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This study presents the shear wave velocity (VS) structures of sedimentary sequences and a section of the upper crustal layer in the Fukushima forearc region of the Japan Trench subduction zone, which were obtained by analysing the horizontal-to-vertical (H/V) spectral ratios of ambient vibration records. The H/V curves were derived using 31 d of continuous seismic data from 3 broad-band and 16 short-period ocean bottom seismometer (OBS) stations. Using the broad-band data, H/V ratios from 0.01 to 10 Hz were derived, but the ratios below 0.1 Hz frequencies were unusually large and temporally unstable. Characterization of seismic noise energy from ∼1 yr of seismic data of three broad-band OBSs revealed variable and elevated energy conditions below 0.1 Hz due to typical long-period oceanic noise; we link these observations with the unstable H/V ratios below this frequency. Therefore, H/V analysis was performed in the frequency range of 0.1–10 Hz for both broad-band and short-period OBSs to obtain subsurface VS profiles. For the forward calculation of the H/V ratios in the inversion process, we used the recently developed 'hvgeneralized' method, which is based on the diffuse field assumption, and accounts for the water layer on top of stratified media. Moreover, available prior geological and geophysical information was utilized during the inversion of the H/V curves. We found that subsurface VS ranged from approximately 30 m s−1 at the seabed to approximately 4900 m s−1 at 7000 m below the sea floor (mbsf). Starting with the best model candidate at each OBS location, the effect of the water layer on the H/V curve in the deep ocean was investigated by comparing synthetic H/V curves with and without the water layer. The synthetic H/V analysis revealed that the water layer had a significant effect on H/V amplitudes at higher frequencies (>1 Hz), whereas comparatively little effect was observed at lower frequencies (<1 Hz). This study provides an empirical basis for H/V analysis using OBS data to determine VS down to several kilometres of sedimentary sequences to the upper crust with high-resolution. [ABSTRACT FROM AUTHOR]

Temporal seismic observations from pop-up type ocean-bottom seismometers were used to detect tectonic tremors immediately following the 2011 Tohoku-Oki earthquake in the northern periphery of the aftershock area. Near-field observations clearly distinguished tremors from regular earthquakes based on their spectral shape in the frequency band of 1–4 Hz. In addition to tremors accompanied by very low-frequency earthquakes (VLFEs), we detected 130 tremors without known VLFE activity during April–October 2011. The newly detected tremors were in the vicinity of a sequence of small repeating earthquakes, indicating a mixed distribution of tremors and regular interplate earthquakes in the region. Tremor activity was high immediately after the deployment of seismometers and gradually decreased. In addition, the tremor activity fluctuated with two activations with an interval of approximately 90 days, similar to the intervals between tremor bursts after 2016. The results of the study suggest that the observed tremors occurred under the influence of aseismic slip caused by the decaying afterslip of the preceding Tohoku-Oki and Mw 7.4 interplate earthquakes and episodic accelerations with a quasi-periodicity unique to the area. [ABSTRACT FROM AUTHOR]

Characterizing shallow interplate slip is essential for modeling the potential generation of megathrust earthquakes and tsunamis. Large postseismic crustal deformation caused by the 2011 Tohoku‐Oki earthquake suggests the occurrence of shallow afterslip, but it is unclear what kind of slip is dominant at the shallow plate interface. Here we report episodic tremor activity south of the primary rupture area based on ocean bottom seismometer observations. Five tremor episodes with recurrence intervals of ~60 days show migration behavior and shear mechanisms consistent with plate subduction, indicating the occurrences of the episodic slow slip events in the vicinity of the afterslip area. The association of slow slip events and afterslip sheds new light on the frictional nature and stress state of shallow megathrusts and may help predict the possible rupture extent of future earthquakes. Plain Language Summary: Understanding the slip characteristics in the shallow part of subduction zone is essential to evaluate the potential risk of megathrust earthquakes and tsunamis. It remains unclear, however, what type of slip occurs at the shallow plate interface due to the difficulty in observing offshore seismic activity. In this paper, we report newly discovered tremor and slow slip activity in the afterslip region where coseimic rupture did not happen during the 2011 Tohoku‐Oki earthquake, based on a new campaign ocean bottom seismometer observation. We propose that the tremor and slow slip region illuminate the boundary between fast coseismic slip and stable aseismic slip that could limit the rupture extent of future megathrust earthquakes. The observed tremor and slow slip activities also suggest that the interplate stress state has been recovered to the level of that before the Tohoku earthquake. Key Points: We discover tectonic tremor activity in the Japan trench subduction zone based on a new campaign ocean bottom seismometer observationThe characteristics of tremor strongly suggest slow slip events in the vicinity of the afterslip area of the 2011 Tohoku earthquakeTremor and slow slip region may delineate the boundary between seismic and aseismic region along the plate interface [ABSTRACT FROM AUTHOR]

A slow slip event (SSE) and tectonic tremors occurred approximately 1.5 month prior to the occurrence of the 2011 Tohoku‐Oki earthquake near the Japan Trench. Although we cannot completely rule out other sources of noise near the observation site, slow earthquakes would demonstrate the nucleation of a fast slip event from prior slow earthquake activity. Here we report on tectonic tremor activity detected from short‐period ocean‐bottom seismometer (OBS) data with a modified frequency scanning method that allows robust tectonic tremor detection by incorporating information from multiple frequency passbands in order to suppress false detections. We have identified at least five tectonic tremor sequences on the OBS records from the end of January 2011 to just before the occurrence of the Tohoku‐Oki earthquake in March 2011. To reveal the spatial distributions of tectonic tremors before the Tohoku‐Oki earthquake, we then calculated tremor energy, with corrected site amplification, from the identified tectonic tremor time window at each OBS that detected tremors. Possible sources of tremors were distributed in trenchward side of the SSE fault prior to the main shock. These tremors could be modulated by SSE to take into account consistent timing between tremor and SSE term, and consistency of spatial locations between estimated tremor sources and SSE faults. This spatiotemporal pattern in tectonic tremor activity demonstrates a significant separation of occurrence sources between tectonic tremors and regular earthquakes. Key Points: A modified frequency scanning method allowed investigation of tectonic tremors occurring before the largest Tohoku‐Oki foreshockThis method revealed details of tremor activity in the rupture area of Tohoku‐Oki earthquake, especially close to the Japan TrenchTemporal and spatial changes in tectonic tremor activity reflected slow slip event expansion processes that occur before large earthquakes [ABSTRACT FROM AUTHOR]

We determined the three-dimensional structure of the crust and upper mantle off the Boso Peninsula, Japan, by analyzing seismograms recorded by ocean-bottom seismometers and land stations between 2011 and 2013. We employed seismic tomography to determine the P- and S-wave velocity structures and earthquake locations simultaneously. The tomographic image shows that the mantle parts of the Pacific and the Philippine Sea plates have high-velocity anomalies. The upper boundary of the Philippine Sea plate is delineated as approximately 2-6 km shallower than that previously estimated from land-based data for the area 140.5°E-141.5°E and 35°N-35.5°N. A pronounced low-velocity anomaly in P- and S-waves with low- V / V ratio (1.5-1.6) was observed at depths shallower than 20 km in the overriding North American plate. This anomaly may be caused by the presence of rocks with a low- V / V ratio, such as quartzite, and the water expelled from the subducted Pacific and Philippine Sea plates. [ABSTRACT FROM AUTHOR]

In 1997, a seismic experiment using an airgun array and ocean bottom seismographs (OBSs) was performed in the forearc region of the northeastern Japan (NEJ) arc. The objectives of this experiment were to clarify seismic structures of the forearc region off Sanriku, Japan using airgun–OBS data and to understand the nature of the Japan trench seismogenic zone. Fundamental features of the structure are as follows:(i)The subduction angle becomes steeper from east to west, from 3 to 8 to 11°, with the top of the plate located at depths of 11 km (40 km west from the trench), 21 km (120 km west from the trench) and 28 km (140 km west from the trench), respectively.(ii)The NEJ Moho beneath the forearc is defined at a depth of approximately 20 km.(iii)P-wave velocity of the island arc upper mantle is approximately 8 km s−1.(iv)TheP-wave velocity of the Cretaceous sedimentary layer exhibits significant lateral variability (4.0–5.5 km s−1) in the E–W direction.(v)The upper crust of NEJ forearc also has the significant lateral heterogeneity (5.0–6.3 km s−1) in the E–W direction, but not in the N–S direction.(vi)The middle crust defined on land extends in some locations 40 km east of the coastline.(vii)Lateral variability in theP-wave velocity of the lower crust (6.7–7.0 km s−1) is small compared with that of the upper crust.According to these results, it is suggested that the rupture areas of large earthquakes (>M7) in this region are limited within the contact zone between the NEJ arc crust and the subducting oceanic crust. Although a thin layer with low velocity is probably present within this contact zone, the serpentinization of uppermost mantle wedge of the NEJ arc by reaction between the mantle materials and the water from the thin layer beneath forearc region is limited. This might indicate that a significant volume of water and sediments subduct with the slab to depths of over 20 km. [ABSTRACT FROM AUTHOR]

The article presents the observations of the 2011 Tohoku-Oki Earthquake at 19 ocean bottom seismometers (OBS) sites located off Miyagi region. The researchers retrieved 17 OBS from March to November 2011 in emergency research cruises. The retrieved OBSs have recorded the seismic wave data, which were used for relocation of the hypocenters of the foreshock, mainshock, and aftershocks.

Shear-wave anisotropy in Earth's mantle helps constrain the lattice-preferred orientation of anisotropic minerals due to viscous flow. Previous studies at the Japan Trench subduction zone using land-based seismic networks identified strong anisotropy in the mantle wedge, reflecting viscous flow induced by the subducting slab. Here we map anisotropy in the previously uninvestigated offshore region by analyzing shear waves from interplate earthquakes that are recorded by a new seafloor network (the S-net). The newly detected anisotropy is not in the mantle wedge but only in the overlying crust (∼0.1 s time delay and trench-parallel fast direction). The distinct lack of anisotropy indicates that the forearc mantle wedge offshore is decoupled from the slab and does not participate in the viscous flow, in sharp contrast with the rest of the mantle wedge. A stagnant forearc mantle wedge provides a stable and cold tectonic environment that is important for the petrological evolution and earthquake processes of subduction zones. Knowledge of shear-wave anisotropy is important to understanding the structure and dynamics of the subduction zone mantle wedge. Here, the authors find unambiguous evidence that forearc anisotropy resides in the upper-plate crust, while weak anisotropy in the most seaward part of the mantle wedge indicates decoupling from the slab [ABSTRACT FROM AUTHOR]

To study the physical properties along the subducting plate boundary at the Japan Trench, a seismic study was carried out using ocean bottom seismometers (OBSs) and artificial sources was carried out. The 250 km long survey line with an almost N–S strike crosses the two major focal zones of the 1968 Tokachi-Oki earthquake and the 1994 Sanriku-Haruka-Oki earthquake. Ray tracing, and non-linear inversion were used to analyze the data obtained by the OBSs. P-wave velocity structure down to the island-arc-Moho was obtained.The result shows distinct heterogeneities along the plate boundary at 40°10′N, which are located exactly at the southern boundary of major moment release regions of the above two earthquakes. P-wave velocities for the crust north of 40°10′N are approximately 7% slower than those for the south. The depths of the island-arc-Moho vary from 15 km in the south to 21 km in the north below the sea surface. Three explanations of the velocity heterogeneities are presented in terms of the migration of water. [Copyright &y& Elsevier]

Abstract: The March 2011 9.0 Tohoku-Oki earthquake activated shallow aftershocks with normal faulting focal mechanisms near the trench axis. To investigate stress state in the shallow subduction zone near the trench axis, we used ocean bottom seismographs (OBSs) to record earthquakes from August to October 2011. We estimated hypocenter locations and focal mechanisms of the recorded earthquakes by using a grid-search method in a 2-D velocity model based on an active seismic survey. The results show that most of the earthquakes occurred within both the overriding and subducting plates, with very few on the plate interface and none in the most seaward 45 km of the overriding plate (aseismic wedge). The low seismicity along the plate interface is consistent with the nearly complete stress drop on the megathrust fault in the 9.0 earthquake. Focal mechanisms indicate that trench-normal tension is pre-dominant in both plates and extends to a depth of about 30 km at least. On the other hand, several trench-normal compressional earthquakes were located at the landward end of the aseismic wedge. These earthquakes suggest horizontal compression of the overriding prism that may be caused by a strengthening of the shallowest part of the megathrust. [Copyright &y& Elsevier]

Abstract: The 2007 Niigata-ken Chuetsu-oki earthquake occurred on July 16, 2007 with a magnitude of 6.8. Immediately after the mainshock, a dense network of ocean-bottom seismometers and temporary land seismic stations were deployed to obtain the accurate aftershock location. A seismic survey using ocean-bottom seismometers, land stations and controlled sources at sea and on land was also conducted along three profiles to estimate the detailed velocity structure of the source region. A thick sedimentary layer covers the crust, and this layer is thickest near the coast. The upper crust has a P-wave velocity of 6km/s and a large lateral heterogeneity with respect to thickness and velocity. The lower crust, with P-wave velocity of 7km/s, has a thickness of approximately 10km. The thickness of the crust is estimated to be approximately 24km. The precise aftershock distribution was obtained by using the high-resolution velocity structure. The aftershocks in the upper crust form a plane dipping to the southeast. Most of the aftershocks are located in the upper crust; in addition, a small number of aftershocks in the lower crust seem to be positioned on the same plane formed by the aftershocks in the upper crust. The mainshock fault estimated from the aftershock distribution is positioned in the high-velocity body of the upper crust. A large deformation of the sediments above the epicentral region is interpreted to be due to the repeated occurrence of large earthquakes. The lower crust immediately below the fault has low velocity. The source region of the 2007 event in the upper crust is sandwiched by a weakening structure. We suggest that the ductile deformation of both the sediments and the lower crust causes stress accumulation in the source region. [Copyright &y& Elsevier]

Abstract: Large earthquakes have repeatedly occurred in the area off southeastern Hokkaido Island, Japan, as the Pacific Plate subducts beneath the island, which is on the North American Plate. The most recent large earthquake in this area, the 2003 Tokachi-oki earthquake (Mw=8.0), occurred on September 26, 2003. In order to investigate aftershock activity in the rupture area, 47 Ocean Bottom Seismometers (OBSs) were quickly deployed after the main shock. In the present study, we simultaneously estimate the hypocenters and 3-D seismic velocity models from the P- and S-wave arrivals of the aftershocks recorded by OBSs. The subducting plate is clearly imaged as a northwest dipping zone in which Vp is greater than 7 km/s, and the relocated hypocenters also show the subducting Pacific Plate. The aftershock distribution reveals that the dip angle of the plate boundary increases abruptly around 90 km from the Kuril Trench. The bending of the subducting plate corresponds to the southeastern edge of the rupture area. The island arc crust on the overriding plate has P-wave velocities of 6–7 km/s and a Vp/Vs of 1.73. A region of Vp/Vs greater than 1.88 was found north of the epicenter of the main shock. The depth of the high Vp/Vs region extends about 10 km upward from the plate interface. The plate boundary just below the high Vp/Vs region has the largest slip at the main rupture. A high Vp anomaly (~7.5 km/s) is found in the island arc crust in northeast part of the study area, which we interpret as a structural boundary related to the arc–arc collisional tectonics of the Hokkaido region, as the rupture of the main shock terminated at this high Vp region. We suggest that the plate interface geometry and the trench-parallel velocity heterogeneity in the landward plate are principal factors in controlling the rupture area of the main shock. [Copyright &y& Elsevier]

• 3D traveltime mapping shows two strong reflection areas from the top of the PHS. • One is near the main slip area of the Boso SSEs and the other is about 60 km to the east. • The reflections from the former area are generated by a thin low-velocity layer. • The latter area contains a high-velocity structure (HVS) near the top of the PHS. • This HVS may represent boninite or partially serpentinized peridotite or gabbro. The region off the Boso Peninsula, Japan, is a tectonically complex area where the Pacific plate is subducting beneath both the landward plate and the Philippine Sea plate (PHS) from the Japan trench and the Izu-Bonin trench as the PHS is subducting under the landward plate from the Sagami trough. It is important to better determine the structure of this region to deepen our understanding of its seismicity. Previous seismic reflection studies have shown that reflections from the upper surface of the PHS vary with depth, being stronger in the main slip area of the slow slip events beneath the Boso Peninsula (Boso SSEs). However, the spatial relationship between the reflective area and the SSEs is poorly constrained. This study mapped the distribution of the reflective area using data recorded by ocean bottom seismometers during an active-source seismic experiment. We constructed a 3D P-wave velocity structure by using traveltimes of first arrivals from 18 ocean bottom seismometer records. We also adapted the traveltime mapping method to reflection traveltimes, projecting them to the depth–distance domain, to map the 2D distribution of strong reflections from the top of the PHS. These reflections were concentrated in two areas, one near the main slip area of the Boso SSEs and the other about 60 km to the east. In the first area, the absence of strong velocity contrasts near the top of the PHS suggests that the reflections were generated by a thin low-velocity layer. In contrast, the structure of the second area has a convex shape of high velocity with a high velocity gradient near the top of the PHS. This structure may represent boninitic material of the outer-arc high, partially serpentinized peridotite, or gabbro displaced by intraoceanic reverse faults. [ABSTRACT FROM AUTHOR]