Masayuki Obayashi

The 15 January 2022 submarine volcanic eruption of Hunga Tonga‐Hunga Ha'apai released immense energy throughout the ocean, solid Earth, and atmosphere. We analyze mid‐oceanic column acoustic pressure recordings from 24 freely drifting Mobile Earthquake Recorder in Marine Areas by Independent Divers sensors, and from 11 moored hydrophones in the International Monitoring System. We focus on the pulsed hydroacoustic phase which propagated horizontally through the ocean as a 30‐min T wave with energy around 2.5–10 Hz. The records show high correlation between some receivers, significant variation among others, and varying amplitudes that cannot be explained by distance alone. We investigate the origin of this heterogeneity via the influence of bathymetric features that may block, or occlude, T ‐wave propagation, affecting both shape and amplitude of the records received. We count the number of seafloor obstacles within the horizontal plane of the first (ray‐theoretical) Fresnel zone at a depth of 1,350 m, where the fundamental‐mode T ‐wave eigenfunction is maximal. Adjusted for geometric spreading, the cross‐correlations and sound pressure level differences between receivers systematically relate to differences in occlusion count. Our model of signal loss due to seafloor interactions predicts a 5.6 dB reduction in sound pressure level per logarithm of occlusion count, explaining 88% of the T ‐wave sound pressure variance across the ocean. Source characterization requires adequate path models. Our findings describe how to correct signal amplitudes for seafloor roughness. This is important for constraining volcanic or explosive yield estimates and earthquake magnitudes, and useful to model detectability through various oceanic corridors when designing hydroacoustic monitoring networks of the future.

The 15 January 2022 submarine volcanic eruption of Hunga Tonga‐Hunga Ha'apai released immense energy throughout the ocean, solid Earth, and atmosphere. We analyze mid‐oceanic column acoustic pressure recordings from 24 freely drifting Mobile Earthquake Recorder in Marine Areas by Independent Divers sensors, and from 11 moored hydrophones in the International Monitoring System. We focus on the pulsed hydroacoustic phase which propagated horizontally through the ocean as a 30‐min T wave with energy around 2.5–10 Hz. The records show high correlation between some receivers, significant variation among others, and varying amplitudes that cannot be explained by distance alone. We investigate the origin of this heterogeneity via the influence of bathymetric features that may block, or occlude, T ‐wave propagation, affecting both shape and amplitude of the records received. We count the number of seafloor obstacles within the horizontal plane of the first (ray‐theoretical) Fresnel zone at a depth of 1,350 m, where the fundamental‐mode T ‐wave eigenfunction is maximal. Adjusted for geometric spreading, the cross‐correlations and sound pressure level differences between receivers systematically relate to differences in occlusion count. Our model of signal loss due to seafloor interactions predicts a 5.6 dB reduction in sound pressure level per logarithm of occlusion count, explaining 88% of the T ‐wave sound pressure variance across the ocean. Source characterization requires adequate path models. Our findings describe how to correct signal amplitudes for seafloor roughness. This is important for constraining volcanic or explosive yield estimates and earthquake magnitudes, and useful to model detectability through various oceanic corridors when designing hydroacoustic monitoring networks of the future.

We have inverted a very large combination of delay times for P, pP, PcP, PKPab, PKPbc from the ISC-EHB catalog, augmented with hand-picked OBS and MERMAID onset data and crosscorrelation delays from broadband stations to derive a new global model, UNICA25, of P-wave velocity variations in the mantle. This paper presents the model, summarizes the data processing and inversion, and shows resolution tests at different scales. After rejecting 109 917 outliers (1% of the total), the data set comprises 10 571 152 arrival times. The misfit of the observed delays, with respect to a background model consisting of the crust of LITHO1.0 on top of model AK135, is on average 2.14 standard errors (σ), which is reduced to 0.99σ by inverting for P-wave velocity variations using both ray theory and finite frequency theory, and 5000 iterations of the iterative solver LSQR. Extensive resolution tests show a resolution at 500 km scale or better in most of the lower mantle. An upper mantle resolution of the order of 300 km is obtained under densely instrumented continents, but upper mantle resolution remains deficient under much of the oceans except were covered by MERMAIDs. Linearized tomography is at least three orders of magnitude faster than recent full waveform inversions (FWI), and reaches much higher frequencies for cross-correlated delays. Model UNICA25 has a P-velocity resolution that rivals that of FWI and shows a superior performance in fitting delay times, notably the ISC-EHB P-delays.

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.

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

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.

This study presents new seismic imaging of the Andean subduction zone through P‐wave hybrid finite‐frequency and ray‐theoretical tomography. We measured both differential and absolute traveltimes using broadband seismic waveforms from stations in an array of ocean‐bottom seismographs near the Chile Triple Junction (CTJ) and stations within 30° of the array. These data were combined with the global traveltime data set to obtain a global P‐wave velocity structure with a focus on central to southern South America. The new tomographic image showed the Nazca slab geometry as a continuous fast anomaly, which is consistent with seismic activity and prior slab models. Furthermore, two notable structures were observed: a broad extension of the fast anomaly beneath the Nazca slab at 26–35°S and a slow anomaly east of the CTJ. The checkerboard resolution and recovery tests confirmed the reliability of these large‐scale features. The fast anomaly, isolated from the Nazca slab, was interpreted as a relic Nazca slab segment based on its strong amplitude and spatial coincidence with the current Pampean and past Payenia flat slab segments. The slow anomaly near the CTJ was consistent with the previously inferred extent of the Patagonian slab window. Moreover, the active adakitic volcanoes are aligned with the southern edge of the anomaly, and the plateau basalts are located within the anomaly. Our model showed that the slow anomaly extended to a depth of up to 250 km, suggesting a depth limit that the asthenospheric window can influence.

On 8 October 2023 (UTC), unique earthquakes occurred in the Izu‐Ogasawara Arc, Japan, in which the P‐ and S‐phases were barely visible and only the T‐phases were evident, followed by tsunamis that reached islands in the Izu‐Ogasawara Arc and a wide area of the Pacific coast of southwest Japan. Our estimated T‐phase source area coincides with the Sofu Seamount, which was previously unrecognized as an active submarine volcano. A bathymetric survey of the seamount conducted 1 month after the event revealed characteristics of the seamount with a caldera and a central cone. Compared to the bathymetry in 1987, the topography in the caldera had changed significantly such as a crater forming in the central cone. This seamount is likely to be an active volcano. The topographic changes on the caldera‐sized scale that occurred at the caldera can be explained as a source of the October tsunami.

On October 8, 2023 (UTC), unique earthquakes occurred in the Izu-Ogasawara Arc, Japan, in which the P- and S-phases were barely visible and only the T-phase was evident, followed by tsunamis that reached islands in the Izu-Ogasawara Arc and a wide area of the Pacific coast of southwest Japan. In the location where the T-phase source was estimated, there is the Sofu Seamount which was previously unrecognized as an active submarine volcano. A bathymetric survey of the seamount conducted one month after the event revealed characteristics of the seamount with a caldera and a central cone. Compared to the bathymetry in 1987, the topography in the caldera had changed significantly such as a crater forming in the central cone. This seamount is likely to be an active volcano. The topographic changes on the caldera-sized scale that occurred at the caldera can be explained as a source of the October tsunami.

This study presents new seismic imaging of the Andean subduction zone through P-wave hybrid finite-frequency and ray-theoretical tomography. We measured both differential and absolute traveltimes using broadband seismic waveforms from stations in an array of ocean-bottom seismographs near the Chile Triple Junction (CTJ) and stations within 30° from the array. These data were combined with the global traveltime dataset to obtain a global P-wave velocity structure with a focus on central to southern South America. The new tomographic image showed the Nazca slab geometry as a continuous fast anomaly, which is consistent with seismic activity and prior slab models. Furthermore, two notable structures were observed: a broad extension of the fast anomaly beneath the Nazca slab at 26–35° S and a slow anomaly east of the CTJ. The checkerboard resolution and recovery tests confirmed the reliability of these large-scale features. The fast anomaly, isolated from the Nazca slab, was interpreted as a relic Nazca slab segment based on its strong amplitude and spatial coincidence with the current Pampean and past Payenia flat slab segments. The slow anomaly near the CTJ was consistent with the previously inferred extent of the Patagonian slab window. Moreover, the active adakitic volcanoes are aligned with the southern edge of the anomaly, and the plateau basalts are located within the anomaly. Our model showed that the slow anomaly extended to a depth of up to 250 km, suggesting a depth limit that the asthenospheric window can influence.