Explore the vast range of titles available.

The evolution history of the Sea of Japan back-arc basin remains under debate, involving the opening of sub-basins such as the Japan and Yamato Basins. Detailed knowledge of the lithospheric structure will provide the key to understanding tectonic history. This study identifies the lithosphere–asthenosphere boundary (LAB) beneath the Sea of Japan back-arc basin using S-receiver functions (S-RFs). The study area, including the Japan and Yamato Basins, has been instrumented with broadband ocean-bottom seismometers (OBSs). S-RFs from these OBSs show negative Sp phases preceding the direct S arrivals, suggesting the LAB. The S-RFs also show abnormally reduced amplitudes. For further qualitative interpretation of these findings, we conduct transdimensional Bayesian inversion for S-wave velocity models. This less-subjective Bayesian approach clarifies that the low-velocity seafloor sediments and damped deconvolution contribute to the amplitude reduction, illuminating the necessity of such considerations for similar receiver function works. Inverted velocity structures show a sharp velocity decrease at the mantle depths, which we consider the LAB. The obtained LAB depths vary among sites: ~ 45 km beneath the Japan and Yamato Basins and ~ 70 km beneath the Yamato Rise, a bathymetric high between the two basins. The thick lithosphere beneath the Yamato Rise most likely reflects its continental origin. However, the thickness is still thin compared to that of eastern Asia, suggesting lithosphere extension by rifting. Notably, the Japan and Yamato Basins show a comparable lithospheric thickness, although the crustal thickness beneath the Yamato Basin is known to be anomalously thick. This consistency in the lithospheric thickness implies that both basins undergo similar back-arc opening processes.

We conducted seismic tomography for entire Japanese Islands including the Sea of Japan and the Pacific Ocean using arrival times from reflection survey as well as the routine seismic network. We successfully imaged the shallow zone along the Sea of Japan from offshore Yamagata to the Noto Peninsula by using air gun data. An extremely low-V shallow zone is imaged between Sado Island and Noto Peninsula. We also obtained detailed seismic velocity structure beneath the Pacific Ocean at depths of 20–50 km using S-net data. The 2007 Noto Peninsula, the 2007 offshore Chuetsu, and the 2019 offshore Yamagata earthquakes occurred at the boundary between high-Vp and low-Vp zones. The west side of the hypocenter of the 2019 offshore Yamagata earthquake at depths of 10–30 km has high-V corresponding to the Mogami Trough. This high-V zone passes through Awa Island and reaches Sado Basin between Sado Island and Honshu. A major rift zone in the Tohoku Arc extending from the Akita region to the Niigata region along the coast of Sea of Japan corresponds to high-V lower crust and a shallow Moho.

Cabled seafloor seismic and tsunami observation systems are ideal for marine geophysical monitoring because the data can be obtained in real time. We have developed a new compact seafloor‐cable seismic and tsunami observation system using Information and Communication Technology (ICT). Our system achieves reliability through redundancy using ICT. A software‐based system using up‐to‐date electronics technology contributes to cost reduction and production sustainability. The system named the Ocean Bottom Cable Seismic and Tsunami (OBCST) observation system was installed on the Pacific Ocean floor off Sanriku, northeast Japan, in September 2015, in the source area of the 2011 Tohoku‐oki earthquake. The purpose of the installation is to monitor seismic activity better and to observe tsunami activity through spatially dense observation. The system has been continuously collecting the seismic and pressure data since the deployment. For the seismic data, ambient seismic noise is comparable to that from a previous cable observation system. One observation node is buried 1 m below the seafloor and has a lower noise level compared to nodes on the seafloor. Because the noise levels are stable, many local earthquakes and teleseismic events have been recorded by the system. The data obtained by high‐precision pressure gauges have a resolution of <1 hPa which is limited to environmental noise. The effectiveness of the buried pressure gauge is demonstrated by a recording of a tsunami. The system also continuously monitors its operating environment. The operating temperature is reasonably low and stable, which is optimal for long‐term operation. Plain Language Summary Large magnitude earthquakes and tsunamis are significant hazards in marine subduction zones. A cable seafloor observation system is essential for research on and the mitigation of such hazards because it can provide observations in real time. We have developed a new compact cable seismic and tsunami observation system to increase the number of observation stations. The system uses Information and Communication Technology which also provides the flexibility to monitor the system and change observational parameters after deployment. In 2015, our system was deployed in the source region of the 2011 Tohoku‐oki earthquake, and data have been gathered continuously since then. Long‐term observational data show that the system consistently records high‐quality seismic and pressure data. The system operates at a low and stable temperature which is ideal for reliable long‐term observations. Key Points We developed a seafloor‐cable seismic and tsunami observation system using Internet technology. Reliability is ensured by redundancy The practical system with three stations and a total length of 105 km was deployed, and observation began in 2015 The seismic data have a low‐noise level, and the pressure data from the buried observation node are comparable to those on the seafloor

Recent studies on the stress field in a seismogenic zone revealed a heterogeneous feature around a fault. To model the stress field around a fault zone, we have developed an inversion method that can be applied to focal mechanism data on microearthquakes and slip data on faults. The modeling scheme takes an objective approach without an a priori model such as the existence of faults or a magma source. The stress variation resulting from inelastic deformation in a medium (e.g., fault slip and magma intrusion) can be expressed as equivalent body forces in the medium. Thus, we attempted to model the stress field through the estimation of parameters of the regional stress and spatially distributed moment sources. The method was applied to the focal mechanism data of the aftershocks of the 2005 Fukuoka earthquake (M7.0) in Japan. The direction obtained for the minimum regional principal stress (i.e., NNW‐SSE) was as expected from the general tendency of the focal mechanisms. The results revealed that the stress field was distorted by the fault slip in the middle segment of the earthquake fault. The slip detected at the deep part of the fault is located away from the coseismic slip area, suggesting a possibility of either preseismic or postseismic slip around the initiation point of the main shock rupture. In contrast, the stress accumulation rises at the folding point of the aftershock alignment, and the stress relaxation area is found at the deeper edge where the largest aftershock occurred and strong heterogeneous medium exists. Key Points A new method to model heterogeneous stress field inferred from slip data The heterogeneous field expressed by regional stress tensor and moment tensors Fault slip was detected by the method using focal mechanism data of Fukuoka EQ

We present high-resolution three-dimensional tomographic images of the crust beneath the entire Kyushu arc, and particularly the western portion. Our results reveal a velocity anomaly that correlates well with an upper crustal gravity anomaly. Significant low-velocity anomalies exist beneath the Miyazaki plane and along the Beppu-Shimabara Graben. Another extensive low-velocity region near the bottom of the crust is located just below the volcanic front and between active volcanoes. The low-velocity anomalies exhibit low V p and V p / V s characteristics, and the spatial relationship between these anomalies, the Bouguer gravity anomaly, and the Moho suggests that low-density material at the base of the crust is responsible for both the seismic and gravity signatures. We interpret this material to constitute a relict ridge subducting below the Kyushu Mountains.

The existence of a static fault segment boundary has been proposed for segmentation of the historical great earthquakes along the Nankai Trough, southwest of Japan. Due to the extremely low seismicity, the seismic characteristics around the boundary have remained too uncertain to allow detailed discussion of the cause of the fault segmentation. We collected four years of continuous seismic data around the segment boundary through repeated marine observations and determined accurate hypocenters, magnitudes and focal mechanisms of the observed earthquakes. The Tokai segment to the east of the boundary shows particularly low seismicity. An abrupt change in the P‐axis orientation of intra‐slab earthquakes coincides with heterogeneous structure within the subducting Philippine Sea Plate. The boundaries between regions of different seismic character are parallel to the magnetic anomaly lineation over the Shikoku Basin of the subducting plate, implying that they are determined by the formation process of the basin.

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 Noto Hanto earthquake in 2007 ( M j 6.9) occurred on March 25, 2007 near the west coast of the Noto peninsula, Honshu, Japan. To study the aftershock activity under the sea, we deployed pop-up type ocean bottom seismometers (OBSs) from April 5 to May 8, 2007. We combined data from ten ocean bottom and four onshore seismic stations located around the rupture area of the earthquake and determined the preliminary distribution of the aftershocks. Most of the offshore aftershocks are located in a depth range between 2 and 10 km, and no earthquakes are observed in the lower crust. Hypocenters of deep events occurring at depths greater than 5 km are confined to an area northeastward from the largest aftershock in offshore region. Most of the aftershocks aligned along a high angle and southeast dipping plane, which is consistent with the geometry of the active faults revealed by previous seismic reflection surveys.

The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followed by an anomalously intense aftershock activity that included nine Mj ≥5.5 aftershocks. We deployed three temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary stations with those from permanent stations located around the aftershock area, and determined the hypocenters of the mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on 27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular to each other. This might be due to heterogeneous structure in the source region. The strain energy was considered to have been enough accumulated on the individual fault planes. These features are probably a cause of the anomalous intensity of the aftershock activity.

An S -wave reflector is considered to relate to the existence of liquid in the seismogenic zone of the crust, which plays an important role in understanding the mechanism of earthquakes. We studied a distribution of S -wave reflectors in and around the hypocentral zone of the 2004 mid Niigata Prefecture Earthquake (M6.8). The earthquake was followed by several aftershocks that were greater than M6. Moreover, the aftershocks were not only located on the fault plane of the main shock but also on conjugate fault planes and on a parallel plane to that of the main shock. In order to discuss the relationship between this complex activity and the crustal heterogeneities, we analyzed the seismograms observed at the seismic stations in this region. Normal moveout processing (NMO) was applied to the data of the aftershock. Several S -wave reflectors could be identified from the NMO sections for every station. In particular, relatively strong S -wave reflectors exist in the lower crust at a depth of approximately 20-25 km in the middle part of the aftershock region. Additionally, reflectors were found beneath the fault planes of the main shock and the largest aftershock. This suggests a possibility of the correlation of the crustal heterogeneities to the occurrence of an earthquake.