Estimation of azimuthal anisotropy in the NW Pacific from seismic ambient noise in seafloor records

Takeo, Akiko; Forsyth, Donald W.; Weeraratne, D. S.; Nishida, Kiwamu

doi:10.1093/gji/ggu240

Cited by 44 publications

(50 citation statements)

References 47 publications

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“…This Figure 1. The thin arrows are the azimuths at depths shallower than~20 km (blue) and deeper depths of~50-80 km (red) by previous studies: S08 (Shinohara et al, 2008) near area A and S14 (Shintaku et al, 2014) and T14 (Takeo et al, 2014) at southwest of the Shatsky Rise. The thick arrows represent fastest azimuths at a depth shallower than~20 km (blue) and deeper depths of~80-100 km (red) determined in this study.…”

Section: Broadband Dispersion Analysismentioning

confidence: 85%

In Situ Characterization of the Lithosphere‐Asthenosphere System beneath NW Pacific Ocean Via Broadband Dispersion Survey With Two OBS Arrays

Takeo

Kawakatsu

Isse

et al. 2018

Geochem Geophys Geosyst

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We conducted broadband dispersion survey by deploying two arrays of broadband ocean bottom seismometers in the northwestern Pacific Ocean at seafloor ages of 130 and 140 Ma. By combining ambient noise and teleseismic surface wave analyses, dispersion curves of Rayleigh waves were obtained at a period range of 5–100 s and then used to invert for one‐dimensional isotropic and azimuthally anisotropic βV (VSV) profiles beneath each array. The obtained profiles show ~2% difference in isotropic βV in the low‐velocity zone (LVZ) at a depth range of 80–150 km in spite of the small difference in seafloor ages and the horizontal distance of ~1,000 km. Forward dispersion‐curve calculation for thermal models indicates that simple cooling models cannot explain the observed difference and an additional mechanism, such as sublithospheric small‐scale convection, is required. In addition, the fastest azimuths of azimuthal anisotropy in the LVZ significantly deviate from the current plate motion direction. We infer that these observations are consistent with the presence of small‐scale convection beneath the study area. As for azimuthal anisotropy in the Lid, the peak‐to‐peak intensity is 3–4% at the depth from Moho to ~40 km. The fastest direction is almost perpendicular to magnetic lineation in area A at 130 Ma and oblique to magnetic lineations in area B at 140 Ma, suggesting complex mantle flow beneath the infant Pacific Plate surrounded by three ridge axes. The intensity of azimuthal anisotropy in the LVZ is ~2%, indicating that radial anisotropy is stronger than azimuthal anisotropy therein.

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Section: Broadband Dispersion Analysismentioning

confidence: 85%

In Situ Characterization of the Lithosphere‐Asthenosphere System beneath NW Pacific Ocean Via Broadband Dispersion Survey With Two OBS Arrays

Takeo

Kawakatsu

Isse

et al. 2018

Geochem Geophys Geosyst

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“…With a magnitude of 4.6%, the percent anisotropy beneath the JdF plate falls between values characteristic of slow‐ (3–4%) and fast‐spreading (6–7%) environments (Gaherty et al, ; Toomey et al, ; Song & Kim, ; Shintaku et al, ; Takeo et al, ). We speculate that the positive correlation between spreading rate and percent mantle anisotropy may be related to the partial off‐axis realignment of olivine LPO.…”

Section: Discussionmentioning

confidence: 99%

“…Seismic anisotropy measurements in the oceanic lithosphere away from spreading centers yield many observations of skew between the seismically fast propagation and paleo‐spreading directions (Morris et al, ; Raitt et al, ; Shearer & Orcutt, ; Shintaku et al, ; Takeo et al, ). While the magnitude of skew observed in these studies is typically >10° and exceeds the measurement error, its significance is not always discussed.…”

Section: Discussionmentioning

confidence: 99%

Shallow Mantle Anisotropy Beneath the Juan de Fuca Plate

VanderBeek

Toomey

2017

Geophysical Research Letters

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The anisotropic fabric of the oceanic mantle lithosphere is often assumed to parallel paleo‐relative plate motion (RPM). However, we find evidence that this assumption is invalid beneath the Juan de Fuca (JdF) plate. Using travel times of seismic energy propagating through the topmost mantle, we find that the fast direction of P wave propagation is rotated 18° ± 3° counterclockwise to the paleo‐spreading direction and strikes between Pacific‐JdF relative and JdF absolute plate motion (APM). The mean mantle velocity is 7.85 ± 0.02 km/s with 4.6% ± 0.4% anisotropy. Synthesis of the plate‐averaged Pn anisotropy signal with measurements of Pn anisotropy beneath the JdF Ridge and SKS splits across the JdF plate suggests that the anisotropic structure of the topmost mantle continues to evolve away from the spreading center to more closely align with APM. We infer that the oceanic mantle lithosphere may record the influence of both paleo‐RPM and paleo‐APM.

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“…Recent studies based on the cross-correlation analysis of ocean bottom records have shown several features of the seismic wavefield recorded at the ocean bottom [e.g., Harmon et al, 2007;Yao et al, 2011;Harmon et al, 2012;Takeo et al, 2013Takeo et al, , 2014Tian and Ritzwoller, 2015]. In particular, it has been seen that the fundamental mode of Rayleigh waves is always present on the vertical component of ocean bottom noise records with periods between 3 and 10 s, whereas the first overtone appears only at short periods [Harmon et al, 2007;Yao et al, 2011].…”

Section: Introductionmentioning

confidence: 99%

On the shaping factors of the secondary microseismic wavefield

et al. 2015

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Seismic noise in the period band 3–10 s is known as secondary microseism, and it is generated at the ocean surface by the interaction of ocean gravity waves coming from nearly opposite directions. In this paper, we investigate the seismic content of the wavefield generated by a source at the ocean surface and three of the major wavefield shaping factors using the 2‐D spectral element method: the ocean‐continent boundary, the source site effect, and the thickness of seafloor sediments. The seismic wavefield recorded on the vertical component seismograms below the seafloor is mainly composed of the fundamental mode and the first overtone of Rayleigh waves. A mode conversion from the first overtone to the fundamental mode of Rayleigh waves occurs at the ocean‐continent boundary. The presence of a continental shelf at the ocean‐continent boundary produces a negligible effect on land‐recorded seismograms, whereas the source site effect, i.e., the source location with respect to the local ocean depth and sediment thickness, plays the major role. A source in shallow water mostly enhances the fundamental mode of Rayleigh waves, whereas a source in deep water mainly enhances the first overtone of Rayleigh waves. Land‐recorded long‐period signals (T > 6 s) are mostly due to deep water sources, whereas land‐recorded short‐period signals (T < 6 s) are due to sources in relatively shallow water, located close to the shelf break. Seafloor sediments around the source region trap seismic waves reducing the amplitude of land‐recorded signals, especially at long periods (T > 6 s).

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Estimation of azimuthal anisotropy in the NW Pacific from seismic ambient noise in seafloor records

Cited by 44 publications

References 47 publications

In Situ Characterization of the Lithosphere‐Asthenosphere System beneath NW Pacific Ocean Via Broadband Dispersion Survey With Two OBS Arrays

In Situ Characterization of the Lithosphere‐Asthenosphere System beneath NW Pacific Ocean Via Broadband Dispersion Survey With Two OBS Arrays

Shallow Mantle Anisotropy Beneath the Juan de Fuca Plate

On the shaping factors of the secondary microseismic wavefield

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