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This study presents an approach to better characterize the P‐wave and S‐wave velocity structure of the seafloor sediment layer using ocean bottom seismometers. The presence of low‐velocity seafloor sediment layers influences the observed seismic record at the seafloor over a broad frequency range, such that detailed knowledge of this sediment structure is essential to predict its effect on teleseismic records. We use the radial component of teleseismic P waves and autocorrelation functions of the radial, vertical, and pressure components of teleseismic P and S waves to obtain sediment layer models using the Markov chain Monte Carlo approach with parallel tempering. Synthetic tests show that the body waves constrain the P‐ and S‐wave impedances and travel times and the P‐ to S‐wave velocity ratio of the sediment layers. The proposed method resolves thin layers at a high resolution, including the uppermost thin (∼50 m to a few hundred meters) low S‐wave velocity layer. Real data applications at sites across the Pacific Ocean that are coincident with previous in situ studies demonstrate the effectiveness of this method in characterizing the seafloor sediment unit. The sediment models characterized by this new approach will allow us to more accurately predict and correct the effects of sediment layers in generating P‐ and S‐wave reverberations. Plain Language Summary: Ocean bottom seismometers are powerful tools for investigating the detailed structure of the oceanic mantle. However, most ocean bottom seismometers are placed above the low‐velocity water‐saturated sediment layer, such that their seismic records are inevitably affected by this seafloor sediment layer. Detailed constraints on the structure of the seafloor sediment layer allow us to accurately predict its effects on the observed seismic records. The seafloor sediment layer may consist of multiple discontinuities and vary regionally. This study proposes a new approach for characterizing the seismic properties of the seafloor sediment layer using teleseismic body waves recorded at ocean bottom seismometers. Synthetic tests show that the method can resolve thin sublayers within the sediment layer, such as a very thin low shear‐wave velocity layer and sharp seismic velocity jumps. Real data applications show that the sediment models obtained using this approach are in good agreement with those obtained from direct measurements and previous body‐wave analyses. Our proposed method will enable more detailed prediction and correction of the influence of the sediment layer on ocean bottom seismometer records, thereby providing more accurate constraints on deeper Earth structures. Key Points: The seismic properties of the seafloor sediment layer are characterized using ocean bottom seismometer teleseismic body‐wave recordsThe P‐ and S‐wave impedances and travel times and the VP/VS values of each sublayer within the sediment layer are resolvedIn situ seafloor sediment structure constraints will enable accurate correction and prediction of shallow structure effects [ABSTRACT FROM AUTHOR]

Editors of JGR‐Solid Earth express their appreciation to those who served as peer reviewers for the journal in 2022. Plain Language Summary: The editorial board of JGR‐Solid Earth thanks those who served as peer reviewers for the journal in 2022. Key Points: Thank you to the reviewers of articles submitted to JGR‐Solid Earth [ABSTRACT FROM AUTHOR]

We constrain olivine fabrics in the oceanic lithosphere using active and passive seismic observations of Pn azimuthal anisotropy. We first analyze active‐source data derived from a broadband ocean bottom seismometer array that was deployed in the Northwestern Pacific. We infer the azimuthal dependence of the Pn velocity, including the 2ϕ and 4ϕ terms of the sinusoidal functions, where ϕ is the back azimuth. We observe a skewed azimuthal dependence of the Pn velocity, with a large peak‐to‐peak amplitude of about 12%. Similar features are observed using an independent data set of teleseismic Pn waves. We constrain the direction of the crystallographic axes of olivine to explain the observed azimuthal dependence and identify A‐type olivine with a slightly dipping a‐axis and slightly tilting b‐axis being compatible with our observations. In contrast, we find that D‐type olivine with any direction of crystallographic axes cannot explain our observations. Secondary deformation and recrystallization in the older Pacific may be responsible for this strong and complex seismic anisotropy in the study region. Plain Language Summary: Olivine is the most abundant mineral in the oceanic upper mantle and exhibits preferred crystallographic orientations when it experiences plastic strain‐induced deformation (Nicolas & Christensen, 1987, https://doi.org/10.1029/GD016p0111). Olivine in the oceanic lithosphere is considered to retain the orientations from its formation at the mid‐ocean ridge unless it experiences tectonic perturbations such as reheating by hotspot volcanisms. Seismic anisotropy observations in the oceanic lithosphere therefore provide clues to plate tectonics processes. In this study, we analyze two independent data sets (active‐ and passive‐source data) and reveal the detailed azimuthal dependence of the P velocity traversing the oceanic lithosphere in the northwestern Pacific. We find that the two data sets consistently show a strong and skewed anisotropic signal. We infer that secondary deformation and recrystallization in the Pacific Plate may be responsible for these observations. Key Points: Strong and skewed azimuthal dependence of Pn velocity is robustly observedCompatible direction of the crystallographic axes of olivine is constrainedSecondary deformation and recrystallization in the older Pacific is suggested [ABSTRACT FROM AUTHOR]

We analyze seismic records collected at the oldest (170–180 Ma) Pacific seafloor using broadband dispersion array analysis. Using ambient noise and teleseismic waveforms, we measure Rayleigh‐wave phase velocities in a period range of 5–200 s that are inverted for array‐average one‐dimensional isotropic and azimuthally anisotropic shear‐wave velocity depth profiles from the crust to a depth of 300 km. The high‐velocity Lid and the low‐velocity zone are well‐resolved with velocity difference of ∼4%, whose transition occurs at depths between 80 and 100 km. The profile is compared with that obtained at the 130‐ and 140‐Ma seafloor. Accounting for the cooling effect due to the plate age difference, the low‐velocity zone of the oldest Pacific seafloor is ∼1.3% slower (∼110°C warmer) than that beneath the 140‐Ma seafloor, suggesting the occurrence of some reheating process beneath the oldest lithosphere. The azimuthal anisotropy at shallow depths (<50 km) is significantly different between the western and eastern areas of the array where the peak‐to‐peak amplitudes are estimated to be ∼2.8% and ∼1.6%, respectively. The fast direction is nearly parallel to the past seafloor spreading direction (perpendicular to the magnetic lineation) in the west, while it largely deviates in the east. The observed difference in azimuthal anisotropy may represent complicated evolution dynamics of the infant Pacific plate that involved a ridge‐ridge‐ridge triple junction. Plain Language Summary: One of the classical enigmas of plate tectonics is the cause of seafloor flattening where the seafloor depth becomes nearly constant for old ages (>∼70 Ma) while a simple cooling model of plate evolution predicts the age dependence. To solve this enigma, it is essential to resolve the subsurface structure of the oceanic plate and the asthenosphere below. As a part of an international effort, Pacific Array, that aims to cover the Pacific seafloor by arrays of ocean bottom seismometers, a team of Japanese and Korean scientists deployed an array in the oldest (170–180 Ma) Pacific seafloor. We determine isotropic and anisotropic shear‐wave velocity structures as a first step toward resolving the enigma. The results indicate that the asthenosphere beneath the oldest Pacific lithosphere is significantly slower than the prediction of the simple cooling and that the occurrence of some reheating process may be needed. Furthermore, azimuthal anisotropy (the dependence of the wave velocity on the horizontal propagation direction) in the shallow lithosphere shows that the anisotropy patterns differ between the eastern and western areas of the array, and simulation results indicate that the evolution process of the infant Pacific plate is not merely a simple triangular plate spreading. Key Points: Broadband Rayleigh‐wave dispersion array analysis has been conducted in the oldest (∼170 Ma) Pacific seafloorThe low‐velocity zone is significantly slower to indicate the occurrence of some reheating process beneath the oldest Pacific lithosphereThe spatial variation of the shallow azimuthal anisotropy indicates a complicated evolution of the infant Pacific plate [ABSTRACT FROM AUTHOR]

Editors of JGR‐Solid Earth express their appreciation to those who served as peer reviewers for the journal in 2021. Plain Language Summary: The editorial board of JGR‐Solid Earth thanks those who served as peer reviewers for the journal in 2021. Key Points: Thank you to the reviewers of articles submitted to JGR‐Solid Earth [ABSTRACT FROM AUTHOR]

Members of the editorial board of JGR‐Solid Earth express their appreciation to those who served as peer reviewers for the journal in 2020. Plain Language Summary: The editorial board of JGR‐Solid Earth thanks those who served as peer reviewers for the journal in 2020. Key Points: Thank you to the reviewers of articles submitted to JGR‐Solid Earth [ABSTRACT FROM AUTHOR]

We performed waveform inversion of the P waveforms recorded by our BroadBand Ocean Bottom Seismometers (BBOBSs) deployed in the Northwestern Pacific. Consequently, the depth profile of the P velocity of the oceanic upper mantle, which has not been well resolved by previous surface wave or receiver function analyses, was revealed. We considered the azimuthal anisotropy in the lithosphere, which significantly improved the variance reduction from 34% to 44%. The resulting P model exhibited higher and lower velocities in the lithosphere and asthenosphere, respectively. The velocity contrast was found to be second/third of that observed in the previous S models; however the obtained model appeared to have some trade‐off with the VS structures in the vicinity of the source. We compared our P model with the previous S model obtained using our BBOBSs and obtained the VP/VS model. The resulting VP/VS model has two notable features. First, the lithosphere is characterized by a rapid increase in the VP/VS values with depth, which implies chemical stratification. Second, the VP/VS values in the vicinity of the lithosphere‐asthenosphere boundary (LAB) are larger than the synthetic values of any major upper mantle mineral predicted by considering the anharmonic effects, which suggests the effects of anelasticity or melt. Key Points: We inverted P waveforms recorded by our BBOBSs to obtain the depth profile of the P velocity beneath the older Pacific plateCombining our P model with a previous S model revealed two notable features in Vp/VS structuresThe obtained Vp/Vs model suggests chemical stratification in the lithosphere and the effects of anelasticity or melt at the LAB [ABSTRACT FROM AUTHOR]

No abstract is available for this article. [ABSTRACT FROM AUTHOR]

The Earth's outer core is believed to be laterally homogeneous because of its low viscosity. However, a hemispherical difference in the inner core likely exists due to its uneven growth, which may be accompanied by localized light‐element releases to the outer core. A few seismological studies proposed heterogeneous lowermost outer core (called F layer) but using methods that are not very sensitive to F layer structures. In a previous study we developed a new method sensitive to the F layer structure and insensitive to others. The method analyzes differences in P wave traveltimes reflected on the inner core boundary and those that turn above the boundary as well as dispersion in waves bottoming or diffracting in the F layer, and was applied to obtain an F layer model of P wave velocity for the northeastern Pacific Ocean. In this paper, we examine the F layer structure beneath Australia using the same method. The observed dispersion requires a lower‐velocity gradient beneath Australia than beneath the northeastern Pacific, whereas the observed traveltime differences require higher average velocities beneath Australia. The results obtained for the two regions indicate that the F layer is laterally heterogeneous and that the layer beneath Australia has a higher velocity and velocity gradient in its upper part and a much smaller gradient in its lower part than beneath northeastern Pacific. The maximum velocity difference between the two regions is 0.04 km/s, which corresponds to 0.8 wt% excess oxygen according to ab initio calculations of elastic properties. These results suggest regional light‐element concentration beneath Australia. Key Points: We analyze the seismic velocity structure of lowermost outer core (F‐layer) using dispersion and differential travel time of core phasesThe F‐layer velocity beneath Australia is 0.04 km/s higher at maximum and more complicated than that beneath the northeastern PacificOur results suggest laterally heterogeneous F‐layer and regional light‐element concentration in the layer beneath Australia [ABSTRACT FROM AUTHOR]