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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. [ABSTRACT FROM AUTHOR]

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 redundancyThe practical system with three stations and a total length of 105 km was deployed, and observation began in 2015The seismic data have a low‐noise level, and the pressure data from the buried observation node are comparable to those on the seafloor [ABSTRACT FROM AUTHOR]

Highlights: [•] We conducted seismic survey using ocean bottom seismometers in Japan Sea. [•] We obtained seismic structure in the Yamato Basin, Japan Sea. [•] The crust is neither that of a typical continental nor that of a typical oceanic crust. [•] We interpreted the crust was formed by the rifting/extension of continental crust. [Copyright &y& Elsevier]

A number of seismoacoustic T-wave events were observed between January 2003 and January 2004 by broadband ocean-bottom seismometers installed on the French Polynesia seafloor at depths of 4,000-5,000 m, well below the conjugate depth of the SOFAR channel. Using T-wave arrival times, we located 89 T-wave events along the Pacific-Antarctic Ridge. Among these, 63 events were not detected by land-based seismic observations of the United States Geological Survey (USGS), which was nearly twice the number of earthquakes reported by the USGS in the area during the observation period. We used a simple method to estimate earthquake magnitude from T-wave energy. The magnitudes of the events unidentified by the USGS ranged from 2.3 to 5.5, whereas magnitudes of the earthquakes reported by the USGS ranged from 4.1 to 6.2. Our study suggests that T-wave observations with abyssal ocean-bottom seismometers can improve the detection of small earthquakes and help our understanding of the seismotectonics of remote oceanic areas. [ABSTRACT FROM AUTHOR]

The mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics, and thus the understanding of the processes at the [ithosphere-asthenosphere boundary (LAB) is fundamental to understand how our planet works. It is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the LAB. Data from borehole broadband ocean bottom seismometers show that the LAB beneath the Pacific and Philippine Sea plates is sharp and age-dependent. The observed large shear wave velocity reduction at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in melt-rich mantle, which accounts for the large viscosity contrast at the LAB that facilitates horizontal plate motions. [ABSTRACT FROM AUTHOR]

Abstract: Stable power supply is essential for various long-term sea floor geophysical observations. Due to a simple structure and a large energy capacity, sea-water batteries have been developed and used for such observations. However, the characteristics of sea-water batteries have not been well known in the case of installations at depths more than 5000m in deep-sea basin. In 2000, a sea floor borehole broadband seismic observatory was installed in the northwestern Pacific basin where the water depth is 5577m. For electric power supply, a Sea-Water Battery (SWB) system with monitoring and control was developed and used. The SWB system consists of four sea-water battery cells, a DC/DC converter, the Power Control System, the Data Logger, and an accumulator. The conditions of the SWB system were recorded more than 1 year, and the monitoring data was recovered. The SWB system generated enough power for the observation system consuming power of 6W in average and continued to supply power for at least 5 years. From the monitoring data, the SWB can supply up to the long-term average of at least 13W. The energy density is estimated to be 318Whkg−1. The sea-water battery is useful for application of long-term power consumption even in the deep-sea basin. [Copyright &y& Elsevier]

Subducting seamounts are thought to increase the normal stress between subducting and overriding plates. However, recent seismic surveys and laboratory experiments suggest that interplate coupling is weak. A seismic survey in the Japan Trench shows that a large seamount is being subducted near a region of repeating earthquakes of magnitude M ∼ 7. Both observed seismicity and the pattern of rupture propagation during the 1982 M 7.0 event imply that interplate coupling was weak over the seamount. A large rupture area with small slip occurred in front of the seamount. Its northern bound could be determined by a trace of multiple subducted seamounts. Whereas a subducted seamount itself may not define the rupture area, its width may be influenced by that of the seamount. [ABSTRACT FROM AUTHOR]

The geological evolution of the volcanic passive margin off mid-Norway, NE Atlantic, and the active volcanic arc in northern Honshu, related to the subduction of the Pacific plate beneath Japan, are similar in the sense that long periods of extensional deformation has shifted to recent compression with formation of domal structures. Seismic wide-angle data have been acquired across six contractional domes in the Vøring Basin, mid-Norwegian Margin, and over the Ou Backbone Range ‘pop-up’ structure, northern Honshu. The models derived from the seismic data reveal that the location of all investigated structures in the Vøring Basin is between, or in the vicinity of pre-Eocene high-velocity structures, which may act as rigid blocks during compression. It is proposed that the existence and trend of these structures, subject to mild NW–SE compression, is the most important factor controlling the formation, spatial distribution and trend of the domes. Structures in the high-velocity lower crust may be the single most important element in controling the domal formation; all modelled highs in the lower crustal early Tertiary intrusive layer seem to be related to the formation of domes in the NW–SE direction. The Ou Backbone Range pop-up structure is localised at a low rigidity zone mapped as low upper crustal seismic velocities. The structure coincides with the present day volcanic front, which may be interpreted as the main reason for the existence of low velocities and the main factor controling the localization of compressional deformation. The Ou Backbone Range is squeezed between a high velocity upper crustal block to the east and a high velocity lower crustal block to the west, and it is speculated that the corresponding lateral and vertical variations in crustal rigidity may have contributed to the actual localization of the present day volcanic front. [Copyright &y& Elsevier]