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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]

Devastating megathrust earthquakes and slow earthquakes both occur along subducting plate interfaces. These interplate seismic activities are strongly dependent on the nature of the plate interface, such as the shape of the plate interface and the materials and physical conditions along the plate interface. The oceanic plate, which is the input to the subduction zone, is the first order control on the nature of the plate interface. To reveal the nature of the subduction inputs to the northeastern Japan arc, we have conducted large-scale controlled-source seismic surveys of the northwestern part of the oceanic Pacific plate. The obtained seismic data have revealed (1) oceanic plate structural evolution caused by plate bending prior to subduction, suggesting the promotion of the oceanic plate hydration; (2) spatial variation of the oceanic plate structure, such as variations in the thickness of sediment and crust; (3) that the spatial variations are caused by both ancient plate formation processes and more recent volcanic activities; and (4) that spatial variations of the nature of the subduction inputs show good correlation with the along-strike variations in the seismic structure and seismic activities after subduction, including the coseismic slip distribution of the 2011 Tohoku earthquakes and the structural differences between the northern and the southern Japan Trench. These observations indicate that the incoming oceanic plate structure is much more spatially variable than previously thought and also imply that the spatial variation of the subduction inputs is a key controlling factor of the spatial variation of various processes in subduction zones, including interplate seismic activities and evolution of the forearc structure. [ABSTRACT FROM AUTHOR]

The structure of the incoming plate is an important element that is often considered to be related to the occurrence of great earthquakes in subduction zones. In the Japan Trench, where the 2011 Tohoku earthquake occurred, we collected seismic profiles along survey lines separated by 2–8 km to examine the structural characteristics of the incoming Pacific plate in detail. The average thickness of the incoming sediments was < 500 m along most of the Japan Trench, and it was < 300 m at ~ 38° N, where the large shallow megathrust slip occurred during the 2011 Tohoku earthquake. We mapped bending-related normal faults, including their dip direction and amount of throw. The numbers of eastward (oceanward) and westward (trenchward) dipping normal faults were generally comparable in the Japan Trench. Eastward dipping normal faults were dominant in the northern and southern parts of the Japan Trench, whereas westward dipping normal faults were more numerous in the central part. Graben-fill sediments deposited at the landward edge of the graben were bounded by eastward dipping normal faults. Trench-fill sediments were also observed along the trench axis. The sediment fills locally increased the thickness of the input sediments where they were deposited. The along-axis variation in input sediment and sediment fill distribution, and the variations in normal fault dip direction between the central and other parts of the Japan Trench may correspond to different slip styles along the plate boundary. [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]

Lateral spatial variations of weak portions at the plate boundary in subduction zones have been estimated primarily by the distribution of slow earthquakes mainly occurring around seismogenic zones. However, the detailed depth profile of weak faults remains elusive. Here, we deployed six ocean bottom seismometers in the Nankai subduction zone, Japan, to observe reflections originated from drilling vessel Chikyu ship noise (hydroacoustic P wave) that was persistently radiated from a fixed position at the sea surface, and retrieved P-to-s (Ps) reflections from multiple dipping faults near the plate boundary. The Ps amplitudes were stacked and compared according to the degrees of tidal stresses, and high amplitudes were observed at high tide (compression). A migration technique shows that the locations where velocity contrasts fluctuate were estimated at both the megasplay fault and another fault between the megasplay fault and the top of the oceanic crust. This indicates that the physical properties of these faults are altered by tidal stress. The physical-property changes are attributed to fluid connections and isolations within the faults due to tidal stress fluctuations, inducing the variation of seismic anisotropy. Such a variation was confirmed by a three-dimensional numerical simulation for wave propagation with anisotropic medium. Our observation suggests that multiple weak faults are present around the plate boundary, and the obtained changes of fault physical properties may have implications for in-depth understanding of tidal triggering of earthquakes. [ABSTRACT FROM AUTHOR]

Oceanic plates play an important role in transporting water from Earth's surface into the mantle. The trench–outer rise region is one of the places where hydration of the oceanic plate occurs. We investigated oceanic plate hydration in the trench–outer rise region of the Japan Trench on the basis of the seismic velocity structure as estimated by a tomographic analysis of data from ocean bottom seismograph arrays. This analysis demonstrates a reduction in P- wave and S- wave velocity (Vp and Vs) within the uppermost oceanic mantle of the incoming Pacific plate in the direction towards the trench axis. This seismic velocity reduction, which was observed in at least the uppermost 15 km of the mantle, is consistent with hydration of the uppermost mantle due to infiltration of water through the oceanic crust along bending-related normal faults. Under the assumption that the velocity reduction is caused solely by serpentinization of the mantle, the Vp changes suggest that water content increases by about 1.1 to 2.9 wt per cent at depths as great as 15 km below the oceanic Moho. An increase in water-filled fractures can also account for this velocity reduction with a smaller amount of water. Aside from the general trend of velocity reduction towards the trench axis, the Vp structure of the incoming plate shows spatial heterogeneity related to mantle seismicity, in that earthquakes within the uppermost mantle generally coincide with areas of low Vp. This spatial seismic heterogeneity is consistent with localized hydration or formation of water-filled fractures along bending-related normal faults. Hence, a low- Vp anomaly within the uppermost oceanic mantle could be an indicator of possible sites of large outer-rise normal-faulting earthquakes in which the rupture extends into the mantle. [ABSTRACT FROM AUTHOR]

Rainfall-induced failures of natural and artificial slopes such as cut slopes, which are subjected to freezing and thawing, have been frequently reported in Hokkaido, Japan. In particular, many failures occur intensively from spring to summer seasons. Despite numerous field studies, explanation of their mechanical behavior based on in situ data has not yet been completely achieved due to the difficulty in grasping failure conditions. This study aims at clarifying the aspects of in-situ volcanic slopes subjected to rainfall and freeze-thaw action. The changes in soilmoisture, pore pressure, deformations, and temperatures in the slope were investigated using soil moisture meters, tensiometers, thermocouple sensors, clinometers, settlement gauges, an anemovane, a snow gauge, and a rainfall gauge. The data generated from these measures indicated deformation in the slope examined mainly proceeded during the drainage process according to changes in soil moisture. Based on this data, a prediction method for failures is discussed in detail. [ABSTRACT FROM AUTHOR]

Abstract: The southwestern Kuril Arc, which is not affected by a series of accretion and collision processes, is one of the most important experimental sites among the world arc–trench systems for investigating the evolution of arc crust. Published petrological and seismic studies have shown that, in south-central Hokkaido, the upper part of the Kuril Arc crust is thrusting upward toward the west while the lower part is descending downward in the same direction. Among several hypotheses for formation of continental crust, we discuss the role of delamination of the island arc lower crust based on geochemical and seismic velocity structure models. Seismic velocity structure is obtained from an onshore–offshore seismic survey across the southwestern Kuril Arc–Trench system. The structure in the back-arc area shows a thick accretionary complex formed by Paleogene accretion and collision. Based on published seismic results, this accretionary complex is considered to continue on land to the Cretaceous accretionary complex observed in the Hidaka Belt. The structure of the Kuril Arc appears characteristic of upper crustal andesitic rocks (6.0–6.2 km/s) and thick middle to lower crust (6.5–7.3 km/s). Based on published geochemical study, we consider a geochemical model of simple crustal evolution explaining the mechanism of generation of andesitic rocks on the Kuril Arc. Each volume of andesitic, basaltic, and mafic layers estimated from the geochemical model is consistent with those obtained from the seismic velocity model. Crustal composition estimated from the geochemical model and major-element chemistry of the volcanic rocks in the Kuril Arc indicate that mechanical delamination is insufficient to cause the oceanic island arc crust to evolve into continental crust. [Copyright &y& Elsevier]

Presents information on a study which investigated the rupture process of the 1946 Nankaido earthquake. Information on zone earthquakes along the Nankai Trough in Southwest Japan; Topography of the Western part of Nankai; Proposed rupture of the earthquake.