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The accurate location of tectonic tremors helps improve understanding of their underlying physical processes. However, current location methods often do not statistically evaluate uncertainties to a satisfactory degree and do not account for potential biases due to subsurface structures not included in the model. To address these issues, we propose a novel three-step process for locating tectonic tremors. First, the measured time- and amplitude differences between station pairs are optimized to obtain station-specific relative time and amplitude measurements with uncertainty estimates. Secondly, the time– and amplitude–distance relationships in the optimized data are used to roughly estimate the propagation speed (i.e. shear wave velocity) and attenuation strength. Linear regression is applied to each event, and the resulting velocity and attenuation strength are used for quality control. Finally, the tremor location problem is formulated within a Bayesian framework where the model parameters include the source locations, local site delay/amplification factors, shear wave velocity and attenuation strength. The Markov chain Monte Carlo algorithm is used to sample the posterior probability and is augmented by a parallel tempering scheme for an efficient global search. We tested the proposed method on ocean-bottom data indicating an intense episode of tectonic tremors in Kumano-nada within the Nankai Trough subduction zone. The results show that the range of the 95 per cent confidence interval is typically <7 km horizontally and <10 km vertically. A series of experiments with different inversion settings reveals that adopting amplitude data and site correction factors help reduce random error and systematic bias, respectively. Probabilistic sampling allows us to spatially map the probability of a tremor occurring at a given location. The probability map is used to identify lineaments of tremor sources, which provides insights into structural factors that favour tremor activity. [ABSTRACT FROM AUTHOR]

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]

Shear wave velocity (Vs) estimations of accretionary prisms can pose unique constraints to the physical properties of rocks, which are hard to obtain from compressional velocities (Vp) alone. Thus, it would help better understand the fluid processes of the accretion system. This study investigates the Vs structure of the Hyuga‐nada accretionary prism using an array of ocean‐bottom seismometers (OBSs) with a 2 km radius. Teleseismic Green's functions and a surface wave dispersion curve are inverted to one‐dimensional Vs structures using transdimensional inversion. The results indicate the presence of a low‐velocity zone 3–4 km below the seafloor. The reduced Vs corresponds to a reduced Vp feature obtained in a previous seismic refraction survey, and the high Vp/Vs ratio suggests the presence of high pore fluid pressure. In addition, the resolved lithological boundary exhibits a sharp offset that extends laterally across the OBS array. We attribute this offset to a blind fault below while acknowledging other possibilities, such as due to mud diapirism or intense fracturing. The predicted fault is located at the Kyushu–Palau Ridge flank, oriented roughly parallel to the ridge axis, and thus likely caused by ridge subduction. This fault may act as a fluid conduit, contributing to the formation of a fluid reservoir beneath the compacted sediment layers. Plain Language Summary: Propagation speeds of seismic S‐waves offer unique constraints on physical properties in shallow subduction zones, which is hard to know from only seismic P‐wave velocity. This study investigates the subsurface structure in Hyuga‐nada in the southwestern Japan subduction zone by exploring S‐wave speeds. For this purpose, we use natural seismic and noise data recorded by densely installed ocean‐bottom seismometers. The results reveal a region with a reduced S‐wave velocity at a depth of ∼3–4 km below the seafloor, which may be a water reservoir. The depth of the potential water reservoir changes abruptly across the array. This offset may suggest the presence of a hidden fault below, although we cannot exclude other possibilities. We propose that a fault created by subducting seamounts acts as a conduit that transports water to the reservoir. Key Points: The shear wave velocity structures of the shallow Hyuga‐nada accretionary prism were derived using a passive seismic arrayA low shear‐wave velocity zone exists ∼3–4 km below the seafloor, possibly indicative of a fluid reservoirA Fault induced by the subducting Kyushu–Palau Ridge may act as a fluid pathway, supplying fluids to the reservoir [ABSTRACT FROM AUTHOR]

The formation of a new ocean basin provides a unique observational window into the structure and deformation of the oceanic lithosphere. This study offers a detailed 1‐D vertically polarized shear‐wave velocity model of the young Japan Basin by interpreting S receiver functions, seafloor Rayleigh wave ellipticities and phase velocities in a Bayesian trans‐dimensional framework. The models suggest a distinct discontinuity in the mid‐lithosphere. Additional analyses indicate that the upper layer is characterized by strong positive radial anisotropy, while the lower layer is more isotropic. We interpret that the upper layer was formed during the back‐arc opening at 20–15 Ma, where the olivine lattice‐preferred orientation (LPO) fabric induced by passive upwelling contributes to the positive radial anisotropy. The lower, less anisotropic layer may be solidified after the sudden cessation of the opening. Small‐scale convection (SSC) from such a major tectonic event could randomize the LPO enough to weaken radial anisotropy before cooling down the asthenosphere, leaving a radially anisotropic discontinuity in the present‐day mid‐lithosphere. The proposed layered evolution history of the lithosphere beneath the Japan Basin illuminates an important role of the small‐scale convection in modifying the structural fabrics of the asthenosphere in terms of radial anisotropy. Such an SSC‐induced radial anisotropy drop could also occur at the base of the lithosphere in regions with ongoing seafloor spreading and enhance the velocity drop of horizontally polarized shear waves. Thus, the role of SSC is non‐negligible in explaining the sharpness of the Gutenberg discontinuity detected by SS precursors beneath the oceanic lithosphere. Plain Language Summary: The formation of the oceanic lithosphere is associated with a relatively simple mantle convection process called seafloor spreading, but may also be affected by more complex, localized processes such as small‐scale mantle convection. To decipher the evolution history of the lithosphere of the Japan Basin, the Sea of Japan, this study collects earthquake waveforms from broadband ocean‐bottom seismometers and offers a detailed 1‐D seismic structural model beneath the Japan Basin by interpreting the multiple seismological observables in a trans‐dimensional Bayesian framework. Our seismic velocity model and associated analysis reveal that the oceanic lithosphere of the Japan Basin is seismically more complex than expected. The upper and lower parts of the lithosphere show distinct seismic anisotropy properties. We propose a layered evolution model for the lithosphere of the Japan Basin, in which small‐scale convection followed a normal seafloor spreading, and resulted in a structural discontinuity in the present‐day mid‐lithosphere. The proposed model highlights the role of the small‐scale convection in modifying the structural fabrics of the asthenosphere in terms of radial anisotropy. Key Points: We construct detailed 1‐D Vsv models of the Japan Basin by trans‐dimensional inversion of multiple observables extracted from ocean‐bottom seismometers recordsOur analysis reveals a mid‐lithosphere discontinuity arising from strong positive radial anisotropy in the upper lithosphereA layered evolution mode associated with small‐scale convection is proposed for the lithosphere of the Japan Basin [ABSTRACT FROM AUTHOR]

To obtain an internal S -wave velocity structure, we conducted a passive seismic campaign with 21 1-Hz seismometers on and around the Showa-Shinzan lava dome, which emerged during the 1943–1945 eruption of Usu Volcano, Japan. Before the campaign, we calibrated seismometers and found slight phase-response differences between seismometers of less than 1–2°. After the campaign, we extracted seismic wavefield by taking cross-correlations of vertical-component ambient noise records between seismic sites. We developed a new method to measure phase velocities of the Rayleigh wave automatically by assuming layered structure and finally obtained 1-D S -wave velocity models in summit, roof and base regions. The obtained S -wave velocity right beneath the intruded lava dome is higher than that in surrounding areas by a few tens of per cent down to a few hundred metres below sea level, indicating narrow but deep existence of the root of the lava dome. The obtained S -wave velocity at depths shallower than ∼50 m inside the lava dome in the summit area was ∼1 km s−1, significantly lower than that predicted from the density of  2.3 × 103 kg m−3 estimated in previous muon-radiography studies and a conventional scaling, indicating the effect of cracking in the lava dome. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Active‐source seismic surveys have resolved the fine‐scale P‐wave velocity (Vp) of the subsurface structure in subduction forearcs. In contrast, the S‐wave velocity (Vs) structure is poorly resolved despite its usefulness in understanding rock properties. This study estimates Vp and Vs structures of the Nankai Trough forearc, by applying transdimensional inversion to high‐frequency teleseismic waveforms. As a result, a thin (∼1 km) low‐velocity zone (LVZ) is evident at ∼6 km depth beneath the sea level, which is located ~3 km seaward from the outer ridge. Based on its high Vp/Vs ratio (∼2.5) and comparison to an existing seismic reflection profile, we conclude that this LVZ reflects a high pore pressure zone at the upper portion of the underthrust sediment. We infer that this overpressured underthrust sediment hosts slow earthquake activities and that accompanied strain release helps impede coseismic rupture propagation further updip. Plain Language Summary: Many geophysical surveys have investigated the subsurface structures of shallow subduction zones by estimating the propagation speed of compressional waves emitted from artificial explosive sources. Although shear and compressional wave speeds are necessary to understand rock properties (e.g., water pressure) of the subsurface, it has been difficult to constrain the shear wave speed with a high‐resolution. In this study, we estimate both compressional and shear wave speeds by applying an advanced technique to earthquake waveforms recorded at ocean‐bottom seismometers deployed at the Nankai subduction zone, Japan. The results show a sufficiently high spatial resolution to detect a thin (~1 km) layer, which we interpret as water‐rich subducted sediment. This water‐rich zone may promote slow slips on the megathrust fault and work as a barrier against rupture propagations during large earthquakes. Key Points: We applied transdimensional inversion of teleseismic waveforms to seafloor cabled stations at the Nankai subduction zoneThe resultant high‐resolution velocity structures allow a direct comparison to active source surveysA low‐velocity zone beneath the outer ridge is evident, which is interpreted as an overpressured portion of underthrust sediment [ABSTRACT FROM AUTHOR]