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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]

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]

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]

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]

Reference 1 Yamaya L, Mochizuki K, Akuhara T, Takemura S, Shinohara M, Yamada T. CMT inversion for small-to-moderate earthquakes applying to dense short-period OBS array at off Ibaraki region. 3 Result of the CMT inversions for the earthquake (Mw 4.0) that occurred at 9:06, March 16, 2011 (JST). a Centroid location of the earthquake and the OBS array. [Extracted from the article]

We conducted centroid moment tensor (CMT) inversion for small-to-moderate aftershocks in the off Ibaraki region of the 2011 off the Pacific coast of Tohoku earthquake. In this study, we used high-frequency (0.4–1.0 Hz) seismograms from a dense array of short-period ocean bottom seismometers (OBSs) and a reliable three-dimensional (3-D) seismic velocity model. Higher frequency analysis and dense OBS arrays offer CMT solutions with high spatial resolutions. Since our OBS array observed aftershocks occurring immediately following the 2011 off the Pacific coast of Tohoku earthquake, we determined 536 CMT solutions for small-to-moderate aftershocks in the off Ibaraki region (JMA-scale magnitudes of 2.5–4.0). According to our CMT solutions, characteristics of the aftershock activities in the off Ibaraki region are classified into 5 groups: (1) thrust earthquakes, which are considered as interplate earthquakes, separated by the large slip area of the 2011 Ibaraki-oki earthquake (the largest M w 7.6 aftershock of the 2011 off the Pacific coast of Tohoku earthquake) and the tectonic tremors; (2) intraslab strike-slip earthquakes located at the north of the fault area of the 2011 Ibaraki-oki earthquake; (3) intraslab normal-fault earthquakes, which suggest a tensional stress field within the subducting Pacific Plate due to the plate bending by the overriding Philippine Sea Plate; (4) various earthquake focal mechanisms above a subducting seamount, which suggest the 3-D complex fractures; and (5) normal-fault earthquakes shallower than the interplate earthquakes, which were possibly caused by the heterogeneity of the subducting seamount. [ABSTRACT FROM AUTHOR]

This study presents reflectivity images of the northeast (NE) Japan subduction zone continuous across the ocean and land. As nearly half of its forearc region is under the ocean, data from ocean bottom seismometers (OBSs) must be utilized to fully image the region by passive seismic analysis. The use of OBS data has been a challenge due to inherent characters of the ocean bottom observations: high noise level and effects of seafloor sediment. Now, decent imaging is possible in NE Japan overcoming the high level noise due to the accumulated data set of the OBSs. The low‐velocity of seafloor sediment significantly delays and amplifies S waves passing through them, and thus complicates teleseismic waveforms. We identify and correct these effects to produce coherent receiver function images throughout the amphibious subduction zone. Our images provide a potential for discussing new structural features and will help better understanding of the dynamics of the NE Japan subduction zone. Plain Language Summary: In northeast Japan, the Pacific plate is subducting beneath the Japanese arc, and delineating how the actual subduction is taking place at depths has been the target of many seismological studies. This study presents our effort of continuous imaging of subsurface interfaces across the ocean and land employing an amphibious data set. On the land side, there are dense seismic networks that have been used to reveal the seismic interfaces using converted phases of distant earthquakes. On the ocean side, the accumulation of data from temporary ocean bottom seismometers in the last decade and the recently installed ocean bottom cable network has made it possible to use a similar approach as on the land. However, the low‐velocity sediment beneath the seafloor causes systematic slower arrivals and larger amplitudes of the seismic phases of interest that result in highly disturbed images if we analyze them in the same way as the land. Here, we identify and correct the effects of the seafloor sediment to analyze the seismic interfaces across the ocean and land. As a result, we succeed in imaging the interfaces of the subducting oceanic crust throughout the region, as well as previously unidentified features under the ocean that may deserve future investigation. Key Points: The northeast Japan subduction zone is imaged using receiver functions (RFs) of an amphibious data setWe identify significant amplitude and time delay effects on RFs from seafloor sedimentCorrections of the sediment effects significantly improve images across an amphibious region [ABSTRACT FROM AUTHOR]

Slow earthquakes occur at deep and shallow plate boundaries along the Nankai Trough. Deep slow earthquakes are continuously distributed along the 30–40 km depth contours of the upper surface of the subducted Philippine Sea Plate. In contrast, shallow slow earthquakes occur in limited regions: Hyuga-nada, off Cape Muroto, and southeast off the Kii Peninsula. This review provides an overview of the up-to-date seismological, geodetic, geological, and experimental results in the shallow Nankai area for a unified understanding of the spot-like occurrence of shallow slow earthquakes. Shallow slow earthquakes tend to be distributed in transitional regions between the frictionally locked and stably sliding zones on the plate boundary. Based on geological and experimental studies, the lithology of incoming sediments and their friction coefficients can be variable along the Nankai Trough. Laboratory friction experiments revealed that sediments under shallow plate boundary conditions often exhibit positive (a − b) values, while negative (a − b) is possible via several processes. Subducted seamounts create complex fracture networks and stress shadows in their surrounding areas; however, not all subducted seamounts are related to shallow slow earthquake activities. This incomplete correlation suggests that alternative factors are required to explain the spot-like distribution of shallow slow earthquakes in the Nankai subduction zone. High pore fluid pressure conditions around shallow slow earthquake zones were interpreted based on seismological structural studies. In addition, ambient noise monitoring revealed temporal changes in seismic velocity structures associated with shallow slow earthquake migrations. This result suggests a close link between pore fluid migration and shallow slow earthquake episodes. Because transient changes in pore fluid pressure can lead to various slip behaviors, the episodic migration of pore fluid around the plate boundary could promote shallow slow earthquake activity along the Nankai Trough. [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]

Seismic waves from earthquakes recorded on the seafloor are composed of complex multiple arrivals. Here, distributed acoustic sensing (DAS) observations along a cable located offshore the Sanriku Coast, Japan, show that the local earthquake wavefield is particularly rich in Scholte waves. We introduce a processing pipeline to extract these surface waves from DAS records. We then invert hundreds of dispersion curves along a section of the cable to form a shallow high‐resolution shear‐wave velocity model. Moreover, we focus on the possible generation mechanisms of Scholte waves through a series of 2D and 3D full‐wavefield numerical simulations. We show that water phase reverberations greatly contribute to the generation of Scholte waves on the ocean floor. This study demonstrates the potential of DAS to observe and better understand a poorly known marine wave phenomenon and image the offshore shallow seismic structure with an unprecedented spatial resolution. Plain Language Summary: Distributed acoustic sensing (DAS) is a measurement technique that has recently demonstrated its utility for marine geophysics. DAS offers the possibility to observe the seismic wave at a scale and an extent previously unattainable with traditional passive seismic surveys. Here, we use a linear DAS array located offshore the Sanriku Coast, Japan. We propose a processing pipeline to extract surface waves from local earthquake DAS records and obtain hundreds of measurements along the cable. These measurements are used to infer a high‐resolution model of the near‐shore subsurface. Supported by a series of numerical simulations, we suggest that acoustic reverberations in the water column greatly contribute to the generation of surface waves on the ocean floor. This study further demonstrates that DAS can be used to understand marine wave phenomena better and image offshore seismic structures. Key Points: Passive high‐resolution Vs imaging of shallow sedimentary layers with ocean‐bottom distributed acoustic sensing and low‐magnitude earthquakesGridded slant‐stack method to extract Scholte waves from local earthquake wavefieldsFull‐wavefield simulations suggest that water phase reverberations generate Scholte waves at the ocean floor [ABSTRACT FROM AUTHOR]

An earthquake with a magnitude of 6.7 occurred in the Japan Sea off Yamagata on June 18, 2019. The mainshock had a source mechanism of reverse-fault type with a compression axis of WNW–ESE direction. Since the source area is positioned in a marine area, seafloor seismic observation is indispensable for obtaining the precise distribution of the aftershocks. The source area has a water depth of less than 100 m, and fishing activity is high. It is difficult to perform aftershock observation using ordinary free-fall pop-up type ocean bottom seismometers (OBSs). We developed a simple anchored-buoy type OBS for shallow water depths and performed the seafloor observation using this. The seafloor seismic unit had three-component seismometers and a hydrophone. Two orthogonal tiltmeters and an azimuth meter monitored the attitude of the package. For seismic observation at shallow water depth, we concluded that an anchored-buoy system would have the advantage of avoiding accidents. Our anchored-buoy OBS was based on a system used in fisheries. We deployed three anchored-buoy OBSs in the source region where the water depth was approximately 80 m on July 5, 2019, and two of the OBSs were recovered on July 13, 2019. Temporary land seismic stations with a three-component seismometer were also installed. The arrival times of P- and S-waves were read from the records of the OBSs and land stations, and we located hypocenters with correction for travel time. A preliminary location was performed using absolute travel time and final hypocenters were obtained using the double-difference method. The aftershocks were distributed at a depth range of 2.5 km to 10 km and along a plane dipping to the southeast. The plane formed by the aftershocks is consistent with the focal mechanism of the mainshock. The activity region of the aftershocks was positioned in the upper part of the upper crust. Focal mechanisms were estimated using the polarity of the first arrivals. Although many aftershocks had a reverse-fault focal mechanism similar to the focal solution of the mainshock, normal-fault type and strike–slip fault type focal mechanisms were also estimated. [ABSTRACT FROM AUTHOR]