Overpressured Underthrust Sediment in the Nankai Trough Forearc Inferred From Transdimensional Inversion of High‐Frequency Teleseismic Waveforms

Akuhara, Takeshi; Tsuji, Takeshi; Tonegawa, Takashi

doi:10.1029/2020gl088280

Cited by 41 publications

(37 citation statements)

References 51 publications

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“…This condition can be achieved when the λ value is the same as the λ b value for the given wedge geometry at the prism toe (Figure 6d). Because previous studies suggested that the prism toe area is unconsolidated and fluid can easily escape, the prism toe region is not considered to be overpressured (Akuhara et al., 2020; Screaton et al., 2009); therefore, λ = λ b = 0.5 is the likely pore pressure condition at the prism toe. Of our three candidate samples from C0006, C0006E‐36X‐1 with a μ w value of 0.57 would be the most representative for the prism toe because its clay mineral content of 40.9% may be reasonable to represent the Nankai sediment near the trench, which possibly contains detrital‐origin sediment in addition to hemipelagic sediment.…”

Section: Discussionmentioning

confidence: 99%

Spatial Patterns in Frictional Behavior of Sediments Along the Kumano Transect in the Nankai Trough

et al. 2021

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Section: Discussionmentioning

confidence: 99%

Spatial Patterns in Frictional Behavior of Sediments Along the Kumano Transect in the Nankai Trough

et al. 2021

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“…For RF inversion, it can eliminate the subjective choice of the layer number. Many RF studies have used the RJMCMC algorithm for inverse problems in recent years Malinverno 2010, 2018;Akuhara et al 2020). Some of them have performed joint inversion with surface wave dispersion curves (Bodin et al 2012;Calò et al 2016), whereas detailed inversion settings differ among studies.…”

Section: Transdimensional Inversionmentioning

confidence: 99%

Lithosphere–asthenosphere Boundary Beneath the Sea of Japan From Transdimensional Inversion of S Receiver Functions

Akuhara

Nakahigashi

Shinohara

et al. 2021

Preprint

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The evolution history of the Sea of Japan back-arc basin remains under debate, involving the opening of sub-basins such as the Japan and Yamato Basins. Detailed knowledge of the lithospheric structure will provide the key to understanding tectonic history. This study identifies the lithosphere–asthenosphere boundary (LAB) beneath the Sea of Japan back-arc basin using S-receiver functions (S-RFs). The study area, including the Japan and Yamato Basins, has been instrumented with broadband ocean-bottom seismometers (OBSs). S-RFs from these OBSs show negative Sp phases preceding the direct S arrivals, suggesting the LAB. The S-RFs also show abnormally reduced amplitudes. For further qualitative interpretation of these findings, we conduct transdimensional Bayesian inversion for S-wave velocity models. This less-subjective Bayesian approach clarifies that the low-velocity seafloor sediments and damped deconvolution contribute to the amplitude reduction, illuminating the necessity of such considerations for similar receiver function works. Inverted velocity structures show a sharp velocity decrease at the mantle depths, which we consider the LAB. The obtained LAB depths vary among sites: ~45 km beneath the Japan and Yamato Basins and ~70 km beneath the Yamato Rise, a bathymetric high between the two basins. The thick lithosphere beneath the Yamato Rise most likely reflects its continental origin. However, the thickness is still thin compared to that of eastern Asia, suggesting lithosphere extension by rifting. Notably, the Japan and Yamato Basins show a comparable lithospheric thickness, although the crustal thickness beneath the Yamato Basin is known to be anomalously thick. This consistency in the lithospheric thickness implies that both basins undergo similar back-arc opening processes.

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“…For the low frequency band, because the turning depth is close to the plate boundary, if seismic velocity discontinuities are present near the bottom of the prism, refracted and head waves can be generated. Indeed, the presence of a low velocity layer has been reported at the bottom of the accretionary prism in the southern DONET1 region (Park et al, 2010;Kamei et al, 2012;Akuhara et al, 2020). In this study, for the low frequency observations from DONET1, the retrieved P waves may contain such multiple P phases.…”

Section: Discussion P Wave Retrievalmentioning

confidence: 57%

Near-Field Body-Wave Extraction From Ambient Seafloor Noise in the Nankai Subduction Zone

2021

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Ambient noise correlation is capable of retrieving waves propagating between two receivers. Although waves retrieved using this technique are primarily surface waves, the retrieval of body waves, including direct, refracted, and reflected waves, has also been reported from land-based observations. The difficulty of body wave extraction may be caused by large amplitudes and little attenuation of surface waves excited by microseisms, indicating that body wave extraction using seafloor records is very challenging because microseisms are generated in ocean areas and large amplitudes of surface waves are presumably observed at the seafloor. In this study, we used a unique dataset acquired by dense arrays deployed in the Nankai subduction zone, including a permanent cabled-network of 49 stations, a borehole sensor, and 150 temporary stations, to attempt to extract near-field body waves from ambient seafloor noise observed by multivariate sensors of broadband and short-period seismometers, differential pressure gauges (DPGs), and hydrophones. Our results show that P waves are extracted only in the DPG-record correlations at a frequency of 0.2–0.5 Hz, which can be seen up to a separation distance of two stations of 17 km with an apparent velocity of 3.2 km/s. At 1–3 Hz, P waves are observed only in the vertical-record correlations up to a separation distance of 11 km with an apparent velocity of 2.0 km/s. These velocity differences reflect the vertical velocity gradient of the accretionary prism, because the P waves at low frequencies propagate at relatively long distances and therefore the turning depth is greater. Moreover, the long-period and short-period P waves are observed at the slope and flat regions on the accretionary prism, respectively. To investigate the retrieved wavefield characteristics, we conducted a two-dimensional numerical simulation for wave propagations, where we located single sources at the sea surface above the flat and slope bathymetry regions. Based on our observations and simulations, we suggest that the retrieval of near-field body waves from ambient seafloor noises depends on the relative amplitudes of P and other surface waves in the ambient noise wavefield, and those are controlled by the subseafloor velocity structure, seafloor topography, and water depth.

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Overpressured Underthrust Sediment in the Nankai Trough Forearc Inferred From Transdimensional Inversion of High‐Frequency Teleseismic Waveforms

Cited by 41 publications

References 51 publications

Spatial Patterns in Frictional Behavior of Sediments Along the Kumano Transect in the Nankai Trough

Spatial Patterns in Frictional Behavior of Sediments Along the Kumano Transect in the Nankai Trough

Lithosphere–asthenosphere Boundary Beneath the Sea of Japan From Transdimensional Inversion of S Receiver Functions

Near-Field Body-Wave Extraction From Ambient Seafloor Noise in the Nankai Subduction Zone

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