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Shearing along subduction zones, laboratory experiments on analogue faults, and sliding along glacier beds are all associated with aseismic and co-seismic slip. In this study, an ocean-bottom seismometer is deployed near the terminus of a Greenlandic tidewater glacier, effectively insulating the signal from the extremely noisy surface seismic wavefield. Continuous, tide-modulated tremor related to ice speed is recorded at the bed of the glacier. When noise interference (for example, due to strong winds) is low, the tremor is also confirmed via analysis of seismic waveforms from surface stations. The signal resembles the tectonic tremor commonly observed during slow-earthquake events in subduction zones. We propose that the glacier sliding velocity can be retrieved from the observed seismic noise. Our approach may open new opportunities for monitoring calving-front processes in one of the most difficult-to-access cryospheric environments. Anomalously slow earthquakes play a critical role in the earthquake cycle and fault sliding. Here, the authors detect continuous seismic radiation from a glacier sliding over its bed and show persistent coastal shaking to represent an addition to the family of slow earthquakes.

The 2024 Noto-Hanto earthquake with a magnitude of 7.6 occurred in the Noto Peninsula on January 1, 2024. The mainshock had a reverse fault focal solution and direction of compression axis was the northwest–southeast. In the Noto Peninsula, earthquake swarms have been observed since December 2020. In contrast to this swarm activity, the mainshock had extending to the marine area. Therefore, we performed a rapid response seafloor seismic observation in the source region and its vicinity. We deployed 34 free-fall pop-up type ocean bottom seismometers (OBSs) in January 2024, recovered 26 short-period OBSs (SPOBSs) after a month. The arrival times of the P- and S-waves were manually read from the data of SPOBSs and land seismic stations based on the event list by a land seismic network. We relocated the hypocenters of the events by combination of the location programs using absolute travel times with station corrections and the double-difference method. A velocity model was derived from the velocity structure by the marine seismic survey. Focal mechanisms were estimated using the grid search method based on the polarities of the first Pwave arrivals. The aftershock depths mostly ranged from 0.2 km to 17 km. Although the aftershock activity seems to be confined in the upper crust, relatively deep events occurred in the easternmost source region. The aftershocks formed several dipping planes corresponding to the multiple faults described in the offshore active fault model constructed before the mainshock. The upper boundaries of the planes of the hypocenter distribution coincide with the upper edges of the modeled faults, and the lower boundary of the aftershock distribution also aligns well with the lower edges of the faults. This consistency indicates that the rupture at the mainshock propagated to faults with different geometries. Although 70 events had a reverse fault focal mechanism similar to the mainshock, we identified 87 strike-slip events. Most of the events involving both reverse and strike-slip faults had P -axes perpendicular to the fault strike. This finding suggests that the aftershock activity was affected by a northwest–southeast compressional stress. Graphical Abstract

Aftershocks of the 2024 Noto earthquake exhibited a notable discrepancy between focal mechanisms estimated from P-wave polarities and those derived from routine moment tensor (MT) analyses based on waveform inversion of regional data. While first-motion solutions suggest prevalent strike-slip events, the MT solutions indicate thrust-type mechanisms. This study attributes the discrepancy to the influence of non-double-couple (non-DC) components in the MTs and verifies this hypothesis through Bayesian inversion of polarity and amplitude data. To ensure the reliability of amplitude information, we applied regression-based corrections for attenuation and site amplification, isolating the radiation patterns required for our MT inversion. Using data from ocean-bottom and onshore seismic stations, we obtained 79 well-constrained MT solutions for aftershocks. Many of these exhibit significant non-DC components, with positive isotropic and negative compensated linear vector dipole (CLVD) components being particularly common. We show that these non-DC components can bias first-motion solutions and lead to the misidentification of thrust-type events as strike-slip events. Our findings highlight the necessity of incorporating both amplitude data and non-DC components to accurately resolve focal mechanisms. The observed non-DC characteristics, particularly volumetric expansion implied by positive ISO components, may suggest involvement of deep fluid migration. However, the dominance of negative CLVD components cannot be explained by simple tensile crack opening alone, implying that the non-DC components originate from more complex faulting processes or the presence of seismic anisotropy. Graphical Abstract

The structure of the oceanic crust generated by the ultraslow-spreading mid-ocean Knipovich Ridge still remains relatively uninvestigated compared to the other North Atlantic spreading ridges further south. The complexity of the Knipovich Ridge, with its oblique ultraslow-spreading and segmentation, makes this end-member of Spreading Ridge Systems an important and challenging ridge to investigate. The Ocean Bottom Seismometer (OBS) data along a refraction/reflection profile (ca. 280 km) crossing the Knipovich Ridge off the western Barents Sea was acquired during cruise of RV G.O. Sars on July 24 to August 6, 2019. The seismic energy was emitted by air-guns with total volume of 80 l. To receive and record the seismic waves at the seafloor, ocean bottom seismometers were deployed. Seismic energy from airgun shots were recorded up to 50 km from the OBSs. The profile provides information on the seismic structure of the oceanic crust in the Knipovich Ridge area. Seismic record sections were analyzed with 2D trial-and-error forward seismic modeling. The crust thickness is variable and the Moho boundary depth changes between 7 and 12 km with P-wave velocity below the interface 7.9–8.0 km/s. The Moho discontinuity attains its minimum depth not directly beneath the Knipovich Ridge, but roughly 30 km to the southeast.

A huge plate boundary earthquake eventuated between the Pacific plate and the landward plate in 2011 and was named the 2011 off the Pacific coast of Tohoku earthquake. Following the mainshock, many aftershocks were generated. It is essential to obtain spatiotemporal variation of aftershock activity for understanding of mechanism of the earthquake generation. Because extensive seafloor aftershock observations using many ocean bottom seismometers (OBSs) were performed following the mainshock until September 2011, a precise aftershock distribution just after the mainshock had been revealed. After the urgent OBS aftershock observation, we started seismic monitoring using long-term OBSs (LTOBSs) in the source area. Thirty-nine LTOBSs were deployed in September 2011, and the observation had been continued for approximately ten months. In September 2013, thirty LTOBSs were deployed in the northern source region and recovered one year later. From long-term observations, spatiotemporal variation of the aftershock activity in the northern source region of the mainshock was revealed. Because we had carried out a LTOBS observation in the study area before the mainshock, the seismicity before the mainshock and aftershock activities was compared. Source positions of the events were determined by P and S-wave arrival times, and focal mechanisms of them were obtained using polarities of vertical components of the first arrivals. Large slip areas have little seismicity during the whole period of the observations. It is deduced from a low seismicity area that the stress in the large slip region had not been recovered 3 years after the mainshock. In contrast, a seismic activity become high just after the mainshock comparing the seismicity before the mainshock in the region off Iwate prefecture. Although many earthquakes in the inside of the plates had normal and strike-slip focal mechanisms from 2011 to 2012, some thrust focal mechanism events also occurred. A ratio of earthquakes occurring near the plate boundary with thrust focal mechanism seems to increase in 2013 and 2014 compared to 2011 and 2012. We interpret the increase in thrust-type earthquakes as indicative of stress change starting at the edge of the large slip region of the mainshock.

The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of the travel-time tomography analysis of the seismic profile across the caldera acquired in 2017 and 2018. A substantial structural difference in thickness in the subsurface low-velocity areas in the Aira caldera between the eastern and western sides, suggest that the Aira caldera comprises at least two calderas, identified as the AT and Wakamiko calderas. The most interesting feature of the caldera structure is the existence of a substantial high-velocity zone (HVZ) with a velocity of more than 6.8 km/s at depths of about 6–11 km beneath the central area of the AT caldera. Because no high ratio of P- to S-wave velocity zones in the depth range were detected from the previous three-dimensional velocity model beneath the AT caldera region, we infer that the HVZ is not an active magma reservoir but comprises a solidified and cool remnant. In addition, a poorly resolved low-velocity zone around 15 km in depth suggests the existence of a deep active magma reservoir. By superimposing the distribution of the known pressure sources derived from the observed ground inflation and the volcanic earthquake distribution onto the 2D velocity model, the magma transportation path in the crust was imaged. This image suggested that the HVZ plays an important role in magma transportation in the upper crust. Moreover, we estimated that the AT magma reservoir in the 30 ka Aira caldera-forming eruptions has the total volume of 490 km3 DRE and is distributed in a depth range of 4–11 km.

The Kuril Trench subduction zone is one of the most seismogenic regions, where underthrust earthquakes with M> 8 recur along the trench. The seismic gap between the source areas of the 1973 Nemuro‐oki and 2003 Tokachi‐oki earthquakes, which are typical underthrust earthquakes faulting with rupture velocities of ∼3 km/s, has been ruptured by the 1952 Tokachi‐oki earthquake. The seismic gap has also slipped incidental to neighboring asperities. The difference in slip pattern on the plate interface generally appears as a spatial difference in seismic structure on the plate interface, such as a reflectivity of the plate interface. We estimated the crustal velocity structure and analyzed the reflectivity of the plate interface to investigate the physical properties of the plate interface by performing an air gun–ocean bottom seismometer experiment on the along‐trench profile across the seismic gap. Strong reflections from the plate interface were observed in the 1952 Tokachi‐oki source area including the seismic gap, rather than in the 1973 Nemuro‐oki source area. The strong reflectivity of the plate interface in such the seismic gap with an incidental slip suggests that a slip pattern in the corresponding seismic gap would be conditionally stable. The coupling condition in the source areas of the eastern part of the source area of the 1952 earthquake is different from that in source areas of typical underthrust earthquakes, such as the 2003 Tokachi‐oki and 1973 Nemuro‐oki earthquakes. Our results suggest that the 1952 Tokachi‐oki earthquake was a complex earthquake with the characteristic of a tsunami earthquake. Key Points Reflectivity survey in the Kuril Trench subduction zone, SE off Hokkaido, Japan Strong variation of the reflectivity along the Kuril Trench Two features of the 1952 Tokachi‐oki earthquake: tsunami and typical underthrust

The ultra-slow, asymmetrically-spreading Knipovich Ridge is the northernmost part of the Mid Atlantic ridge system. In the autumn of 2002 a combined ocean-bottom seismometer multichannel seismic (OBS/MCS) and gravity survey along the spreading direction of the Knipovich Ridge was carried out. The main objective of the study was to gain an insight into the crustal structure and composition of what is assumed to be an amagmatic segment of oceanic crust. P-wave velocity and Vp/Vs models were built and complemented by a gravity model. The 190 km long transect reveals a much more complex crustal structure than anticipated. The magmatic crust is thinner than the global average of 7.1 ± 1.0 km. The young fractured portion of Oceanic Layer 2 has low seismic velocities while the older part has normal seismic velocities and is broken into several rotated fault blocks seen as thickness variations of Layer 2. The youngest part of Oceanic Layer 3 is also dominated by low velocities, indicative of fracturing, seawater circulation and thermal expansion. The remaining portion of Layer 3 exhibits inverse variations in thickness and seismic velocity. This is explained by a sequence of periods of faster spreading (estimated to be up to 8 mm/year from interpretation of magnetic anomalies) when more normal gabbroic crust was being generated and periods of slower spreading (5.5 mm/year) when amagmatic stretching and serpentinization of the upper mantle occurred, and crust composed of mixed gabbro and serpentinized mantle was generated. The volumetric changes and upward fluid migration, associated with the process of serpentinization in this part of the crust, caused disruption to the overlying sedimentary layers.

The Chuetsu-Oki Earthquake occurred on July 16, 2007. To understand the mechanism of earthquake generation, it is important to obtain a detailed seismic activity. Since the source region of the 2007 Chuetsu-oki Earthquake lies mainly offshore of Chuetsu region, a central part of Niigata Prefecture, it is difficult to estimate the geometry of faults using only the land seismic network data. A precise aftershock distribution is essential to determine the fault geometry of the mainshock. To obtain the detailed aftershock distribution of the 2007 Chuetsu-oki Earthquake, 32 Ocean Bottom Seismometers (OBSs) were deployed from July 25 to August 28 in and around the source region of the mainshock. In addition, a seismic survey using airguns and OBSs was carried out during the observation to obtain a seismic velocity structure below the observation area for precise hypocenter determination. Seven hundred and four aftershocks were recorded with high spatial resolution during the observation period using OBSs, temporally installed land seismic stations, and telemetered seismic land stations and were located using the double-difference method. Most of the aftershocks occurred in a depth range of 6–15 km, which corresponds to the 6-km/s layer. From the depth distribution of the hypocenters, the aftershocks occurred along a plane dipping to the southeast in the whole aftershock region. The dip angle of this plane is approximately 40°. This single plane with a dip to the southeast is considered to represent the fault plane of the mainshock. The regions where few aftershocks occurred are related to the asperities where large slip is estimated from the data of the mainshock. The OBS observation is indispensable to determine the precise depths of events which occur in offshore regions even close to a coast.

The Tokachi-Oki earthquake occurred on September 26, 2003. Precise aftershock distribution is important to understand the mechanism of this earthquake generation. To study the aftershock activity, we deployed forty-seven ocean bottom seismometers (OBSs) and two ocean bottom pressure meters (OBPs) at thirty-eight sites in the source region. We started the OBS observation four days after the mainshock for an observation period of approximately two months. In the middle of the observation period, nine OBSs near the epicenter of the mainshock were recovered to clarify the depth distribution of aftershocks near the mainshock. From the data overall OBS, seventy-four aftershocks were located with high spatial resolution. Most of the aftershocks were located in a depth range of 15–20 km and occurred within the subducting oceanic crust, the 5.5-km/s layer of the landward plate and the plate boundary. No aftershocks were found in the mantle of the subducting plate. The low seismic activity beneath the trench area where the water depth is greater than about 2000 m suggests a weak coupling between the two plates. The depth of the mainshock is inferred to be 15–20 km from the aftershock distribution.