Seismic structure and geomagnetic anomaly in the Nankai Trough related to subduction of the Philippine Sea plate.

Kinoshita, Hajimu; Matsuda, Naoto

doi:10.5636/jgg.41.161

Cited by 12 publications

(7 citation statements)

References 15 publications

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“…They observed gradual decreases in magnetization, but they did not observe a sudden drop in magnetization by a depth of 15 km to the slab. Concerning the Nankai Trough, Kinoshita and Matsuda [1989] previously discussed such magnetization attenuation phenomena relative to available seismic structural information. They assumed that some kind of physical destruction causes the rapid weakening in the magnetization and discussed geometrical disordering using model experiments based on the then available, relatively low‐resolution seismic structural studies.…”

Section: Discussionmentioning

confidence: 99%

Regional variation of magnetization of oceanic crust subducting beneath the Nankai Trough

Kido

Fujiwara

2004

Geochem Geophys Geosyst

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[1] We present the characteristics of marine magnetic anomalies in the Nankai Trough and the northern Shikoku Basin. The marine magnetic anomalies are clearly visible over the Shikoku Basin but gradually disappear across the Nankai Trough. The expected reason for the decrease in the marine magnetic anomaly amplitudes across the trough is the increase in depth of the magnetic source: the subducting oceanic crust. Forward and inverse magnetic modeling results, which take into account the slab depth distribution, show variations in magnetization that cannot be explained solely by increasing depth of the slab. We identify five zones along the trough axis on the basis of the pattern of the magnetic amplitude profile: two ridges, a seamount chain, and two basement zones. The magnetic inversion analysis indicates cyclic intensity variations along the ridge zones. The largest contrast in magnetization along the N-S lineation before the deformation front occurs off Ashizuri (AS). Low-temperature chemical oxidation effects are most likely the main cause of the reduction in magnetization. There are considerable differences in the original carrier of magnetic intensity in the AS and Kii Peninsula (KP) zones, and the original carrier in AS is characterized by much higher original magnetic intensities. This indicates several possibilities: the oceanic crust originated from asymmetric spreading; there was magma injection during the late stage volcanism; or the AS zone was affected by an anomalous heat source. It would be concluded from the current data that the basement in AS originally had a much stronger magnetic carrier and suffered stronger low-temperature oxidation effects than the eastern part of the Shikoku Basin. The regional differences in intensity strongly suggest a variety of magnetic minerals and phase changes associated with temperature and mechanical dislocation.

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Section: Discussionmentioning

confidence: 99%

Regional variation of magnetization of oceanic crust subducting beneath the Nankai Trough

Kido

Fujiwara

2004

Geochem Geophys Geosyst

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“…The thickness and width of the V p = 5.0 to 7.2 km/s body are 7 to 13 km and 50 km wide, respectively, and it thins to 6 km on either side. According to previous conventional seismic refraction studies at the northern edge of the Philippine Sea Plate (7,8), P-wave velocities of the oceanic crust range between 5.0 and 7.0 kmls with 6 km thickness. The thick 5.0 to 7.2 km/s body in the middle of the model may be locally thickened oceanic crust.…”

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confidence: 94%

Subducted Seamount Imaged in the Rupture Zone of the 1946 Nankaido Earthquake

Kodaira

Takahashi

Nakanishi

et al. 2000

Science

287

235

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The Nankai Trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century, with a recurrence time of 100 to 200 years. The 1946 Nankaido earthquake was unusual, with a rupture zone estimated from long-period geodetic data that was more than twice as large as that derived from shorter period seismic data. In the center of this earthquake rupture zone, we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers. We propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture.

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“…Velocity models around the eastern part of area A were constructed [ Yoshii et al , 1973; Kinoshita and Matsuda , 1989; Kodaira et al , 2000], and Kodaira et al [2000] estimated the crustal structure across area A using controlled source seismic data and the rupture area of the 1946 Nankaido earthquake proposed by Ando [1975]. The crustal structure is characterized by a gently sloping subducting oceanic crust and a thick overlying unit of thick low‐velocity sediments (9 km thick at a distance of 70 km from trough axis).…”

Section: Introductionmentioning

confidence: 99%

Seismic structure of western end of the Nankai trough seismogenic zone

Takahashi¹,

Kodaira²,

Nakanishi³

et al. 2002

J. Geophys. Res.

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[1] In 1998 a seismic experiment along a 320-km-long profile using an air gun array and ocean bottom seismographs (OBSs) was collected in the on shore-offshore, forearc region of the western Nankai trough seismogenic zone, southwestern Japan. This area is outside the coseismic rupture area of the 1946 Nankaido earthquake (M s = 8.2). The main objectives of this paper are to clarify the P wave and the Poisson's ratio profile around the forearc region of southwestern Japan, and estimate differences between this area and the Nankaido coseismic rupture area. The P wave velocity model estimated by two-dimensional (2-D) ray tracing and travel time inversion indicates three units with distinctive velocity patterns. The lens-shaped unit has a maximum thickness of 5 km and velocity of 3.4-4.0 km/s. The underlying unit has velocity of 5.4-5.6 km/s, and its P wave velocity increases on land (V p = 6.2-6.4 km/s). The relative homogeneous unit with P wave velocity of 6.6-6.9 km/s exists just beneath secondary unit with P velocity of 5.4-5.6 km/s. The angle of subduction is $7°. We also obtained a Poisson's ratio of the crustal structure of the forearc region. The Poisson's ratio of the southern part of secondary unit is larger than that of the northern part. We conclude that these data indicate clay-rich metamorphic rocks plus oceanic basaltic rocks, rather than high-porosity materials, comprise the majority of the layer. We suggest that the region of high Poisson's ratio probably corresponds to underplating and the metamorphism of the clay-rich sediments and possibly the oceanic basalt just below the decollement.

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Seismic structure and geomagnetic anomaly in the Nankai Trough related to subduction of the Philippine Sea plate.

Cited by 12 publications

References 15 publications

Regional variation of magnetization of oceanic crust subducting beneath the Nankai Trough

Regional variation of magnetization of oceanic crust subducting beneath the Nankai Trough

Subducted Seamount Imaged in the Rupture Zone of the 1946 Nankaido Earthquake

Seismic structure of western end of the Nankai trough seismogenic zone

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