Satoru Tanaka

Melt re-injection after a giant caldera eruption was quantitatively investigated for the Kikai Caldera Volcano in Japan, which erupted 7300 years ago (Kikai-Akahoya eruption). Our seismic refraction survey revealed a low-velocity anomaly directly beneath the Kikai Caldera Volcano, indicating the existence of a large magma reservoir at a shallow depth of 2.5–6 km. The reservoir can be approximated by a trapezoidal shape in this 2D section, with its width being at least the same as the width of the inner caldera, and its melt fraction was estimated as 3–6%, but could be limited to 10% at most. We propose a melt re-injection model in which new melt is re-injected into this large magma reservoir at the shallow depth just beneath the caldera, which is the same magma reservoir for the Kikai-Akahoya eruption. This model may demonstrate a common feature of volcanoes that have experienced a giant caldera eruption.

The Ontong Java Plateau (OJP) is believed to have formed through large‐scale Cretaceous volcanism, but the impact of the massive volcanism on the lithosphere remains largely unknown. In this study, the ascent process of the thermochemical plume and its impact on the physicochemical properties of the pre‐existing lithosphere were explored. High‐frequency seismic wave analysis revealed that the internal structure of the lithosphere beneath the OJP is a hybrid structure comprising dike swarms that are superimposed on the laminar structure. Moreover, the lithosphere exhibits lower seismic wave velocities than normal oceanic lithosphere, suggesting that the lithospheric mantle was physicochemically altered by the intrusion of dike swarms filled with magma from a large‐scale thermochemical plume. Our results advance the current understanding of the alterations in the lithosphere caused by a thermochemical plume.

Melt re-injection after a giant caldera eruption was quantitatively investigated for the Kikai Caldera Volcano in Japan, which erupted 7300 years ago (Kikai-Akahoya eruption). A seismic refraction survey was used to derive the P-wave velocity structure. Results revealed that the low-velocity anomaly was located directly beneath the central lava dome of the Kikai Caldera Volcano, allowing us to identify the existence of a giant magma reservoir with a melt fraction of 3–8%. The magma reservoir can be approximated by a trapezoidal shape in this 2D section, with upper and lower bases of 4 and 17 km, respectively, and a height of 4 km (2–6 km depth). Assuming that this magma reservoir is an axial object, its volume would be as large as 390 km ³ , indicating the large size of the magma reservoir; its total melt volume was calculated to be 21 km ³ . We therefore propose a melt re-injection model: the magma reservoir for the Kikai-Akahoya eruption was at the same location as the magma reservoir found in this study. Almost all the melt in this reservoir was ejected during the Kikai-Akahoya eruption; the new melt was re-injected into the same magma reservoir. Our estimated melt injection rate was more than 8.2 km ³ /kyr.

An increase in seismic activity was observed near the Izu-Torishima and Sofugan islands in the Izu-Ogasawara arc of Japan in early October 2023. Several M6-class earthquakes occurred from October 3 to 6, and tsunamis were recorded on October 5 and 6, associated with M6-class normal faulting earthquakes. On October 8, tsunamis were observed without any significant earthquakes but accompanied by distinct T-phase signals. Analyses of T-phases, tsunami waveforms, and bathymetry surveys suggested that the October 8 tsunamis were linked to the volcanic activity at the Sofu Seamount, located west of Sofugan Island. However, the precise location of the entire activity and its connection to the tsunamis on October 5 and 6 remain unclear. We conducted earthquake observations near the Sofu Seamount from November to December 2023 using ocean bottom seismographs (OBSs). These observations revealed that seismic activity extended approximately 50 km in the NNW direction, from the Sofu Seamount to the Torishima Rift, a back-arc rift between the Izu-Torishima and Sofugan Islands. Although it is difficult to constrain the hypocenter depths owing to the limited number of OBSs and their large separations, earthquakes in the Torishima Rift were likely located at depths of 15–20 km or even shallower, at approximately 5 km. The tsunami observed at Hachijo-jima Island on October 5 can be attributed to the rupture of a normal fault in the Torishima Rift, as inferred from OBS observations. Shallow normal faults and intrusions reaching the seafloor were previously imaged in the back-arc rift zone near the Torishima Rift. Moreover, the earthquake activity in early October showed a northward migration from the Sofu Seamount to the Torishima Rift, followed by a rapid decrease after the volcanic activity of the Sofu Seamount, which was accompanied by tsunamis on October 8. The results suggest that the seismic activity in early October 2023, accompanied by tsunamis, is likely related not only to the volcanic activity at the Sofu Seamount but also to the tectono-magmatic activity in the Torishima Rift. Graphical

We collected approximately 500 ScS–S differential travel times passing beneath the Philippines with various azimuths to discuss whether there were azimuthal variations in the ScS–S time residuals. By correcting for mantle heterogeneity using a three-dimensional (3D) mantle velocity model, we found a large variance reduction in the ScS–S residuals. In addition, the strong negative correlation between the S and ScS–S time residuals was greatly reduced, while the positive correlation between the ScS and ScS–S time residuals moderately improved, indicating that the corrected ScS–S residuals are manifestations of the lower half of the lower mantle structure. Next, we corrected for the local-scale heterogeneity in the lower mantle by subtracting the bin-averaged ScS–S residuals, and we experimented with fitting the trigonometric functions in terms of the propagation azimuth θ to the ScS–S residuals, suggesting that a 2θ variation is significant. If we accept the hypothesis of azimuthal anisotropy in the lowermost mantle, the fastest direction of the S-wave velocity was east-southeast–west-northwest (N97.5° E– N82.5° W), and the amplitude of the azimuthal anisotropy was approximately 1.4% anisotropy if we assume a D″ thickness of 300 km.

International Monitoring System hydrophone arrays are powerful tools for monitoring submarine volcanic activity. In this study, we routinely analyzed the signals of the triplet H11S, located off the coast of Wake Island, beginning in July 2022, to monitor submarine volcanic activity, particularly in the Izu–Bonin–Mariana arc. We removed the instrument response and applied a band-pass filter with cutoff frequencies of 4 and 8 Hz. Subsequently, the hydrophone signals were divided into 10 s time windows without overlapping. Semblance analysis was performed to determine the incoming direction and apparent velocity. The maximum obtained semblance values typically ranged from 0.5 to 0.9, and only values ≥0.7 were interpreted as significant. In our observation period (July 2022 to August 2024), we detected significant hydroacoustic signals from six sites (Ahyi, Daikoku, Kaitoku, Sofugan, Sumisu-jima, and Hachijo-jima) and compared them with those obtained from the past eruptions at Fukutoku-Oka-no-Ba. We discussed the characteristics of individual waveforms and spectrograms of some detected signals and found that frequency index values, defined as the ratio of mean spectral amplitudes between higher and lower frequency components, could be helpful in objectively discriminating the origins of hydroacoustic signals.

The structure of the lowermost mantle and the core‐mantle boundary (CMB) has profound implications for Earth's evolution and current‐day dynamics. Whilst tomographic studies of V _S show good agreement in the lowermost mantle, consensus as to V _P and especially CMB radius has not yet been reached. We perform a hierarchical Bayesian inversion for V _P in the lowermost 300 km of the mantle and the radius of the CMB using differential travel time data. Concurrent with finding V _P perturbations of 0.56% RMS amplitude that spatially agree with previous studies in areas of low posterior variance, we find 4.5 km RMS amplitude CMB radius perturbations with a broadly north‐south hemispherical character, with spherical harmonic power evenly distributed between degrees 1–3. These results suggest that CMB radial processes are set by a longer scale process than the V _P perturbations.

The structure of the lowermost mantle and the core-mantle boundary (CMB) has profound implications for Earth’s evolution and current-day dynamics. Whilst tomographic studies of Vs show good agreement in the lowermost mantle, consensus as to Vp and especially CMB radius has not yet been reached. We perform a hierarchical Bayesian inversion for Vp in the lowermost 300 km of the mantle and the radius of the core-mantle boundary using differential travel time data. Concurrent with finding Vp perturbations of 0.56% RMS amplitude that spatially agree with previous studies in areas of low posterior variance, we find 4.5 km RMS amplitude core-mantle boundary radius perturbations with a broadly north-south hemispherical character, with spherical harmonic power evenly distributed between degrees 1-3. These results suggest that CMB radial processes are set by a longer scale process than the Vp perturbations.

The Ontong Java Plateau in the western Pacific Ocean is the world’s largest oceanic plateau. It was formed 122 million years ago by a massive volcanic event that significantly affected Earth’s environment. The cause of the magmatic event remains controversial because the upper mantle structure beneath the plateau is poorly known. Here we use passive seismic data obtained through seafloor observations, alongside existing seismic data, to determine the three-dimensional radially anisotropic shear wave velocity to depths of up to 300 km. We find that the lithosphere–asthenosphere boundary is approximately 40 km deeper beneath the centre of the Ontong Java Plateau than beneath the surrounding seafloor. Based on our results and petrological and rheological constraints, we propose that the lithosphere–asthenosphere boundary has deepened as a result of underplating of dehydrated residual material beneath the pre-existing lithosphere during formation of the Ontong Java Plateau by a thermochemical mantle plume.

Strong long‐period seismic signals at periods around 25 and 18 s appear in the ambient noise cross‐correlation functions recorded by an array of ocean bottom seismometers (OBSs) in the western‐central Pacific. The signal amplitude varies from time to time, and the apparent travel times of the signals are typically smaller than those expected for the Rayleigh waves propagating along the great circle connecting the station pairs. From the cross‐correlation functions, the signal sources are located in the Vanuatu Arc. Local data analysis suggests the signals originate from two different sources possibly located at depths of ~0–1 km below the sea level beneath the active cones of Ambrym volcano.