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.

The giant 2011 Tohoku earthquake (M9.0) could be expected to induce an 8-class outer-rise earthquake at the Japan Trench. In order to assess the risk of tsunamis from outer-rise events, we carried out tsunami simulations using 33 simple rectangular fault models with 60 degrees dips based on geophysical studies of the Japan Trench. The largest tsunami resulting from these models, a fault 332 km long producing a 8.66 normal-faulting event, had a maximum height of 27.0 m. We tested variations of the predictions due to the uncertainties in the assumed parameters. Because seismic observations and surveys show that the dip angles of outer-rise faults range from 45 to 75 degrees, we calculated tsunamis from events on fault models with 45-75 degree dips. We tested a compound fault model with 75 degrees dip in the upper half and 45 degrees dip in the lower half. Rake angles were varied by plus-minus 15 degrees. We also tested models consisting of small subfaults with dimensions of about 60 km, models using other earthquake scaling laws, and models including dispersive tsunami effects. Predicted tsunami heights changed by 5-10% for dip angle changes, about 5-10% from considering tsunami dispersion, about 2% from rake angle changes, and about 1% from using the model with subfaults. The use of different earthquake scaling laws changed predicted tsunami heights by about 50% on average for the 33 fault models. We emphasize that the earthquake scaling law used in tsunami predictions for outer-rise earthquakes should be chosen with great care.

Seismic velocity structures have been estimated using ambient noise records observed by distributed acoustic sensing (DAS) technology. Previous studies have obtained S-wave velocity (Vs) structures along submarine cables using surface waves; however P-wave velocity (Vp) structure have seldom been constructed because ambient noise primarily contains surface waves. Here, we show P and Rayleigh wave extractions from ambient seafloor noise observed by DAS, and also estimate the Vp and Vs structures along a submarine cable deployed off Cape Muroto in the Nankai subduction zone, Japan. To extract the P waves, we calculated the cross-correlation functions (CCFs) of the ambient noise and applied a frequency-wavenumber filter to the CCFs to remove the effects of slower surface waves. The results indicate that similar features can be obtained in the Vp and Vs profiles, and that the P wave extractions can be performed on days with poor weather conditions.

Seismic wave extractions have been performed using ambient noise records observed by distributed acoustic sensing (DAS) technology. Extractions of microseisms can be investigated at a local scale using such DAS records observed in the ocean. Here, we show P and Scholte wave extractions from ambient seafloor noise observed by DAS along a submarine cable deployed off Cape Muroto in the Nankai subduction zone, Japan. The P waves can be observed at a frequency band of 0.1–0.3 Hz and up to a distance of 6–7 km. The distance in which the P waves can be observed is controlled by the P incident angle and the DAS sensitivity to the observable apparent velocity. The effective extractions of P and Scholte waves are performed in correspondence to large significant wave heights of the sea surface, which indicate that these waves are originated from fluid distturbances at the sea surface.

At the northern Cascadia subduction zone, the subducting Explorer and Juan de Fuca plates interact across a transform deformation zone, known as the Nootka fault zone (NFZ). This study continues the Seafloor Earthquake Array Japan Canada Cascadia Experiment to a second phase (SeaJade II) consisting of nine months of recording of earthquakes using ocean-bottom and land-based seismometers. In addition to mapping the distribution of seismicity, including an MW 6.4 earthquake and aftershocks along the previously unknown Nootka Sequence Fault, we also conducted seismic tomography that delineates the geometry of the shallow subducting Explorer plate (ExP). We derived hundreds of high-quality focal mechanism solutions from the SeaJade II data. The mechanisms manifest a complex regional tectonic state, with normal faulting of the ExP west of the NFZ, left-lateral strike-slip behaviour of the NFZ, and reverse faulting within the overriding plate above the subducting Juan de Fuca plate. Using data from the combined SeaJade I and II catalogs, we have performed double-difference hypocentre relocations and found seismicity lineations to the southeast of, and oriented 18° clockwise from, the subducted NFZ, which we interpret to represent less active small faults off the primary faults of the NFZ. These lineations are not optimally oriented for shear failure in the regional stress field, which we inferred from averaged focal mechanism solutions, and may represent paleo-configurations of the NFZ. Further, active faults interpreted from seismicity lineations within the subducted plate, including the Nootka Sequence Fault, may have originated as conjugate faults within the paleo-NFZ.