On 8 October 2023 UTC, significant tsunamis were observed around Japan without any major tsunamigenic earthquake, associated with a series of 14 successive minor earthquakes (mb = 4.5–5.4) near Sofugan in the Izu-Bonin islands. To examine the cause of this tsunami, we estimated the horizontal locations of the tsunami source and temporal history of the seafloor displacement, using the tsunami data recorded by the ocean-bottom pressure gauges > ~600 km away. Our results showed the main tsunami source was an uplift located at a caldera-like bathymetric feature near Sofugan, suggesting the involvement of caldera activity in the tsunami generation. The total seafloor uplift was larger than ~3 m, and the uplift amount of each event gradually increased over time, reflecting an accelerating occurrence of multiple sudden caldera uplifts within only a few hours.

On 9 October 2023 (JST), mysterious tsunamis with a maximum wave height of 60 cm were observed in Izu Islands and southwestern Japan, although only seismic events of body-wave magnitudes mb 4–5 have been documented in the southwest of Torishima Island. To investigate the source process, we analyze tsunami waveforms recorded by an array network of ocean-bottom pressure gauges. A stacked waveform of 16 records suggests recurrent arrivals of multiple wave trains. Deconvolution of the stacked waveform by a tsunami waveform from the first event revealed over 10 source events that intermittently generated tsunamis for ~1.5 hours. The temporal history of this sequence corresponds to the origin times of T-phases estimated by an ocean-bottom seismometer, and the mb 4–5 seismic swarm, implying a common origin. Larger events later in the sequence occurred at intervals comparable to the tsunami wave period, causing amplification of later phases of the tsunami waves.

Ocean bottom pressure-gauge (OBP) records play an important role in seafloor geodesy, but oceanographic fluctuations in OBP data are a major source of noise in seafloor transient crustal deformation observations, including slow slip events (SSEs), so it is important to evaluate them properly. To extract the significant characteristics of the oceanographic fluctuations, we applied principal component analysis (PCA) to the 3-year Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) OBP time series for 40 stations during 2016–2019. PCA can separate several oceanographic signals based on the characteristics of their spatial distributions, although transient tectonic signals could not be clearly confirmed from the observed pressure records. The higher-order modes of the principal component reflected the oceanographic variation along the sea depth, and we interpreted that they were caused by the strength or weakness and meandering of ocean geostrophic currents, based on a comparison to the global ocean model ECCO2 by “Estimating the Circulation and Climate of the Ocean” (ECCO) consortium. In addition, to evaluate the ability of PCA to separate transient crustal deformation from oceanographic fluctuations, we conducted a synthetic test assuming an SSE by rectangular faults. The assumed synthetic tectonic signal can be separated from the oceanographic signals and included in the principal component independently depending on its amplitude. We proposed a transient event-detection method based on the spatial distribution variation of a specific principal component with or without a tectonic signal. This method can detect transient tectonic signals larger than moment-magnitude scale MW 5.9 from OBP records.  

On 15 January 2022, a massive underwater eruption occurred at the Hunga Tonga-Hunga Ha’apai Volcano. The plume reached the mesosphere, and the eruption excited a significant atmospheric Lamb wave, which forced the tsunami. The complicated tsunami waveforms due to ocean-atmosphere coupling prevented inferring the force history of the excitation. To address this, we analyze ocean surface gravity waves (OSWs) from 15 to 40 mHz, which are decoupled from the Lamb wave due to their slower phase velocities. Modeling these OSWs, we infer that the excitation started at 4:00 UTC with an amplitude of 1010 N and lasted for 5 hours, followed by a sub-event at 8:40 UTC. The observations suggest an initial blowout of seawater above the summit and a subsequent outflow that excited a tsunami below 5 mHz. The 2-hour delayed OSW excitation from 6 to 15 mHz may indicate seawater inflow into the crater.

An unexpected major tsunami from the region near the Sofu Seamount was observed on 8 October 2023. The Sofu Seamount is located approximately 600 km from the coast of Japan. Due to far epicentral distances and the successive occurrence of seismic events, the conventional seismic analysis to reveal the accompanying seismic sequence cannot work well. We investigated high-frequency teleseismic P and regional T waves from the accompanying seismic sequence during the tsunamigenic events near the Sofu Seamount. Envelope shapes of teleseismic P and regional T waves were similar, indicating that T-wave envelopes also reflected source properties of seismic sequence. During seismic events near the Sofu Seamount, observed regional envelopes were characterized by weak body waves and large amplitude T waves with durations of 39-68 s. According to numerical simulations of seismic wave propagation using a realistic topography model, characteristics of T waves exhibit weak slope-angle and strong source-depth dependencies. Strong T waves with durations less than 60 s only appeared in results with sources at depths ≤ 0.5 km below the seafloor. We concluded that high-frequency radiation of the accompanying seismic sequence during the tsunamigenic events near the Sofu Seamount possibly occurred at shallower depths just below the seafloor. If seismic and tsunami sources coincide, shallower source depths might cause tsunamigenic uplifts. The observed peak seafloor uplifts and T-wave amplitudes during tsunamigenic events were scaled. This result suggests the possibility of tsunami forecasting based on T-wave amplitudes from submarine volcanoes.

Tsunamis with maximum amplitudes of up to 40 cm, related to the Mw 7.1 normal-faulting earthquake off Fukushima, Japan, on November 21, 2016 (UTC), were clearly recorded by a new offshore wide and dense ocean bottom pressure gauge network, S-net, with high azimuthal coverage located closer to the focal area. We processed the S-net data and found that some stations included the tsunami-irrelevant drift and step signals. We then analyzed the S-net data to infer the tsunami source distribution. A subsidence region with a narrow spatial extent (~40 km) and a large peak (~200 cm) was obtained. The other near-coastal waveforms not used for the inversion analysis were also reproduced very well. Our fault model suggests that the stress drop of this earthquake is ~10 MPa, whereas the shear stress increase along the fault caused by the 2011 Tohoku earthquake was only ~2 MPa. Past studies have suggested that horizontal compressional stress around this region switched to horizontal extensional stress after the Tohoku earthquake due to the stress change.The present result, however, suggests that the horizontal extensional stress was locally predominant at the shallowest surface around this region even before the 2011 Tohoku earthquake. The present study demonstrates that the S-net high-azimuthal-coverage pressure data provides a significant constraint on the fault modeling, which enables us to discuss the stress regime within the overriding plate around the offshore region. Our analysis provides an implication for the crustal stress state, which is important for understanding the generation mechanisms of the intraplate earthquake.

Recent developments in ocean-bottom pressure gauge (OBP) networks have enabled us to continuously monitor various waves in the ocean. On 1 July, 2020, an OBP network, S-net, recorded tsunami-like pressure changes, although no earthquake was reported. These waves were well explained by a numerical simulation supposing a northward-moving atmospheric low pressure system with a maximum pressure depression of −0.5 ± 0.1 hPa and an apparent speed of 100–110 m/s. This simulation suggested that these waves were meteotsunamis. The simulation also suggested that the maximum amplitudes of the sea-surface height of ~ 2 cm were up to ~30% larger than those expected from the observed pressure if we do not consider the effect of the atmospheric pressure change. Our study showed that the S-net can detect the generation and propagation of meteotsunamis, which could not be achieved in the past when OBP networks with only a few stations were available.

Although most tsunamis are generated by the sea-bottom deformation caused by earthquakes, some tsunamis are excited by sea-surface pressure changes. This study theoretically investigated tsunamis generated by sea-surface pressure changes and derived the solutions in 3-D space, whereas most past studies employed 2-D equations. Using the solutions, we simulated and visualized the tsunami generation by a growing pressure change. Negative pressure change made the sea surface uplifted inside the source region and negative leading waves were radiated from the source region. We also simulated the tsunami generation when the pressure change at the sea surface moves with almost the same speed as the tsunami propagation velocity. The tsunami height increased with increasing the travel distance including the dispersion effects. The 3-D solutions in this study, including the vertical velocity distribution, indicate that both the tsunami height and the sea-surface pressure changes contribute to the ocean-bottom pressure changes, suggesting the difficulty in measuring the tsunami height with ocean-bottom pressure observations. When the pressure change source was characterized by short-wavelength components, the dispersion increased tsunami height more extensively than the non-dispersive tsunamis. The 3-D solutions are necessary for describing the tsunami generation where the long-wave approximations are not applicable in open oceans.

Recent studies have shown that ocean-bottom pressure gauges (OBPs) can record seismic waves in addition to tsunamis and seafloor permanent displacements, even if they are installed inside the focal area where the signals are extremely large. We developed a method to extract dynamic ground motion waveforms from near-field OBP data consisting of a complex mixture of various signals, based on an inversion analysis along with a theory of tsunami generation. We applied this method to the OBP data of the 2011 Tohoku-Oki earthquake. We successfully extracted the low-frequency vertical seismograms inside the focal area (f < ~0.05 Hz), although those of the Mw ~9.0 megathrust earthquake had never previously been reported. The seismograms suggested two dominant energy releases around the hypocenter. The seismic wave signals recorded by the near-field OBP will be important not only to reveal earthquake ruptures and tsunami generation processes but also to conduct real-time tsunami forecasts.