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To investigate the dominant deformational patterns and stress conditions in the upper crustal structure of the Kinki region, southern-central Japan, we constructed a high-resolution 3D azimuthal anisotropy model to a depth of ~ 11 km. We used 6-month-long ambient noise data recorded by the densely distributed permanent and temporary stations. From this dataset, cross-correlations were retrieved. We then obtained a 3D isotropic velocity model by inverting Rayleigh wave dispersion data, followed by a direct joint inversion for both 3D azimuthal anisotropy and additional isotropic velocity perturbation. The resolved 3D azimuthal anisotropy reveals significant contrasts of anisotropy across the Kinki region. The predominant fast axes observed in the northwestern Kinki region align with the direction of the maximum horizontal compressional stress and the principal strain rate axes, suggesting that the observed anisotropy is mainly stress-induced. In the southern part of the study area, furthermore, the predominant fast axes trend NE–SW and near E–W, also indicating the presence of stress-induced anisotropy. On the depth profile of the anisotropy, we found depth-dependent variation of azimuthal anisotropy. There exists a significant consistency between the anisotropy observed beneath 3 km depth and the dense distribution of earthquake hypocenters. This interrelationship between anisotropy and seismicity demonstrates that the observed anisotropy could be linked to local crustal stress or fractures relevant to earthquake ruptures. Our 3D anisotropy model therefore contributes towards understanding the locations and features of the seismicity region. [ABSTRACT FROM AUTHOR]

Large megathrust ruptures can create notable tsunamis in tectonic back‐arc basins as was documented during the 2011 Tohoku earthquake in the Sea of Japan. We present a physical analysis of the excitation of back‐arc tsunamis by extending the fore‐arc deformation field from the earthquake centroid into the back‐arc basin and identify fault dip as the main geometrical contributor to the propagation of these events. As such, our theoretical model along with a large number of numerical simulations reveal that the dominant period of back‐arc tsunamis is different from that of the fore‐arc waves and thus they are a new class of tsunamis. Through numerical simulations and analysis of data from the 2011 Tohoku event, we show that a combination of near‐ to intermediate‐field horizontal and vertical deformation as well as transient surface waves is necessary to reconstruct the back‐arc propagation. We find that while seismic surface waves can affect coastal tsunami amplitudes in the back‐arc, their effect comes second to that of the horizontal component of deformation as manifested via bathymetric gradient. We then simulate back‐arc tsunamis and the hazard in the Sea of Japan from several potential future earthquake scenarios in the Japan Trench and Nankai Trough. Our results show that the coseismic excitation of back‐arc tsunamis can result in considerable waves close to 1 m in the Sea of Japan, near the Niigata Prefecture from megathrust earthquakes. Plain Language Summary: The 2011 Japan earthquake created a large tsunami in the Pacific Ocean. But it also made a moderate tsunami in the Sea of Japan, on the far side of the Japanese islands. We call these waves Back‐arc tsunamis and study their properties by looking at the contributions of various earthquake aspects including earthquake size, depth, fault geometry, and seismic surface waves, to the generation of these tsunamis. Our work shows that contrary to regular tsunamis, the hazard from back‐arc tsunamis is affected by the fault dip angle (how steep the fault is). We also find that large earthquakes in eastern Japan can create relatively large back‐arc tsunamis (with wave heights exceeding 1 m) in the Sea of Japan. Key Points: We present theoretical and numerical models of back‐arc tsunamis produced by megathrust earthquakesAmong source parameters, seismic moment and fault dip angles determine the dominant period and the spread of back‐arc tsunamisBack‐arc tsunamis from Japan Trench rupture scenarios can reach amplitudes >1 m in the Sea of Japan, and should be included in hazard models [ABSTRACT FROM AUTHOR]

Research interest in the Kinki region, southwestern Japan, has been aroused by the frequent occurrence of microearthquake activity that do not always coincide with documented active fault locations. Previous studies in the Kinki region focused mainly on deep, large-scale structures and could not efficiently resolve fine-scale (~ 10 km) shallow crustal structures. Hence, characterization of the upper crustal structure of this region at an improved spatial resolution is required. From the cross-correlation of the vertical components of the ambient seismic noise data recorded by a densely distributed seismic array, we estimated Rayleigh wave phase velocities using a frequency domain method. Then, we applied a direct surface wave tomographic method for the measured phase velocity dispersion data to obtain a 3D S-wave velocity model of the Kinki region. The estimated velocity model reveals a NE–SW trending low-velocity structure coinciding with the Niigata–Kobe Tectonic Zone (NKTZ) and the active Biwako-seigan Fault Zone (BSFZ). Also, we identified fine-scale low-velocity structures coinciding with known active faults on the eastern side of the NKTZ, as well as sets of low-velocity structures across the Tanba region. Furthermore, sedimentary basins manifest as low-velocity zones extending to depths ranging from ~ 1.5 to 2 km, correlating with those reported in previous studies. Our results therefore contribute towards fundamental understanding of earthquake faulting as well as tectonic boundary and will be useful for hazard assessment and disaster mitigation. [ABSTRACT FROM AUTHOR]

The reliable determination of earthquake source parameters is a relevant task of seismological investigations which ground nowadays on high-quality seismic waveforms collected by near-source dense arrays of ground motion sensors. Here, we propose a parametric modeling technique which analyzes the time-domain P-wave signal recorded in the near-source range of small-to-large-size earthquakes. Assuming a triangular moment rate function, a uniform speed, and circular rupture model, we develop the equations to estimate the seismic moment, rupture radius, and stress drop from the corner time and plateau level of the average logarithm of the P-wave displacement versus time curves (LPDT). The constant- $Q$ , anelastic attenuation effect, is accounted by a postprocessing procedure that evaluates the $Q$ -unperturbed moment rate triangular shape. The methodology has been validated through application to the acceleration records of the 2016–2017 Central Italy and 2007–2019 Japan earthquake sequences covering a wide moment magnitude range (Mw 2.5–6.5) and recording distance <100 km. After correcting for the anelastic attenuation function, the estimated average stress drop and the confidence interval ($\langle \Delta \sigma \rangle =0.60$ (0.42–0.87) MPa and $\langle \Delta \sigma \rangle =1.53$ (1.01–2.31) for crustal and subcrustal events of Japan and $\langle \Delta \sigma \rangle =0.36$ (0.30–0.44) MPa for Central Italy) show for both regions a self-similar, constant stress drop scaling of the rupture duration/radius with seismic moment. The smaller sensitivity of the spatially averaged, time-varying peak displacement amplitude to the radiation from localized high slip patch on the fracture surface could explain the retrieved smaller average stress drops for subcrustal earthquakes in Japan and $M >5.5$ events in Central Italy relative to previous estimates using spectral methods. [ABSTRACT FROM AUTHOR]

The ocean significantly influences the Earthʼs climate by exchanging heat, gas, and particles with the atmosphere. The radiation budget of the Earthʼs atmosphere is affected by physical, chemical, and biological processes in the atmospheric boundary layer over the ocean via cloud formation and CO2 absorption at the sea surface. This in turn influences the ocean through changes in temperature, precipitation, and solar radiation. The air‒sea boundary has a direct interaction with human life through the observation and prediction of extreme occurrences, such as typhoons and rogue surface waves, in addition to climate issues. Herein, this paper focuses on “the air‒sea boundary,” not limiting as “the air‒sea interface.” The air‒sea boundary encompasses a large vertical region, extending from the euphotic layer, where primary production occurs, to the troposphere. This paper also focuses on satellite observations relating to air‒sea boundary phenomena. Does the deposition of aerosols containing nitrogen, phosphorus, and iron contribute to primary production in terms of nutrient deposition from the atmosphere? What role does the sea surface microlayer play as an air‒sea interface, and what factors influence its physical properties? What information is required to calculate the amount of greenhouse gas and marine biogenic gas exchange? What types of marine aerosols influence cloud formation and the radiation budget? What approaches are required to comprehend the spatiotemporal distributions that affect the amount of matter and energy exchanged during wave processes? To answer these questions and help humans live in harmony with the natural environment, this paper presents a 10-year vision for necessary research and collaboration between the Oceanographic Society of Japan and neighboring societies. [ABSTRACT FROM AUTHOR]

Many significant geological features in central Japan reflect the history of tectonic events such as volcanic eruptions, fault ruptures, and the collision of the Izu‐Bonin arc. Because most previous studies focused on deep, large‐scale structures at relatively low resolution or on local structures using high‐resolution data (e.g., active source seismic data), tectonic structures over a wide area throughout central Japan have not been characterized in detail. In this study, we estimated 3D S wave velocity structures with high spatial resolution by extracting surface waves from 1‐year‐long ambient seismic data recorded by the Hi‐net high‐sensitivity seismograph network. We computed cross spectra of ambient noise data between station pairs and then extracted phase velocity dispersion curves using a frequency domain method. We developed an algorithm to calculate phase velocity and obtained numerous dispersion data. We then estimated the 3D S wave velocity structure by applying a direct surface‐wave inversion method. Our results clearly mapped heterogeneous features such as tectonic lines, sedimentary plains, and volcanic systems. We observed some low‐velocity anomalies below volcanoes. Active fault zones identified as low‐velocity zones were consistent with features on seismic reflection profiles. Several geological blocks that are related to the Izu collision were observed around the Izu Peninsula. We observed previously unreported geological features of the island of Honshu. Our high‐resolution S wave velocity model can be used for not only interpretation of geological structures but also hazard assessment of earthquake. Plain Language Summary: Techniques based on the speed of seismic waves are used to observe features of the Earth's interior in Japan, but most studies using them have focused on deep structures (tens or hundreds of kilometers deep) over a wide area or on local structures with high‐resolution data. The tectonic structures over a wide area throughout central Japan have therefore not been well characterized. Studies of detailed, shallow structures typically use artificial seismic waves; however, these are difficult to do over wide areas due to the cost, safety considerations. In this study, we used only passive sources, generated by ambient (background) vibrations of the Earth, to extract details of the shallow crust and construct a three‐dimensional geologic model based on seismic wave speeds in central Japan. Our results clearly mapped geological features such as tectonic lines, sedimentary plains, and volcanoes. An active fault zone was identified as a low‐speed zone that agreed with features identified in previous studies using active sources. Several geological blocks were identified around the Izu Peninsula, and the features observed on our results reflected processes of the tectonic evolution. Our results can also be used to predict how earthquake waves propagate and thereby contribute to disaster evaluation. Key Points: We estimated a 3D S wave velocity structure for central Japan using surface waves extracted from ambient noiseOur velocity model reflects small‐scale geological features and deformation over a wide areaLow‐velocity anomalies were observed that continue down the dip direction of active reverse faults [ABSTRACT FROM AUTHOR]

We applied advanced surface wave analysis for multichannel and multishot seismic data to estimate S-wave velocity structure with high lateral resolution. Although a horizontally layered structure is typically assumed in surface wave analysis, this assumption might be violated in environments such as geothermal fields because of their heterogeneous geological formation. The lateral variation of phase velocity can be effectively estimated with common midpoint (CMP) cross-correlation (CMPCC) analysis. In this study, we introduced two additional approaches into the CMPCC analysis workflow, to further improve lateral resolution of phase velocity estimates. One approach is window-controlled CMPCC analysis, which applies a spatial window for the CMP gathers while maintaining the accuracy of the phase velocity estimates. In this analysis, we found that it is difficult to improve phase velocity estimates at lower frequencies, as a wider spatial window must be kept to maintain the accuracy of the dispersion curve. Therefore, we introduced another approach in which we consider the direction of surface wave propagation from sources to receivers. Selection of cross-correlations based on source–CMP direction provides a significant improvement in dispersion curve resolution in the presence of lateral velocity variations, for a wide frequency range. We applied direction-controlled CMPCC analysis to seismic data acquired in the Yamagawa geothermal field, Kyushu Island, southwest Japan, and obtained dispersion curves in the heterogeneous geological setting. We then obtained S-wave velocity profiles by applying genetic algorithm inversion to the dispersion curves. The S-wave velocity profiles in the geothermal field resolve shallow and local heterogeneous structures (e.g. volcanic ash, pumice and igneous intrusions) that cannot be identified on reflection seismic profiles. [ABSTRACT FROM AUTHOR]

We reconstructed a 2D seismic wavefield and obtained its propagation properties by using the seismic gradiometry method together with dense observations of the Hi-net seismograph network in Japan. The seismic gradiometry method estimates the wave amplitude and its spatial derivative coefficients at any location from a discrete station record by using a Taylor series approximation. From the spatial derivatives in horizontal directions, the properties of a propagating wave packet, including the arrival direction, slowness, geometrical spreading, and radiation pattern can be obtained. In addition, by using spatial derivatives together with free-surface boundary conditions, the 2D vector elastic wavefield can be decomposed into divergence and rotation components. First, as a feasibility test, we performed an analysis with a synthetic seismogram dataset computed by a numerical simulation for a realistic 3D medium and the actual Hi-net station layout. We confirmed that the wave amplitude and its spatial derivatives were very well-reproduced for period bands longer than 25 s. Applications to a real large earthquake showed that the amplitude and phase of the wavefield were well reconstructed, along with slowness vector. The slowness of the reconstructed wavefield showed a clear contrast between body and surface waves and regional non-great-circle-path wave propagation, possibly owing to scattering. Slowness vectors together with divergence and rotation decomposition are expected to be useful for determining constituents of observed wavefields in inhomogeneous media. [ABSTRACT FROM AUTHOR]

Abstract: Three-dimensional shear wave speed structure of the upper mantle beneath the Japanese Islands and the surrounding regions is constructed from the measurements of inter-station phase speeds of surface waves. We employ the dense broad-band seismic network (F-net), deployed throughout Japan, as well as permanent stations of global seismic network in East Asia. A temporary broad-band seismic network in Far-East Russia, which has been constructed as a part of the Stagnant Slab Project since 2004, is also used in combination with the permanent stations. Using seismic events from 2005 to 2007 with moment magnitude greater than 6.0 and depth shallower than 100km, we measure the phase speeds of the fundamental-mode Rayleigh and Love waves, working with the conventional two-station technique. Phase speed maps of the Japanese Islands and the Sea of Japan are obtained from the inversions of inter-station phase speeds that are measured for a large number of station-pairs. We have collected over 5300 paths for Rayleigh waves and 3800 paths for Love waves. The phase speed maps of Rayleigh waves, in the period range from 25 to 140s, are inverted to obtain an isotropic 3D shear wave speed model in the depth range from 40 to 200km. The isotropic S wave model represents prominent fast wave speed anomalies in northeastern Japan, associated with the subducting Pacific Plate. Fast anomalies of the lithosphere beneath the Sea of Japan are also imaged, implying that the average thickness of lithosphere beneath the Yamato Rise and Japan Basin is about km. Immediately below this oceanic lithosphere, slow anomalies are found in the asthenosphere beneath the entire region of the Sea of Japan in the depth down to about 200km. In southwestern Japan, the low-angle subduction of the Philippine Sea Plate is mapped clearly, indicating that the northern end of the plate extends beyond the northern coast of the Chugoku region, reaching beneath the Oki Islands in the Sea of Japan. A strong low velocity anomaly off the west coast of Kyushu is found in the depth down to about 130km, which supports the existence of an upwelling flow beneath this area. We also invert the phase speed maps of Rayleigh and Love waves in the period range from 30 to 80s simultaneously, to construct the radially anisotropic shear wave speed model. A remarkable anisotropy with SH>SV is found in the mantle wedge beneath northeastern Japan, while only weak anisotropy is seen beneath the Sea of Japan. Such anisotropic properties, which cannot readily be obtained from body wave studies only, will be an important key to understanding the dynamic processes in this tectonically actively region. [Copyright &y& Elsevier]

Abstract: Experiments on the high-speed liquid lithium flow with free surface were conducted using the lithium circulation loop at Osaka University for the International Fusion Materials Irradiation Facility (IFMIF). The test section of the loop has a contraction nozzle which is 1/2.5 scale of IFMIF and produces a flat plane jet of 70mm width and 10mm thickness. In this study, velocity measurements on the liquid lithium flow were performed to develop a diagnostics system in view of its application on the IFMIF lithium target. The measurement invokes the Particle Image Velocimetry (PIV) technique. PIV is based on the statistical correlation of image subregions to determine the local flow velocity distribution. On the other hand, in this study, the local lithium flow velocity distribution was measured by tracking intensity patterns of the surface waves generated on the flow. Therefore, the measurable velocity consisted of the surface velocity and phase speed of the surface waves. As a result of the experiment, the surface-wave velocity was successfully measured. [Copyright &y& Elsevier]

Abstract: Experiments on the high-speed liquid lithium flow with free surface were conducted using the lithium circulation loop at Osaka University for International Fusion Materials Irradiation Facility (IFMIF). The test section of the loop has a contraction nozzle which is 1/2.5 scale of IFMIF and provides a flat plane jet of 70mm width and 10mm thickness. In the present study, new surface velocity measurement was tested, to develop a diagnostics system in view of its application on IFMIF lithium target. The measurement is based on particle image velocimetry (PIV) technique. Patterns of surface waves generated on the flow are tracked using a pair of images with short time delay with using an algorithm called gray level difference accumulation, and a velocity field is attempted to be calculated by correlating the two images. As a result, the surface velocity field was successfully measured even very close to the nozzle exit. It is expected that the present velocity measurement method could be applied to the monitoring and diagnostics system of the IFMIF target. [Copyright &y& Elsevier]