anisotropy, that is, the orientation of the faster polarized shear wave is parallel to the axis of the maximum horizontal compressional strain rate estimated from GPS data. Others show structure induced anisotropy, that is, the orientation is parallel to the strike of active faults. For the stress induced anisotropy, time delays normalized by the path length in the anisotropic upper crust is proportional to the differential strain rate. We estimate a spatial variation in stressing rate of the upper crust beneath the high strain rate zone based on a response of the normalized time delay to a step-wise stress change caused by a moderate-sized earthquake. The variation in the stressing rate of 3 kPa/year estimated from shear wave splitting is coincident with that from GPS data. We conclude, together with other seismological features in the NKTZ reported previously, that the high strain rate in the NKTZ is attributed to the high deformation rate below the brittle-ductile transition zone in the crust.

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Section: Introductionsupporting

confidence: 55%

Spatial variation in shear wave splitting of the upper crust in the zone of inland high strain rate, central Japan

et al. 2010

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“…Nakajima and Hasegawa (2007b) interpreted the low-velocity zone in the mantle wedge as being associated with upwelling flow induced by the subduction of the Philippine Sea slab. Iidaka et al (2009) investigated shear-wave splitting in central Japan and demonstrated the existence of mantle anisotropy in the N-S direction beneath the Atotsugawa fault. They attributed their observations to a preferred orientation of olivine caused by mantle return flow induced by the subduction of the Philippine Sea slab or to the alignment of melt-filled cracks in the return flow, supporting the existence of upwelling flow in the mantle.…”

Section: Deep Origin Of Fluids In the Lower Crustmentioning

confidence: 99%

Deep crustal structure around the Atotsugawa fault system, central Japan: A weak zone below the seismogenic zone and its role in earthquake generation

et al. 2010

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We carried out travel-time tomography around the Atotsugawa fault in central Japan, using arrival-time data obtained from a dense temporary seismograph network in joint observations by the Japanese University group. The observed velocities beneath the fault are 10-13% lower than those of the assumed host rocks (pyroxene amphibolite and hornblende-pyroxene gabbro) in the lower crust. Because the seismogenic layer is thickest in the central part of the fault, reaching a depth of ∼15 km, we infer that the low-velocity anomaly is caused by aqueous uids. Fluid fractions in the lower crust are estimated to be 2-3% and ∼10%, assuming pyroxene amphibolite and hornblende-pyroxene gabbro, respectively. A distinct low-velocity anomaly is imaged in the central part of the Atotsugawa fault at a depth of 10 km, where seismic activity is very low at the upper 7 km and creeplike movement is observed at the surface. This anomaly is horizontally isolated but vertically connected to the lowvelocity anomaly in the lower crust. We interpreted that abundant aqueous uids supplied from the lower crust are responsible for this anomaly. High pore uid pressure may enhance the stability of frictional slip, resulting in aseismic or episodic slip along the fault.

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“…Other groups in the Joint Seismic Observations at the NKTZ are studying seismicity, seismic tomography (Nakajima and Hasegawa, 2007), source mechanism, reflection and scattering studies, shear-wave splitting (Iidaka et al, 2009), low-frequency earthquake, and attenuation structure using data obtained by this project. The results of these analyses will contribute greatly to reveal the mechanisms of inland earthquakes.…”

Section: Introductionmentioning

confidence: 99%

Crust and uppermost mantle structure beneath central Japan inferred from receiver function analysis

et al. 2009

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We apply the receiver function method to estimate the structure of the crust and the uppermost mantle at an area that traverses central Japan including the Niigata-Kobe Tectonic Zone (NKTZ). The resultant receiver function images show clear seismic discontinuities, such as the subducting Philippine Sea plate, the Moho in the overriding plate, and other discontinuities inside the crust around the NKTZ. We also address station corrections for shallow structures using a synthetic receiver function. Crustal discontinuities seem to be complicated at the south side from the northern limit of the NKTZ. The dip of the discontinuities changes around the Atotsugawa active fault located in the NKTZ. The Moho discontinuity in the overriding plate is continuous and gradually dips to the south. The depths of the Moho discontinuity in the receiver function image exceed 40 km at the southern part of the profile line, and are 5-10 km deeper than that indicated by an explosion analysis of the same profile line. It seems that the differences between the estimated depths obtained by the two methods indicate complicated structures around the Moho discontinuity.

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Shear-wave splitting analysis of the upper mantle at the Niigata-Kobe Tectonic Zone with the data of the Joint Seismic Observations at NKTZ

Cited by 7 publications

References 56 publications

Spatial variation in shear wave splitting of the upper crust in the zone of inland high strain rate, central Japan

Spatial variation in shear wave splitting of the upper crust in the zone of inland high strain rate, central Japan

Deep crustal structure around the Atotsugawa fault system, central Japan: A weak zone below the seismogenic zone and its role in earthquake generation

Crust and uppermost mantle structure beneath central Japan inferred from receiver function analysis

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