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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 estimated as ∼4 m, and the uplift amount of each event gradually increased over time, reflecting an accelerating occurrence of volcanic unrest of the submarine caldera within only a few hours. Plain Language Summary On 8 October 2023, a tsunami was widely observed along the Japanese coast without any major tsunamigenic earthquake, while a series of 14 small earthquakes occurred near Sofugan, located in the Izu‐Bonin Islands. Two possible candidates for this tsunami have been proposed, involving submarine volcanic processes or submarine landslides, but the exact cause remains unclarified. Using the tsunami data observed by the seafloor pressure gauges located more than 600 km from the tsunami source region, we analyzed sea height movements to obtain insights into the origin of this enigmatic tsunami. Our analysis showed that the tsunami source consisted of the seafloor uplift that repetitively occurred at a submarine volcanic caldera. Our results also showed an accelerating tsunami excitation, such that the amount of the seafloor uplift movement increased over time and the time intervals of the earthquakes gradually shortened. These results are consistent with the acceleration process of volcanic activity, suggesting the tsunami originated from the multiple sudden uplifts of the submarine caldera. Key Points We revealed the source kinematics of enigmatic tsunamis excited near Torishima on 8 October 2023 with remote (>∼600 km) tsunami data Its tsunami source was identified as repetitive seafloor uplift at the same location with gradually increasing amounts for later events This unique feature of the accelerating caldera uplift within a few hours was brought about by the volcanic unrest of a submarine caldera

Slow earthquakes that are observed in the > 1 Hz frequency band are called tectonic tremor or low-frequency earthquakes (LFEs) and those in the 0.01–0.10 Hz band are called very-low-frequency earthquakes (VLFEs). These two phenomena are separated by large microseismic noise at 0.1–1.0 Hz. However, recent observations of the signal in this microseismic frequency band accompanying LFEs and VLFEs in the shallow part of the Nankai subduction zone suggest that LFEs and VLFEs are parts of the same broadband phenomenon, “broadband slow earthquakes”. Here, we report the observation of slow earthquake signals in the microseismic frequency band in the western Shikoku region of the Nankai subduction zone, Japan, by stacking many seismograms relative to the timing of the high-frequency LFE signals. We relocate LFE events detected by the Japan Meteorological Agency, use these LFE waveforms to construct synthetic templates, perform a matched-filter event detection analysis using these templates, stack the seismograms recorded by broadband high-sensitivity accelerometers relative to the timing of the detections, and compare the amplitude of the stacked waveforms at different frequency bands. The stacked waveforms have a continuous signal in the 0.015625 Hz (64 s) to 8 Hz frequency band, and support the idea that LFEs are just a small part of the broadband slow earthquake spectrum, which extends to the VLFE frequency band. Furthermore, the frequency dependency of the maximum amplitudes in this study is similar to that of slow earthquakes in the Cascadia subduction zone, and this is also explained by a Brownian slow earthquake model. However, the frequency dependency is inconsistent with the omega-square model, which is a model for ordinary earthquakes.

The Nankai Trough in Southwest Japan exhibits a wide spectrum of fault slip, with long-term and short-term slow-slip events, slow and fast earthquakes, all associated with different segments down the plate interface. Frictional and viscous properties vary depending on rock type, temperature, and pressure. However, what controls the down-dip segmentation of the Nankai subduction zone megathrust and how the different domains of the subduction zone interact during the seismic cycle remains unclear. Here, we model a representative cross-section of the Nankai subduction zone offshore Shikoku Island where the frictional behavior is dictated by the structure and composition of the overriding plate. The intersections of the megathrust with the accretionary prism, arc crust, metamorphic belt, and upper mantle down to the asthenosphere constitute important domain boundaries that shape the characteristics of the seismic cycle. The mechanical interactions between neighboring fault segments and the impact from the long-term viscoelastic flow strongly modulate the recurrence pattern of earthquakes and slow-slip events. Afterslip penetrates down-dip and up-dip into slow-slip regions, leading to accelerated slow-slip cycles at depth and long-lasting creep waves in the accretionary prism. The trench-ward migrating locking boundary near the bottom of the seismogenic zone progressively increases the size of long-term slow-slip events during the interseismic period. Fault dynamics is complex and potentially tsunami-genic in the accretionary region due to low friction, off-fault deformation, and coupling with the seismogenic zone.

Slow slip events (SSEs) occur in the deeper extents of areas where large interplate earthquakes are expected in subduction zones, such as the Nankai region of Japan and the Cascadia region of North America. In the Nankai region, SSEs are divided into long‐ and short‐term SSEs, depending on their duration and recurrence interval. We modeled and examined the occurrence of long‐ and short‐term SSEs and changes in their behavior during the seismic cycles of large interplate earthquakes. In these numerical simulations we adopted a rate‐ and state‐dependent friction law with cutoff velocities and assumed that the distribution of pore fluid controls the recurrence interval of both long‐ and short‐term SSEs. The recurrence intervals of reproduced short‐term SSEs decrease during a long‐term SSE, as observed in western Shikoku, in the Nankai region. The recurrence intervals of both types of SSEs become shorter in the later stages of interseismic periods. Large interplate earthquakes nucleate between the region where SSEs occur and the locked region of the large earthquakes, as suggested from observations of the 1944 Tonankai earthquake. Our numerical results suggest that the stress buildup process in a seismic cycle affects the recurrence behavior of SSEs.

With a new location method, we obtained a high‐resolution map of non‐volcanic tremor along the subducting Philippine Sea plate in southwest Japan and clarified the depth‐dependent behavior of the tremor activity. A bimodal distribution of tremor along‐dip is apparent in regions where short‐term slow slip events (SSEs) and very‐low‐frequency earthquakes are frequently detected. The separation of bimodal peaks is 5 to 10 km in depth. The updip tremor activity occurs episodically, coincident with major bursts that often accompany geodetically detectable SSEs. The downdip tremor activity is, however, rather stable with frequent minor bursts. This depth‐dependent tremor activity likely reflects variation of interplate slip properties, specifically weakening with increasing depth and temperature. In westernmost Shikoku, anomalous tremor activity was detected in only the updip section in late 2003. This is interpreted as being triggered by a long‐term SSE in the shallower edge of the tremor zone.

We performed seismic waveform inversions and numerical landslide simulations of deep-seated landslides in Japan to understand the dynamic evolution of friction of the landslides. By comparing the forces obtained from a numerical simulation to those resolved from seismic waveform inversion, the coefficient of friction during sliding was well-constrained between 0.3 and 0.4 for landslides with volumes of 2–8 ×106 m3. We obtained similar coefficients of friction for landslides with similar scale and geology, and they are consistent with the empirical relationship between the volume and dynamic coefficient of friction obtained from the past studies. This hybrid method of the numerical simulation and seismic waveform inversion shows the possibility of reproducing or predicting the movement of a large-scale landslide. Our numerical simulation allows us to estimate the velocity distribution for each time step. The maximum velocity at the center of mass is 12–36 m/s and is proportional to the square root of the elevation change at the center of mass of the landslide body, which suggests that they can be estimated from the initial DEMs. About 20% of the total potential energy is transferred to the kinetic energy in our volume range. The combination of the seismic waveform inversion and the numerical simulation helps to obtain the well-constrained dynamic coefficients of friction and velocity distribution during sliding, which will be used in numerical models to estimate the hazard of potential landslides.

In subduction zones, slip deficit monitoring along the plate interface is important for understanding the seismogenesis of megathrust earthquakes. In the last two decades, aseismic slip transients, such as slow slip events (SSEs), which are usually synchronized with tectonic tremors, have been detected in subduction zones worldwide. Frequent SSEs are particularly important for releasing slip deficits during the inter-seismic periods of megathrust earthquakes. In southwest Japan, deep short-term SSEs have been primarily monitored with strain and tilt records because the SSEs in this region are small. However, strain and tilt records are so sensitive that they record not only SSEs, but also rainfall and local groundwater movements, which temporally affect the quality of data making it difficult to apply an automated detection algorithm. Therefore, previously reported short-term SSE catalogs, based on strain and tilt records, were created by visual inspections, although they are not suitable for generating a long-term catalog. In this study, a quantitative detection algorithm was developed to detect short-term SSEs using strain and tilt records. The problem of temporally varying data quality was solved by introducing the prior probability of log-normal distributions in the fitting variance. This method was applied to an 8-year (2013–2020) dataset of strains and tilts from southwest Japan. A total of 96 events were detected, among which, 78 corresponded with SSEs previously reported by the Geological Survey of Japan (GSJ). Although the GSJ catalog contained more events with smaller magnitudes, such events were difficult to distinguish from noise using the developed method. Three of the remaining 18 events were considered SSEs that were not reported in the GSJ catalog. Others could be artifacts because there were no obvious signals in the global navigation satellite system records (with events of magnitude > 6.0). Previous studies have suggested the existence of aseismic transients deeper or shallower than regular short-term SSEs in southwest Japan. However, detection results from this study did not confirm such events.

We investigated the migration mode of deep non‐volcanic tremor activity beneath Kii Peninsula, southwest Japan. Major tremor episodes are characterized by long‐term migration with a velocity of about 10 km/day, propagating along the strike of the subducting plate. Similar tremor migration in Cascadia is accompanied by reverse propagation at speeds on the order of 100 km/day and much faster slip‐parallel migration at speeds on the order of 1000 km/day. We systematically searched for migrating tremor with clear linearity in space and time. As a result, we found tremor migrations at speeds ranging from 1 to 60 km/hr depending on the along‐dip position in the tremor zone. The observed decrease in migration speed with increasing measurement time scale suggests that migration is controlled by a diffusion process. The along‐strike migration at lower speeds, including both forward and backward directions relative to the long‐term migration episode, is concentrated at the updip side of the tremor zone, whereas the faster slip‐parallel migration is distributed over the entire zone. The long‐term migration seems to consist of and be excited by the propagation of along‐strike creep at the updip part. The concentration of along‐strike migrating tremor sequences at the updip side may reflect the existence of abundant fluid that accumulates at the corner of the mantle wedge. The faster slip‐parallel migrations represent projections of along‐strike fluctuations in slip pulse propagation controlled by striations along the plate interface. Key Points Tremor migration is classified into two modes depending on the depth in the zone The updip‐most along‐strike tremor migration boosts the long‐term propagation The faster slip‐parallel migration and RTR reflect fluctuation in slip pulse

Very low frequency (VLF) seismic signals observed in southwestern Japan are evidently radiated from shear slips on the upper surface of the subducting Philippine Sea Plate. We used grid moment tensor inversion and centroid moment tensor inversion to calculate 242 moment tensor solutions with moment magnitudes between 3.1 and 3.8 from continuous seismograms recorded over a 5 year period by using a very dense broadband seismic network. At least 5–10 sequences of repetitive activity were observed during the 5 years. The VLF events formed clustered distributions along the 35 km isodepth contour of the subducting plate surface. The nodal planes (which dip landward) of moment tensor solutions of the VLF events reflected the configuration of the subducting plate interface. The slip vectors were consistent with the direction of movement of the subducting plate; the dip and strike of the slip vectors clearly reflected the configuration of the upper surface of the subducting plate. We found that the rates of seismic moment release per unit area associated with five major VLF clusters were very similar, although both the seismic magnitudes and sizes of the clusters varied considerably. The rate of seismic moment released from detectable VLF sources was 0.1% of the rates of short‐term slow slip events, suggesting that the source areas occupied only 0.1% of the fault segment on which the short‐term slow slip events occurred.

In the subduction zone of southwest Japan, short‐term slow slip events (SSEs) occur with low frequency tremors (LFTs) at intervals of several months. Recently LFTs have been located with high resolution and their activities have been examined in detail. By setting the generation zones of SSEs such that these zones contain the LFT hypocenters, we simulate SSEs on a 3D plate interface beneath the Kii Peninsula and Tokai regions by using a rate‐ and state‐dependent friction law with a small cut‐off velocity for the evolution effect. Our numerical results show that recurrence intervals of SSEs in the southern and central Kii Peninsula, in the northern Kii Peninsula, and in the Tokai region are 2.5–3.0, 4.8–5.6, and 3.5–4.5 months, respectively, which are consistent with observed SSE activity. Our simulation also produces a multisegment event that propagates from the Kii to Tokai segments at a speed of 10 km/day, which is consistent with observations. The results suggest that generation zones of LFTs coincide with SSE regions and that these two events are different manifestations of the same slip process along the subduction zone. We also perform 3D modeling of faster events accompanied by short‐term SSEs by considering local circular patches with a smaller critical displacement, surrounded by SSE zones with a larger critical displacement. Local circular patches are set at the observed LFT locations. The simulation reproduces fast events that propagate at a speed of 100–200 km/day during a single SSE event. Key Points Model slow slip events and local fast events in the Kii and Tokai regions Zones of slow slip events are set by the observed low‐frequency tremor locations Our model reproduces the observed activity of slow slip events well