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.

Submarine calderas with active magma supply have recently been identified as potential sources of volcanic tsunamis due to sudden meter-scale uplift by trapdoor faulting, occurring every few years to a decade. These trapdoor uplifts are seismically recorded as non-double-couple earthquakes with magnitudes M > 5. Kita-Ioto Caldera, a submarine caldera in the Izu-Bonin arc, caused such earthquakes every 2-5 years. Our previous study (Sandanbata and Saito, 2024) analyzed data from a single ocean bottom pressure (OBP) gauge in the Philippine Sea, confirming trapdoor uplifts during the earthquakes in 2008 and 2015. However, high temporal-resolution data for the earthquakes in 2017 and 2019 were lost, preventing source mechanism investigation. To address this, we examine OBP data of the two recent earthquakes from the array network of DONET, deployed off the southwestern coast of Japan. Despite the poor signal-to-noise ratio in each record, we successfully detect clear tsunami signals associated with the earthquakes using a waveform stacking method, with sea-surface wave heights of only 1-2 mm. By analyzing the data, we propose source models that represent trapdoor uplifts in the submarine caldera and accurately reproduce the detected tsunami waveforms, confirming the recurrence of trapdoor uplifts. Notably, differences in the tsunami waveforms between the 2017 and 2019 earthquakes suggest that different segments of the intra-caldera fault system were activated. This segmentation likely influences the recurrence characteristics of the inflation cycle in calderas, which would be a key to understanding the magma accumulation process and assessing the sizes and timings of future trapdoor uplifts.

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.

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.

Two submarine earthquakes (Mw 5.8) occurred near volcanic islands, Curtis and Cheeseman, in the Kermadec Arc in 2009 and 2017. Following both earthquakes, similar tsunamis with wave heights of about a meter, larger than expected from their moderate seismic magnitudes, were observed by coastal tide gauges. We investigate the source mechanism for both earthquakes by analyzing tsunami and seismic data of the 2017 event. Tsunami waveform analysis indicates that the earthquake uplifted a submerged caldera around the islands. Combined analysis of tsunami and seismic data suggests that trapdoor faulting, or sudden intra-caldera fault slip interacted with magma reservoir deformation, occurred due to magma overpressure, possibly in association with caldera resurgence. The earthquake scaling relationship for trapdoor faulting events at three calderas deviates from that for regular earthquakes, possibly due to the fault-reservoir interaction in calderas.

Submarine volcano monitoring is vital for assessing volcanic hazards but challenging in remote and inaccessible environments. In the vicinity of Kita-Ioto Island, south of Japan, unusual M~5 non-double-couple volcanic earthquakes exhibited quasi-regular repetition near a submarine caldera. Following the 2008 earthquake, a distant ocean bottom pressure sensor recorded a distinct tsunami signal. In this study, we aim to find a source model of the tsunami-generating earthquake and quantify the pre-seismic magma overpressure within the caldera’s magma reservoir. Based on the earthquake’s atypical focal mechanism and efficient tsunami generation, we hypothesize that submarine trapdoor faulting occurred due to highly pressurized magma. To investigate this hypothesis, we establish a mechanical earthquake model that links pre-seismic magma overpressure to the size of the resulting trapdoor faulting, by considering stress interaction between a ring-fault system and a reservoir of the caldera. The model reproduces the observed tsunami waveform data. Our estimates indicate trapdoor faulting with large fault slip occurred in the critically stressed submarine caldera accommodating pre-seismic magma overpressure of ~10 MPa. The model infers that the earthquake partially reduced magma overpressure by 10–20%, indicating that the magmatic system maintained high stress levels even after the earthquake. Due to limited data, uncertainties persist, and alternative source geometries of trapdoor faulting could lead to estimate variations. These results suggest that magmatic systems beneath calderas are influenced much by intra-caldera fault systems. Monitoring and investigation of volcanic tsunamis and earthquakes help to obtain quantitative insights into submarine volcanism hidden under the ocean.

Characterizing the large M4.7+ seismic events during the 2018 Kīlauea eruption is important to understand the complex subsurface deformation at the Kīlauea summit. The first 12 events (May 17 - May 26) are associated with long-duration seismic signals and the remaining 50 events (May 29 - August 02) are accompanied by large-scale caldera collapses. Resolving the source location and mechanism is challenging because of the shallow source depth, significant non double-couple components, and complex velocity structure. We demonstrate that combining multiple geophysical data from broadband seismometers, accelerometers and infrasound is essential to resolve different aspects of the seismic source. Seismic moment tensor solutions using near-field summit stations show the early events are highly volumetric. Infrasound data and particle motion analysis identify the inflation source as the Halema’uma’u reservoir. For the later collapse events, two independent moment tensor inversions using local and global stations consistently show that asymmetric slips occur on inward-dipping normal faults along the northwest corner of the caldera. While the source mechanism from May 29 onwards is not fully resolvable seismically using far-field stations, infrasound records and simulations suggest there may be inflation during the collapse. The summit events are characterized by both inflation and asymmetric slip, which are consistent with geodetic data. Based on the location of the slip and microseismicity, the caldera may have failed in a ‘see-saw’ manner: small continuous slips in the form of microseismicity on the southeast corner of the caldera, compensated by large slips on the northwest during the large collapse events.

Moderate earthquakes (Mw > 5) with moment tensors (MTs) dominated by a vertical compensated-linear-vector-dipole (vertical-CLVD) component are often generated by dip slip along a curved ring-fault system at active volcanoes. However, relating their MTs to ring-fault parameters has been proved difficult. The objective of this study is to find a robust way of estimating some ring-fault parameters based on their MT solutions obtained from long-period seismic records. We first model the MTs of idealized ring-faulting and show that MT components representing the vertical-CLVD and vertical strike-slip mechanisms are resolvable by the deviatoric MT inversion using long-period seismic waves, whereas a component representing the vertical dip-slip mechanism is indeterminate owing to a shallow source depth. We then propose a new method for estimating the arc angle and orientation of ring-faulting using the two resolvable MT components. For validation, we study a vertical-CLVD earthquake that occurred during the 2005 volcanic activity at the Sierra Negra caldera, Galápagos Islands. The resolvable MT components are stably determined with long-period seismic waves, and our estimation of the ring-fault parameters is consistent with the ring-fault geometry identified by previous geodetic studies and field surveys. We also estimate ring-fault parameters of two earthquakes that took place during the 2018 activity at the caldera, revealing significant differences between the two earthquakes in terms of slip direction and location. These results show the usefulness of our method for estimating ring-fault parameters, enabling us to examine the kinematics and structures below active volcanoes with ring faults that are distributed globally.

The main cause of tsunamis is large subduction zone earthquakes with seismic magnitudes Mw > 7, but submarine volcanic processes can also generate tsunamis. At the submarine Sumisu caldera in the Izu–Bonin arc, moderate-sized earthquakes with Mw < 6 occur almost once a decade and cause meter-scale tsunamis. The source mechanism of the volcanic earthquakes is poorly understood. Here we use tsunami and seismic data for the recent 2015 event to show that abrupt uplift of the submarine caldera, with a large brittle rupture of the ring fault system due to overpressure in its magma reservoir, caused the earthquake and tsunami. This submarine trapdoor faulting mechanism can efficiently generate tsunamis due to large vertical seafloor displacements, but it inefficiently radiates long-period seismic waves. Similar seismic radiation patterns and tsunami waveforms due to repeated earthquakes indicate that continuous magma supply into the caldera induces quasi-regular trapdoor faulting. This mechanism of tsunami generation by submarine trapdoor faulting underscores the need to monitor submarine calderas for robust assessment of tsunami hazards.

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.