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Slow earthquakes play a crucial role in understanding stress accumulation and release along plate interfaces in subduction zones. The northern Ryukyu Trench, where the Philippine Sea Plate subducts northwestward beneath the Eurasian Plate, experienced a major earthquake in 1911 and is currently regarded as a low-seismicity area (LSA). Understanding the seismic activity in this region, particularly the relationship between very-low-frequency earthquakes (VLFEs) and regular seismic events, is crucial for understanding subduction zone dynamics. We investigated the spatial and temporal distribution of VLFE activity in the northern Ryukyu Trench using broadband ocean-bottom seismometers deployed around Amami Island between September 2018 and June 2019. Our analysis, employing the envelope correlation method, revealed that VLFE activity is primarily concentrated northeast of Amami Island, an area characterized by low regular earthquake activity, with the distribution of VLFEs spatially segregated from that of regular earthquakes. Furthermore, we observed earthquake swarm activity at the edges of the LSA in the northern Ryukyu Trench following VLFE activity. In November 2018, intense VLFE activity northeast of Amami Island migrated northeastward, which was followed by a regular earthquake swarm at the edge of this LSA. Following VLFE activity in January 2019, additional seismic activity, including foreshocks, occurred at the edges of this LSA approximately 1 month later. The spatial segregation of VLFEs and regular earthquakes suggests that VLFE activity may be influenced by the migration of high-pressure fluids within the subducted slab. This migration appears to trigger related time-delayed seismic activity, similar to mechanisms observed in other subduction zones such as Hikurangi. Understanding these dynamics is essential for assessing the coupling state of subduction zones and associated fluid behaviors, which play a critical role in evaluating seismic hazards in LSAs. [ABSTRACT FROM AUTHOR]

To obtain novel information regarding normal faults that could cause tsunamis and large earthquakes on the outer slopes of the Kuril Trench, recently acquired multichannel seismic reflection (MCS) and bathymetric data are processed in this study to interpret the normal faults on the outer slope of the western Kuril Trench from near the Erimo Seamount to near the Nosappu Fracture Zone. This work also assesses the distribution and characteristics of the faults through comparisons with previous studies on the outer slope faults of the Japan Trench. The results of this study show that faulting starts approximately 100 km seaward of the trench axis, and a comparison with the normal fault distributions on the outer slope of the Japan Trench presented in previous studies reveals that the outer slope region of the Kuril Trench is narrower and more densely faulted than that of the Japan Trench. Additionally, the seismic and bathymetric data show that the Erimo and Takuyo-Daiichi Seamounts are deformed by normal faults. However, the number of faults in the seamount areas is lower than that in areas without them, which also affects the scale of the fault throws. The results of the MCS survey reveal that the thickness of the oceanic crust varies between 6 and 10 km due to the presence of these seamounts. Considering the positions of the outer swells presented in previous studies, the results suggest that the crustal and plate structures in the study area influence the distribution and formation of faults on the outer slope of the Kuril Trench. [ABSTRACT FROM AUTHOR]

The preface for the article collection "10 years after the 2011 Tohoku earthquake: a milestone of solid earth science" highlights the scientific discoveries made since the earthquake, thanks to near-field observations and existing knowledge about the northeastern Japan arc. The earthquake revealed dynamic frictional behaviors of the subduction interface and provided insights into subduction zone dynamics. The collection of research on the Tohoku earthquake is expected to benefit the understanding of subduction earthquakes worldwide. The preface also mentions various research papers included in the collection, covering topics such as coseismic stress drop, afterslip, seafloor observations, slow earthquake activity, tsunami deposits, paleoseismology, and numerical modeling of the earthquake cycle. Ongoing research on postseismic deformation and historical research on earthquake occurrence are also discussed. The authors express their appreciation to the reviewers and the editorial board for their contributions to the special issue. [Extracted from the article]

The southern Kuril Trench is one of the most seismically active regions in the world. In this study, marine surveys and observations were performed to construct fault models for possible outer-rise earthquakes. Seismic and seafloor bathymetric surveys indicated that the dip angle of the outer-rise fault was approximately 50°–80°, with a strike that was slightly oblique to the axis of the Kuril Trench. The maximum fault length was estimated to be ~ 260 km. Based on these findings, we proposed 17 fault models, with moment magnitudes ranging from 7.2 to 8.4. To numerically simulate tsunami, we solved two-dimensional dispersive wave and three-dimensional Euler equations using the outer-rise fault models. The results of both simulations yielded identical predictions for tsunami with short-wavelength components, resulting in significant dispersive deformations in the open ocean. We also found that tsunami generated by outer-rise earthquakes were affected by refraction and diffraction because of the source location beyond the trench axis. These findings can improve future predictions of tsunami hazards. [ABSTRACT FROM AUTHOR]

The westernmost Nankai Trough, southwest Japan, exhibits a rapid along-strike reduction in plate coupling in the proximity to the subducting Kyushu-Palau ridge. Yet how and to what extent the ridge subduction impacts physical properties at the megathrust have not been investigated. Here we present high-resolution seismic P-wave velocity models along the forearc wedge in the western Nankai Trough derived from full-waveform inversion analyses of seismic refraction data. The velocity models show that where the plate coupling is weak and the plate boundary presumably hosts slow earthquakes, the upper plate exhibits lower seismic velocities indicating higher degree of fracturing over a ~ 100 km length along trough. Intriguingly, the extent of the upper-plate low-velocity features is significantly larger than the surficial width of the Kyushu-Palau ridge, and this low-velocity zone is underthrust by the slab with increased crustal thickness by 2–4 km. Seismic reflection images consistently reveal that the thicker slab crust has appreciable basement roughness extending ~ 60 km from the eastern margin of the Kyushu-Palau ridge beneath the western Shikoku basin. We suggest that such a thicker and rugged slab crust, together with the main body of the Kyushu-Palau ridge, can cause significant fracture zones in the overriding plate, decrease the interplate coupling and produce preferable conditions for shallow slow earthquakes to occur when subducted. The results may also provide structural constraints on the western limit of future megathrust earthquakes in the Nankai Trough. [ABSTRACT FROM AUTHOR]

We examined the possible factors affecting the spatial distribution of very low frequency earthquakes and tremors in the shallow megathrust of Nankai Trough (<30 km) using a dense network of prestack depth migrated profiles at the frontal wedge. Geometrical parameters examined were decollement roughness, taper angle, and underthrust thickness. Physical properties such as effective basal friction (μb) and pore pressure ratio (λ*) were calculated from the taper angle and p‐wave velocity. Regions of low λ* (0.39 ± 0.08) and smooth decollement showed no slow earthquake activity. In contrast, high activity of slow earthquakes was observed in areas with a rough decollement due to the presence of subducted seamounts or bathymetric highs. The low taper angle (3.8°) off Muroto where slow earthquakes also occur translates to a wide zone of low μb (0.21 ± 0.06) and high λ* (0.66 ± 0.06). However, our results also show that slow earthquakes don't always occur in areas with high λ*. Plain Language Summary: Slow earthquakes such as very low frequency earthquakes and tremors are different than typical earthquakes because they occur longer and can last for more than several days. They have been reported in numerous subduction zones around the world and the study of slow earthquakes has recently gained attention because they have been suggested as precursors to larger earthquakes albeit on rare occasions. Slow earthquakes show a clustered distribution in the Nankai Trough. We used a dense network of seismic reflection data acquired at the shallow portion of the Nankai Trough to examine why some areas have high activity and some areas have no activity. Our results showed that areas with no slow earthquake activity have a smooth decollement and low pore fluid pressure. On the other hand, areas with high slow earthquake activity have a rough decollement. The occurrence of slow earthquakes has been typically associated with high pore fluid pressure because it allows the two slabs of rock to slide easier. However, our results suggest that high pore fluid pressure may not always be a prerequisite for slow earthquakes to occur. Key Points: A smooth decollement due to absence of subducted seamounts or bathymetric highs, and low pore fluid pressure results in a slow earthquake gapSlow earthquakes don't always occur in areas of high pore pressure but in combination with other variables like decollement roughnessThe low taper angle off Muroto indicates a wide zone of low friction and high pore pressure possibly associated with subducted seamounts [ABSTRACT FROM AUTHOR]

In shallow subduction zones, fluid behavior impacts various geodynamic processes capable of regulating slip behaviors and forming mud volcanoes. However, evidence of structures that control the fluid transfer within an overriding plate is limited and the physical properties at the source faults of slow earthquakes are poorly understood. Here we present high-resolution seismic velocity models and reflection images of the Hyuga-nada area, Japan, where the Kyushu-Palau ridge subducts. We image distinct kilometer-wide columns in the upper plate with reduced velocities that extend vertically from the seafloor down to 10–13 km depth. We interpret the low-velocity columns as damaged zones caused by seamount subduction and suggest that they serve as conduits, facilitating vertical fluid migration from the plate boundary. The lateral variation in upper-plate velocity and seismic reflectivity along the plate boundary correlates with the distribution of slow earthquakes, indicating that the upper-plate drainage system controls the complex pattern of seismic slip at subduction faults. Vertical fluid pathways in the upper plate of the Hyuga-nada subduction zone, Japan, facilitate upward fluid migration from the plate boundary that host slow earthquakes and produce seafloor mud volcanoes. [ABSTRACT FROM AUTHOR]

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, which 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. [ABSTRACT FROM AUTHOR]

Since the 2011 Mw 9.0 Tohoku-oki earthquake, intra-plate normal-faulting earthquakes, including several M7-class earthquakes, have occurred in the outer trench slope area from the trench to the outer rise along the Japan Trench. Concerns regarding large earthquakes and associated tsunamis have also arisen. Based on aftershock distributions, several outer trench slope normal-faulting earthquakes (hereinafter referred to as outer-rise earthquakes) are likely related to the rupture of multiple faults. However, few observations have clearly shown how multiple faults act during outer-rise earthquakes. During the ocean bottom seismograph (OBS) observations in the outer trench slope area of the central Japan Trench from September 2017 to July 2018, three M6-class normal-faulting earthquakes (Mw 6.2 on September 20, Mw 6.2 on October 06, and Mw 6.0 on November 12) occurred around the OBS network. The near-field OBS observations provided detailed information on hypocenter locations and focal mechanisms of the mainshocks and aftershocks, including immediately after the mainshocks. We investigated the fault configurations of normal-faulting earthquakes based on OBS observations. During the September 2017 earthquake, the mainshock ruptured high-angle normal faults with a dip angle of 65°. Off-fault aftershock activities that were not directly related to the mainshock rupture and could be explained by the stress changes caused by the mainshock were confirmed. However, hypocenter distributions and focal mechanisms of the main and aftershocks of the October and November 2017 earthquakes suggest that the mainshock ruptured multiple faults with various dipping directions, angles, and strike orientations. The complicated fault geometry should be considered a possible fault model for large outer-rise earthquakes and related tsunamis. [ABSTRACT FROM AUTHOR]

The 2011 M w 9.0 Tohoku-oki earthquake ruptured the subduction megathrust fault in the central Japan Trench. We investigated the aftershock activity in the southern Japan Trench to the south of the main rupture area using ocean bottom seismographs deployed both landward and seaward of the trench. In the trench-outer rise region seaward of the trench axis, we identified several ∼100-km-long linear earthquake trends both parallel and oblique to the southern Japan Trench. The earthquake trend oblique to the southern Japan Trench is a southward extension of the trench-parallel linear earthquake trend in the central to northern Japan Trench. The trench-parallel normal-faults in the trench-outer rise region could extend linearly, despite the change of the trench strike from N–S to NNE–SSW to the south of the main rupture area. Normal-faults oblique to the trench should be considered as substantial parts of large intraplate normal-faulting earthquakes. In addition, intraplate seismicity coinciding with the lower velocity oceanic mantle suggest that the structure heterogeneity would be indicative of normal-faults extending into the mantle. In the trench landward area, earthquake activity showed along-trench variations. Earthquakes along the shallow megathrust interface near the trench were observed south of 37°N. These shallow near-trench regular earthquakes, which are located close to the episodic tremors and temporally correlated with the tremor activities, suggest that the afterslip on the plate interface likely extended to the shallow plate interface close to the trench axis. Smaller spatial scale structure heterogeneity, such as the thickness variation in the channel-like low-velocity sedimentary unit, likely relate to the proximity of the regular earthquakes and slow slip which results in the formation of diverse slip behaviours in the shallow subduction zone of the southern Japan Trench. [ABSTRACT FROM AUTHOR]

We present a high‐resolution three‐dimensional (3‐D) anisotropic P wave velocity (Vp) model in the northern Hikurangi margin offshore Gisborne, New Zealand, constructed by tomographic inversion of over 430,000 first arrivals recorded by a dense grid of ocean bottom seismometers. Since the study area covers a region where shallow slow slip events (SSEs) occur repeatedly and the subduction of a seamount is proposed, it offers an ideal location to link our understanding of structural and hydrogeologic properties at megathrust faults to slip behavior. The Vp model reveals an ~30‐km‐wide, low‐velocity accretionary wedge at the toe of the overriding plate, where previous seismic reflection studies show a series of active thrust faults branching from the plate interface. We find some locations with significant Vp azimuthal anisotropy >5% near the branching faults and the deformation front. This finding suggests that the anisotropy is not ubiquitous and homogeneous within the overriding plate, but more localized in the vicinity of active thrust faults. The fast axes of Vp within the accretionary wedge are mostly oriented to the plate convergence direction, which is interpreted as preferentially oriented cracks in a compressional stress regime associated with plate subduction. We find that the magnitudes of anisotropy are roughly equivalent to values found at oceanic spreading centers, where the extensional stress regime is dominant and the crack density is expected to be higher than subduction zones. This consideration may indicate that additional effects such as fault foliation and clay mineral alignment also contribute to upper plate anisotropy along subduction margins. Plain Language Summary: We use man‐made sound waves (controlled‐source seismic data) to reveal in three dimensions the speed of sound within rocks and sediments that make up the northern Hikurangi margin off the east coast of North Island, New Zealand. We find that the sediments lying on top of subducting plate (accretionary wedge) have low velocities and host several branching faults. We also find that the speed of sound is faster when measured in the direction that the plates are colliding and slower when measured parallel to the faults (this phenomenon is called seismic anisotropy). This difference in speed suggests that the faults contain cracks that are aligned parallel to the direction of the plate subduction. Key Points: We present a three‐dimensional anisotropic P wave velocity model in northern Hikurangi from traveltime tomography using NZ3D OBS dataWe find evidence for fault‐bound anisotropy that azimuthal anisotropy is larger in the vicinity of the splay faults and deformation frontAnisotropy magnitude >5% may indicate preferentially oriented cracks and along‐fault clay‐rich layers contribute to upper plate anisotropy [ABSTRACT FROM AUTHOR]

To assess the risk of tsunamis from outer‐rise earthquakes, we carried out tsunami simulations using 33 simple rectangular fault models with 60° dip angles based on marine seismic observations and surveys of the Japan Trench. The largest tsunami resulting from these models, produced by a Mw 8.7 normal‐faulting event on a fault 332 km long, had a maximum height of 27.0 m. We tested variations of the predictions due to the uncertainties in the assumed parameters. Because the actual dip angles of the Japan Trench outer‐rise faults range from 45° to 75°, we calculated tsunamis from earthquakes on fault models with 45°, 60°, and 75° dip angles. We also tested a compound fault model with 75° dip in the upper half and 45° dip in the lower half. Rake angles were varied by ±15°. We also tested models consisting of small subfaults with dimensions of about 60 km, models using other earthquake scaling laws, models with heterogeneous slips, and models incorporating dispersive tsunami effects. Predicted tsunami heights changed by 10–15% for heterogeneous slips, up to 10% for varying dip angles, about 5–10% from considering tsunami dispersion, about 2% from varying rake angles, and about 1% from using the model with small 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. Key Points: Tsunamis were accurately predicted using well‐mapped outer‐rise normal faults along the Japan TrenchIt is necessary to carefully select the earthquake scaling law when predicting tsunamis from outer‐rise earthquakes.Heterogeneous slips have a large impact on tsunami predictions for outer‐rise earthquakes. [ABSTRACT FROM AUTHOR]

Shallow very low frequency earthquakes (sVLFEs) have occurred recurrently at the shallow plate interface of the Hyuga-nada region of the western Nankai subduction zone, Japan. Although the locations of sVLFE epicentres have been determined using land-based seismic records with moderate accuracy, it is necessary to determine their locations more precisely to explore the relationship between sVLFEs and other shallow slow earthquakes and examine the structural factors that may control sVLFE activity. Here, we identified sVLFE epicentres using seismic records obtained from temporarily deployed ocean bottom seismometers (OBSs) in the Hyuga-nada region. Seismic observations involved the deployment of 5–13 OBSs for approximately 1 yr, with deployments conducted three times during 2014–2016 each time with changing OBS numbers and array distribution. As a result, one sVLFE episode, containing successive Rayleigh wave pulses with slow velocities due to marine sediments, could be detected at a frequency band of 0.1–0.15 Hz per observation, resulting in a total of three episodes. Rayleigh wave amplitudes of ordinary earthquakes in the continuous records were suppressed using earthquake catalogues. We estimated the dispersion curve for the Rayleigh wave group velocity for each array, which represented the averaged group velocity within the array, using coda interferometry, and applied an envelope correlation method (ECM) using the group velocities to continuous records. These processing provided sVLFE epicentres with horizontal distance errors of <5 km. Our results showed that sVLFEs depths, which were inferred from the contour line of the top of the Phillipine Sea Plate, had increased from <10 km to 10–15 km in the region of the subducted Kyusyu-Palau Ridge (KPR). It was also apparent that migration of sVLFE epicentres occurred in 2015 from a depth of 15 km to shallower depths along the northern margin of the subducted KPR. These results identified the subducted KPR as a structural factor controlling the excitation conditions of sVLFE activities. [ABSTRACT FROM AUTHOR]

The strong ground motions, large crustal deformation, and tsunami generated by the 2011 Tohoku-oki earthquake (Mw 9.1) reveal that a large coseismic slip likely propagated to shallow depth in the Japan Trench. Although data acquired by onshore networks cannot resolve the slip behavior of the updip fault rupture, marine geophysical and geological studies provide direct evidence of coseismic slip to the trench. Differential bathymetry data show ∼50 m of coseismic seafloor displacement extending to the central Japan Trench (38–39.2°N). Seismic data show that coseismic slip ruptured the seafloor within the trench. Pelagic clays may have promoted slip propagation to shallow depths, whereas disturbed/metamorphosed clays may have restricted slip to the main rupture zone. Those observations imply that a smooth, broadly distributed, weak, clay-rich sediment in a shallow part of a subduction zone is a characteristic factor that can foster a large coseismic slip to the trench and, consequently, the generation of a large tsunami. ▪ During the 2011 Tohoku-oki earthquake (Mw 9.1), more than ∼50 m of slip occurred on a fault that ruptured the seafloor in the central Japan Trench. ▪ The fault rupture reaching the seafloor caused a large tsunami. ▪ Marine geophysical explorations revealed that a clay-rich sediment in the subduction zone was one factor fostering the large fault slip. ▪ Understanding of slip behavior in the shallow portion of a subduction zone will help us prepare for future large tsunamis along the Japan-Kuril Trench. [ABSTRACT FROM AUTHOR]

Earthquakes occur by overcoming fault friction; therefore, quantifying fault resistance is central to earthquake physics. Values for both static and dynamic friction are required, and the latter is especially difficult to determine on natural faults. However, large earthquakes provide signals that can determine friction in situ. The Japan Trench Fast Drilling Project (JFAST), an Integrated Ocean Discovery Program expedition, determined stresses by collecting data directly from the fault 1–2 years after the 2011 Mw 9.1 Tohoku earthquake. Geological, rheological, and geophysical data record stress before, during, and after the earthquake. Together, the observations imply that the shear strength during the earthquake was substantially below that predicted by the traditional Byerlee's law. Locally the stress drop appears near total, and stress reversal is plausible. Most solutions to the energy balance require off-fault deformation to account for dissipation during rupture. These observations make extreme coseismic weakening the preferred model for fault behavior. ▪ Determining the friction during an earthquake is required to understand when and where earthquakes occur. ▪ Drilling into the Tohoku fault showed that friction during the earthquake was low. ▪ Dynamic friction during the earthquake was lower than static friction. ▪ Complete stress drop is possible, and stress reversal is plausible. [ABSTRACT FROM AUTHOR]

The rupture areas of the large interplate thrust earthquakes along the Nankai trough, offshore southwestern Japan, are divided into several segments. The 1944 Tonankai earthquake ruptured one of the segments off of the Kii Peninsula. In 2005, we conducted an ocean-bottom seismograph experiment at the eastern end of the rupture area of the 1944 earthquake. Little seismic activity was observed on the plate interface; most earthquake activity was within the subducting and overriding plates. Aftershocks of the 2004 off Kii Peninsula earthquake, which was an intraplate earthquake in the subducting Philippine Sea plate, were mainly located in the subducting oceanic crust and uppermost mantle. However, several earthquakes at the eastern end of the rupture area of the 1944 earthquake were in the sedimentary wedge, with a focal mechanism indicating deformation by a subducting seamount. The earthquakes and faults in the sedimentary wedge show deformation related to the irregular topography of the subducting oceanic crust to the east of the rupture area of the 1944 Tonankai earthquake. In contrast, few earthquakes were observed in the sedimentary wedge in the rupture area of the 1944 earthquake. Difference in topography of the subducting oceanic crust in the two areas might have caused spatial variations of interplate coupling, which, in turn, caused the observed differences in the deformation of the sedimentary wedge. The spatial variation of interplate coupling may affect rupture propagation during large interplate earthquakes and cause the segmentation of large interplate earthquakes along the Nankai trough. [ABSTRACT FROM AUTHOR]

Silent-slip events have been detected at several subduction zones, but the cause of these events is unknown. Using seismic imaging, we detected a cause of the Tokai silent slip, which occurred at a presumed fault zone of a great earthquake. The seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down-dip of a subducted ridge. We propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip. [ABSTRACT FROM AUTHOR]

Along the Nankai trough, southwestern Japan, the Philippine Sea plate (PSP) is subducting beneath the Eurasian plate, and large interplate earthquakes have occurred repeatedly with a recurrence interval of about 100–200 years. The most recent large thrust event in the eastern Nankai trough off Kii Peninsula was the 1944 Tonankai earthquake. In this region, current seismicity is very low and hypocenters are not determined accurately by the land seismic network. We conducted microseismicity observations around the rupture area of the 1944 Tonankai earthquake using ocean bottom seismographs (OBSs). Hypocenters were determined using a 2-D seismic velocity structure model based on an airgun-OBS seismic survey. Results obtained show that the seismicity was relatively active near the trough axis. These earthquakes may relate to deformation of the subducting Philippine Sea plate. On the other hand, microseismicity in the rupture area of the 1944 Tonankai earthquake was very low. This low-level seismic activity in the co-seismic rupture area of the 1944 Tonankai earthquake likely relates to a single large asperity off Kii Peninsula. [Copyright &y& Elsevier]

The article presents the discussion on earthquake and tsunami causing enormous damage along the coast of northeast Japan in the Tohoku region including afterslip and viscoelastic relaxation of the mantle.

Seismic refraction and multi-channel seismic reflection surveys were conducted on the northeastern Hawaiian Arch to examine the effect of hotspot volcanism on the seismic structure of the crust and uppermost mantle. The crustal thickness deduced from the refraction data was typical for oceanic crust, which suggests that magmatic underplating does not occur, at least in our survey area, although the crustal seismic velocity may be influenced by flexure of the lithosphere on the arch. We identified high P-wave velocities (~ 8.65 km/s) in the uppermost mantle parallel to the paleo-seafloor spreading direction, which indicates that the shallower mantle structure immediately below the Moho preserves the original structure formed at a mid-ocean ridge. Moreover, we observed wide-angle reflection waves at large offsets in ocean-bottom seismometer records. The travel time analysis results showed that these waves were reflected from mantle reflectors at depths of 30-85 km below the seafloor, which are considered to represent heterogeneities consisting of frozen melts created during the cooling of the plate. [ABSTRACT FROM AUTHOR]

Megathrust earthquakes have occurred repeatedly at intervals of 100 to 150 years along the Nankai Trough, situated in the southwest of Japan. Given that it has been 70 years since the last event, the occurrence of the next devastating earthquake is anticipated in the near future. On April 1, 2016, a moderate earthquake (Mw 5.9, MJMA 6.5) occurred off the southeastern coast of Mie Prefecture in the source region of the 1944 Tonankai earthquake (Mw 8.2). In this study, we investigated the influence of the 2016 earthquake on future megathrust earthquakes. We first determined the hypocenter distributions using a precise velocity structure obtained from seismic surveys in the source region. Using data obtained from the DONET ocean-bottom observation network, we found that this earthquake occurred along the plate boundary fault, which is also believed to have slipped during past megathrust earthquakes. We then performed a preliminary numerical simulation to reproduce the occurrence of a moderate earthquake in the middle of a megathrust earthquake cycle. We used a hierarchical asperity model, in which smaller asperities causing moderate earthquakes are embedded in a hyper-asperity that serves as the source region of megathrust earthquakes. The simulation shows that moderate earthquakes, caused by ruptures of a smaller asperity, occur as a result of shrinkage of strongly coupled areas in the hyper-asperity. This result is consistent with the observation that the hypocenter of the 2016 earthquake was located at the edge of a strongly coupled region along the plate boundary. The simulation also reproduced postseismic slip (afterslip/slow slip) along the plate boundary updip of the hyper-asperity, which is consistent with the observations of slow slip events and very-low-frequency earthquakes after the 2016 earthquake. Thus, the occurrence of the moderate earthquake offshore southeastern Mie Prefecture in the middle of the megathrust earthquake cycle implies that the shrinkage of the strongly coupled area along the plate boundary is occurring as a preparatory process for the next megathrust earthquake in the region. [ABSTRACT FROM AUTHOR]

The coseismic slip during the 2011 Tohoku earthquake has been revealed to reach the Japan trench axis, but the detailed slip behaviors in different zones along the trench remain an unsolved problem. To investigate the along-trench structures that directly affect the coseismic activities, we collected densely distributed seismic reflection data south of the major coseismic slip zone in the region of 36–37.5°N. Our seismic data document numerous local structural variations along different segments of the study zone: varying thicknesses of incoming sediments; chaotic structures in the north and fold-and-thrust belts in the south at the wedge front; different morphologies of the subducting plate, correlating with the physical properties above the plate interface and influencing, in turn, shallow megathrust slip and tsunami genesis. The southern limit of the 2011 event large coseismic rupture zone at ∼37°N works as a transition zone, which appears to correlate with the landward extension of a subducting channel in the deep places, and the shallower detachment fault structures in the graben beneath the trench axis that are less developed than those in the large slip zone further north. It also appears to correlate with the different elastic properties along the basal layer of the overriding plate caused by regionally various thickness and lithologies. • The seismic data show the structural heterogeneities across the 2011 Tohoku earthquake southern boundary slip zone. • Variations of sediments thickness, prism deformation and subducting plate morphologies lead to different seismic behaviors. • Coseismic propagation weakening may relate to a transition zone without well-developed thrust faults in the trench. • The subcuting graben-related duplex channel features affect the physical properties of the plate interface. [ABSTRACT FROM AUTHOR]

Abstract: From 2011 to 2012, we conducted marine seismic surveys using a multichannel seismic reflection (MCS) system and ocean bottom seismometers (OBSs) in and around the hypocentral region of the 1983 Nihonkai–Chubu earthquake. The survey covered the area from the Japan coast of the Japan Sea to the Yamato Basin and the Japan Basin. We investigated the relationship between the Nihonkai–Chubu earthquake and the crustal structure obtained from seismic reflection imaging and the P-wave velocity structure near the hypocentral region. The focus in the present paper is on seismic data that were acquired using both the MCS system and OBSs. Based on the results, the crustal structure of the Japan Sea around the hypocentral region of the 1983 Nihonkai–Chubu earthquake is divided into three types: island arc crust, thick oceanic crust, and oceanic crust. The Nihonkai–Chubu earthquake occurred near the boundary between the thick oceanic crust and the island arc crust. There is a strong possibility that the east-dipping reflector detected in the crust was the source fault of the Nihonkai–Chubu earthquake. In addition, the location of anticlines formed by the east-dipping reverse fault occurring in the boundary of the crustal structure associated with the source fault of the Nihonkai–Chubu earthquake was confirmed in the range of latitude from to , although they were not found south of this segment. There is also a concentrated area of reverse faults and folds approximately 100 km west of the source region of the Nihonkai–Chubu earthquake. This area is located near the boundary between the thick oceanic crust and the oceanic crust. [Copyright &y& Elsevier]

Abstract: Many destructive earthquakes have occurred in the northern Japan trench region owing to the subduction of the Pacific plate. Two plate bending points, where the dip of the subducting plate changes markedly, have been found, and correlations between the bending points and the updip limits of large earthquake rupture zones have been previously found in the regions off Aomori ( N) and off Miyagi ( N). In this study, we investigate whether sharp plate bending is also present in the region off Iwate ( N), located approximately halfway between the off-Aomori and off-Miyagi regions. We analysed the existing off-Iwate wide-angle seismic experimental data by applying a new method, in which observed reflection phases are directly projected onto the depth–distance domain from the time–distance domain. As a result, we found a plate bending point in the region off Iwate; this is the first time such a point has been found there. Distances between the three bending points and the trench axis are almost uniform and sharp plate bending is a common feature in the northern Japan trench subduction zone. The bending axis, which is defined by connecting bending points, approximately coincides with the updip limit of the rupture zones of large interplate earthquakes. Seismic swarm activities have been observed inside the subducting oceanic crust along the bending axis, and we infer that some of these activities are caused by bending of the subducting plate. In addition, P-wave velocities of the upper plate vary across the bending axis, implying that the position of the bending axis is closely related to physical properties of the upper plate and to seismic activities. [Copyright &y& Elsevier]

We perform a simple subevent analysis of seismic P-waveforms from historical records of the 1946 Nankai earthquake. The results of this analysis establish that the earthquake rupture consisted of two main subevents, one lying within the 1-day aftershock area off the Kii Peninsula, while the other lies much farther to the west in the area of pronounced crustal deformation of western Shikoku. These conform to two segments of the Nankai Trough which have long been referred to in studies of historical earthquakes and geology of the Nankai Trough, but to our knowledge have not previously been established as having distinct megathrust rupture behavior. We show that these two subevents are bounded by pronounced features in the geometry of the subducting Philippine Sea Plate (PSP): a tear in the plate to the east beneath the Kii Peninsula, and a subducting seamount farther to the west. We speculate that these features may be associated with low-strength parts of the upper and/or lower plates that may be susceptible to permanent, anelastic deformation which prevents the accumulation of the elastic strain energy necessary to sustain seismic rupture. [Copyright &y& Elsevier]

We conduct seismic tomography to model subsurface seismicity between 2010 and 2012 and structural heterogeneity off the Kii Peninsula, southwestern Japan, and to investigate their relationships with segmentation of the Nankai and Tonankai seismogenic zones of the Nankai Trough. In order to constrain both the shallow and deep structure of the offshore seismogenic segments, we use both active- and passive-source data recorded by both ocean-bottom seismometers and land seismic stations. The relocated microearthquakes indicate a lack of seismic activity in the Tonankai seismogenic segment off Kumano, whereas there was active intraslab seismicity in the Kii Channel area of the Nankai seismogenic segment. Based on comparisons among the distribution of seismicity, age, and spreading rate of the subducting Philippine Sea plate, and the slip-deficit distribution, we conclude that seismicity in the subducting slab under the Kii Channel region nucleated from structures in the Philippine Sea slab that pre-date subduction and that fluids released by dehydration are related to decreased interplate coupling of these intraslab earthquakes. Our velocity model clearly shows the areal extent of two key structures reported in previous 2-D active-source surveys: a high-velocity zone beneath Cape Shionomisaki and a subducted seamount off Cape Muroto, both of which are roughly circular and of 15–20 km radius. The epicenters of the 1944 Tonankai and 1946 Nankai earthquakes are near the edge of the high-velocity body beneath Cape Shionomisaki, suggesting that this anomalous structure is related to the nucleation of these two earthquakes. We identify several other high- and low-velocity zones immediately above the plate boundary in the Tonankai and Nankai seismogenic segments. In comparison with the slip-deficit model, some of the low-velocity zones appear to correspond to an area of strong coupling. Our observations suggest that, unlike the Japan Trench subduction zone, in our study area there is not a simple correspondence between areas of large coseismic slip or strong interplate coupling and areas of high velocity in the overriding plate. [ABSTRACT FROM AUTHOR]

Abstract: The Nankai and Hyuga-nada seismogenic segments, in the western part of the Nankai subduction zone off southwestern Japan, have sometimes ruptured separately and sometimes simultaneously. To investigate the relationships among heterogeneities of seismic structure, spatial variation of the incoming plate, and the seismogenic segments, we carried out seismic observations in the western Nankai subduction zone and modeled the area with 3D seismic tomography using both onshore and offshore seismic data. Our seismic observations suggested that the pattern of seismicity is related to heterogeneities within the subducted plate rather than the seismogenic segments. The up-dip depth limit of seismicity along the plate boundary and in the oceanic crust is typically around 15 km, corresponding to the depth of dehydration of the oceanic crust. In addition, the seaward-extended seismicity observed where the subducted plate was considered to have rough internal structures. In the resulting velocity model, the up-dip limit of the area where the P-wave velocity just above the plate boundary exceeds 6 km/s corresponds to the up-dip limit of coseismic slip in the 1968 Hyuga-nada and 1946 Nankai earthquakes. Between the two coseismic rupture zones is an area of lower P-wave velocity about 40 km wide that is evidence of lateral heterogeneities in the upper plate along the trough-parallel direction. Structural heterogeneities in the upper plate may explain the variety of coseismic slip patterns in this region. [Copyright &y& Elsevier]

Abstract: We performed 3D seismic tomography of the Hyuga-nada region, western Nankai subduction zone, to investigate the relationship of the subducted part of Kyushu-Palau Ridge (KPR) to coseismic rupture propagation, seismicity, and shallow very low frequency earthquakes. Combining active-source and passive-source data recorded both onshore and offshore, we imaged the deep slab from near the trough axis to the coastal area. Our results show the subducted KPR as a low-velocity belt oriented NW–SE extending down the plate boundary to around 30km depth. At this depth, we suggest that the subducted KPR detaches from the slab and becomes underplated on the overriding continental plate. As the coseismic slip areas of past large earthquakes do not extend into the subducted KPR, we suggest that it may inhibit rupture propagation. The interior of the subducted KPR shows active intraslab seismicity with a wide depth distribution. Shallow very low frequency earthquakes are continuously active above the location of the subducted KPR, whereas they are intermittent to the northeast of the subducted KPR. Thus, the subducted KPR appears to be an important factor in coseismic rupture propagation and seismic phenomena in this region. [Copyright &y& Elsevier]

Abstract: High-resolution 3-D P-wave velocity structure at depths of 0–400 km was determined to investigate the structural heterogeneities and their implications for mountain building, plate collision and tectonics beneath Taiwan by simultaneously inverting a large number of arrival-time data from local and teleseismic earthquakes. In the joint seismic tomography, 227,258 absolute arrival times from 8982 local and regional earthquakes and 18,869 relative arrival-time data collected from 3179 teleseismic events were used. Our tomographic images provide direct geophysical evidence for a tectonic model proposed by previous studies and revealed some new features of structural heterogeneity related to the subducted Eurasian lithosphere. The seismic images show that the Philippine Sea slab descends northwestward from the Ryukyu Trench down to a depth of 200 km, showing agreement with the previous seismic, geochemical and geophysical studies. Low-velocity anomalies beneath the Kueishan volcano are revealed at depths of 0–65 km in North Taiwan, indicating its close relationship to the subducting process of the Philippine Sea slab. The Eurasian lithosphere is generally imaged as a high-velocity zone with a thickness of 65–80 km and it has subducted down to a depth of 300 km under South Taiwan, whilst it is not visible beneath North Taiwan. Low-velocity anomalies above the subducted continental lithosphere are observed at depths of 0–200 km, which might reflect, at least part of the subducted continental crust. Our tomographic results indicate that the plate convergence of the Eurasian plate varies from subduction beneath South Taiwan to collision with the Philippine Sea slab under North Taiwan. These features of the structural heterogeneities in the crust and upper mantle suggest that the mountain building process in the central part of Taiwan, arc magmatism in northeast offshore region and seismotectonics are mainly attributed to the subduction-collision configuration at the boundary between the Eurasian continental lithosphere and the subducting oceanic lithosphere of the Philippine Sea slab. [Copyright &y& Elsevier]

Abstract: We determined the three-dimensional Vp and Vs structures beneath Japan by applying seismic tomography to a large number of arrival times recorded at temporary stations in the Japan Sea and the Pacific Ocean, as well as those at permanent stations on the Japan Islands. As a result, we obtained more precise seismic images than previous studies. In the crust and the uppermost mantle, southwestern Honshu exhibited weaker heterogeneity than the other areas in Japan, corresponding to the distribution of active volcanoes. Stripe-like heterogeneities exist in the subducting Pacific slab. Relatively low-velocity zones correspond to low-seismicity areas in the Pacific slab, suggesting that the slab is possibly torn or thin around the areas. The fact that nonvolcanic deep tremors associated with the subducting Philippine Sea slab beneath Shikoku, Kii, and Tokai do not occur in zones of high Vp, high Vs, and low Vp/Vs ratio may reflect the existence of fluids generated by the dehydration processes of the slab. Prominent and wide low Vp and Vs zones exist beneath central Honshu at the depth range of 30–60km, where the volcanic front related to the subducting Pacific plate is located and seismicity around the Philippine Sea plate is very low. This condition may exist because magma genesis processes related to the subducting Pacific plate activate the same processes around the Philippine Sea plate. [Copyright &y& Elsevier]

We present the results of a seismic refraction experiment conducted off eastern Aomori, northeastern Japan forearc region, where the detailed crustal structure was poorly known. An objective analysis using high-quality survey data allowed us to obtain the seismic velocity structure to about 30 km depth, and in particular, to obtain a well-constrained geometry of the plate boundary in the source region of the 1994 Sanriku-oki earthquake. Our results show that the subducting Pacific oceanic plate increases its dip from about 5° to approximately 15° around the longitude of 143.5° E, 80 km landward of the trench axis near the latitude of 40° N. The location of this bend in the plate is 40 km east of the bending point previously proposed on the basis of hypocenter distribution. A cluster of aftershocks of the 1994 Sanriku-oki earthquake occurred around the bending point. This is because the bending of the subducting plate appears to affect the stress state in the vicinity of the plate boundary. Furthermore, the plate geometry is also considered to be related to the coupling or slip features along the plate boundary as the bending point correlates with the spatial distribution of repeating earthquakes. Our results will lead to more reliable determinations of asperity areas because an accurate dip angle of the fault plane is necessary for slip distribution studies. [Copyright &y& Elsevier]

Megathrust earthquake rupture is highly influenced by seafloor bathymetric features such as oceanic fracture zones, seamounts, and ridges in convergent plate margins where these features subduct. Recent seismic reflection and refraction data reveal a trough-parallel subducted oceanic ridge that is attached to the descending Philippine Sea Plate beneath the accretionary wedge of the overriding Eurasian Plate in the eastern Nankai Trough subduction zone consisting of the Tonankai and Tokai segments. The seismic survey data confirm the existence of subducted paleo-Zenisu ridge that was suggested by just magnetic data, and as a result, expands its distribution to the west. The newly constrained subducted ridge that spans the two segments is estimated to be a maximum of ∼2.5 km high, ∼20–30 km wide, and ∼200 km long. Spatial mapping of the ridge shows that it is located roughly at the seaward edge of the coseismic rupture zone of the 1944 Tonankai earthquake (M=8.1). This ridge appears to be in close contact with the seaward end of the rigid backstop in the Tonankai segment. Both the spatial correlation and the ridge–backstop collision geometry suggest that the subducted ridge might be strongly mechanically coupled and may thus play a significant role as a seaward barrier inhibiting the 1944 earthquake rupture from propagating farther seaward. [Copyright &y& Elsevier]

• Hypocenters within the subducted Explorer plate indicate slab deformation. • The oceanic slab is bending downward toward the northwest. • A complex sequence of focal mechanisms also indicates plate deformation. • Decreased seismic activity in the overriding plate indicates decoupling to the NW. • Deformation and decoupling could limit megathrust rupture propagation. At the northernmost extent of the Cascadia subduction zone, the Explorer plate subducts at approximately 2 cm/yr, less than half the rate of the Juan de Fuca plate to the south. The boundary between these two plates is known as the Nootka fault zone, which is one of the focuses of the Seafloor Earthquake Array Japan-Canada Cascadia Experiment (SeaJade). During this survey, an M W 6.4 earthquake occurred on 24 April 2014. This event and the subsequent aftershocks (referred to as the Nootka Sequence) reveal an approximately 40-km-long subducted fault within the Explorer Plate to the north of the Nootka fault zone. We infer that the fault is a subducted conjugate fault because of its nearly identical orientation to those seaward of the subduction front within the Nootka fault zone. The depth distribution and focal mechanisms of the aftershocks indicate significant margin-parallel deformation of the subducting plate. The subduction interface at the Nootka Sequence fault has been deflected downward to the northwest from a depth of approximately 15 – 25 km over a distance of 25 km. We propose two possible scenarios that are modified from previously suggested slab-tear model with induced margin-parallel mantle flow to explain the significant deformation of the young, warm subducting Explorer plate. To the northwest of this change in slab geometry, a lack of seismic activity above the plate interface indicates that the Explorer plate has partially decoupled from the overriding North America plate. We conclude that the geometric variation separating the southern Explorer plate from the north, along with decoupling and a possible intraslab tear, may be a significant combination to resist the propagation of a megathrust rupture across this boundary. [ABSTRACT FROM AUTHOR]