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  1. Article ; Online: Correction

    Lina Yamaya / Kimihiro Mochizuki / Takeshi Akuhara / Shunsuke Takemura / Masanao Shinohara / Tomoaki Yamada

    Earth, Planets and Space, Vol 74, Iss 1, Pp 1-

    CMT inversion for small-to-moderate earthquakes applying to dense short-period OBS array at off Ibaraki region

    2022  Volume 8

    Keywords Geography. Anthropology. Recreation ; G ; Geodesy ; QB275-343 ; Geology ; QE1-996.5
    Language English
    Publishing date 2022-12-01T00:00:00Z
    Publisher SpringerOpen
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  2. Article ; Online: CMT inversion for small-to-moderate earthquakes applying to dense short-period OBS array at off Ibaraki region

    Lina Yamaya / Kimihiro Mochizuki / Takeshi Akuhara / Shunsuke Takemura / Masanao Shinohara / Tomoaki Yamada

    Earth, Planets and Space, Vol 74, Iss 1, Pp 1-

    2022  Volume 18

    Abstract: Abstract We conducted centroid moment tensor (CMT) inversion for small-to-moderate aftershocks in the off Ibaraki region of the 2011 off the Pacific coast of Tohoku earthquake. In this study, we used high-frequency (0.4–1.0 Hz) seismograms from a dense ... ...

    Abstract Abstract We conducted centroid moment tensor (CMT) inversion for small-to-moderate aftershocks in the off Ibaraki region of the 2011 off the Pacific coast of Tohoku earthquake. In this study, we used high-frequency (0.4–1.0 Hz) seismograms from a dense array of short-period ocean bottom seismometers (OBSs) and a reliable three-dimensional (3-D) seismic velocity model. Higher frequency analysis and dense OBS arrays offer CMT solutions with high spatial resolutions. Since our OBS array observed aftershocks occurring immediately following the 2011 off the Pacific coast of Tohoku earthquake, we determined 536 CMT solutions for small-to-moderate aftershocks in the off Ibaraki region (JMA-scale magnitudes of 2.5–4.0). According to our CMT solutions, characteristics of the aftershock activities in the off Ibaraki region are classified into 5 groups: (1) thrust earthquakes, which are considered as interplate earthquakes, separated by the large slip area of the 2011 Ibaraki-oki earthquake (the largest $${\mathrm{M}}_{\mathrm{w}}$$ M w 7.6 aftershock of the 2011 off the Pacific coast of Tohoku earthquake) and the tectonic tremors; (2) intraslab strike-slip earthquakes located at the north of the fault area of the 2011 Ibaraki-oki earthquake; (3) intraslab normal-fault earthquakes, which suggest a tensional stress field within the subducting Pacific Plate due to the plate bending by the overriding Philippine Sea Plate; (4) various earthquake focal mechanisms above a subducting seamount, which suggest the 3-D complex fractures; and (5) normal-fault earthquakes shallower than the interplate earthquakes, which were possibly caused by the heterogeneity of the subducting seamount. Graphical Abstract
    Keywords CMT inversion ; Ocean bottom seismometer ; Focal mechanism ; Off Ibaraki ; The 2011 off the Pacific coast of Tohoku earthquake ; Geography. Anthropology. Recreation ; G ; Geodesy ; QB275-343 ; Geology ; QE1-996.5
    Subject code 551
    Language English
    Publishing date 2022-11-01T00:00:00Z
    Publisher SpringerOpen
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

    Full text online

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    Inter-library loan at ZB MED

    Your chosen title can be delivered directly to ZB MED Cologne location if you are registered as a user at ZB MED Cologne.

  3. Article ; Online: Performance of Seismic Observation by Distributed Acoustic Sensing Technology Using a Seafloor Cable Off Sanriku, Japan

    Masanao Shinohara / Tomoaki Yamada / Takeshi Akuhara / Kimihiro Mochizuki / Shin’ichi Sakai

    Frontiers in Marine Science, Vol

    2022  Volume 9

    Abstract: Recently, the distributed acoustic sensing (DAS) measurement, which utilizes an optical fiber itself as a sensor, becomes popular for various fields and is being applied to seismic observations. The shortest spatial sampling of DAS observations reaches a ...

    Abstract Recently, the distributed acoustic sensing (DAS) measurement, which utilizes an optical fiber itself as a sensor, becomes popular for various fields and is being applied to seismic observations. The shortest spatial sampling of DAS observations reaches a few meters, and the total measurement distance becomes greater than 50 km. A high temporal sampling rate is achieved. Due to these characteristics, a DAS measurement allows for a dense seismic observation as a linear array. Applying a DAS measurement to the seafloor cable is advantageous because the quantity of data can be significantly increased in a marine area. A seafloor seismic tsunami observation cable system using an optical fiber for data transmission was deployed off Sanriku by the Earthquake Research Institute, the University of Tokyo in 1996. This seafloor cable observation system has spare fibers for extension. Beginning in February 2019, we made several DAS observations using the spare fibers of the seafloor system. Consequently, many earthquakes were recorded. Small earthquakes with a magnitude of 1.8 occurring near the cable system were recorded by the DAS system. The arrivals of P- and S-waves of the earthquake with a magnitude of 3 were clearly seen using the phase data from the DAS measurement. In addition, a teleseismic event with an epicentral distance of approximately 2,300 km and a magnitude of 6.6 was clearly observed. Because there are conventional seismometers in the Sanriku cable system, we compared records from the DAS measurement with those from the seismometer. The DAS records and the data by the seismometer showed a high coherency. The noise levels of the DAS measurement were evaluated, and there was little temporal variation of the noise levels. A spatial variation of ambient seismic noises was revealed using a spatially high-density observation with a long distance. In November 2020, a seismic survey using the DAS system and airguns was carried out, and the DAS system clearly recorded signals from the airguns. We also compared ...
    Keywords distributed acoustic sensing (DAS) ; seafloor cable ; seismic ambient noise level ; optical fiber sensing ; microearthquake observation ; controlled seismic source survey ; Science ; Q ; General. Including nature conservation ; geographical distribution ; QH1-199.5
    Subject code 500 ; 550
    Language English
    Publishing date 2022-04-01T00:00:00Z
    Publisher Frontiers Media S.A.
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  4. Article ; Online: Precise aftershock distribution of the 2019 Yamagata-oki earthquake using newly developed simple anchored-buoy ocean bottom seismometers and land seismic stations

    Masanao Shinohara / Shin’ichi Sakai / Tomomi Okada / Hiroshi Sato / Yusuke Yamashita / Ryota Hino / Kimihiro Mochizuki / Takeshi Akuhara

    Earth, Planets and Space, Vol 74, Iss 1, Pp 1-

    2022  Volume 15

    Abstract: Abstract An earthquake with a magnitude of 6.7 occurred in the Japan Sea off Yamagata on June 18, 2019. The mainshock had a source mechanism of reverse-fault type with a compression axis of WNW–ESE direction. Since the source area is positioned in a ... ...

    Abstract Abstract An earthquake with a magnitude of 6.7 occurred in the Japan Sea off Yamagata on June 18, 2019. The mainshock had a source mechanism of reverse-fault type with a compression axis of WNW–ESE direction. Since the source area is positioned in a marine area, seafloor seismic observation is indispensable for obtaining the precise distribution of the aftershocks. The source area has a water depth of less than 100 m, and fishing activity is high. It is difficult to perform aftershock observation using ordinary free-fall pop-up type ocean bottom seismometers (OBSs). We developed a simple anchored-buoy type OBS for shallow water depths and performed the seafloor observation using this. The seafloor seismic unit had three-component seismometers and a hydrophone. Two orthogonal tiltmeters and an azimuth meter monitored the attitude of the package. For seismic observation at shallow water depth, we concluded that an anchored-buoy system would have the advantage of avoiding accidents. Our anchored-buoy OBS was based on a system used in fisheries. We deployed three anchored-buoy OBSs in the source region where the water depth was approximately 80 m on July 5, 2019, and two of the OBSs were recovered on July 13, 2019. Temporary land seismic stations with a three-component seismometer were also installed. The arrival times of P- and S-waves were read from the records of the OBSs and land stations, and we located hypocenters with correction for travel time. A preliminary location was performed using absolute travel time and final hypocenters were obtained using the double-difference method. The aftershocks were distributed at a depth range of 2.5 km to 10 km and along a plane dipping to the southeast. The plane formed by the aftershocks is consistent with the focal mechanism of the mainshock. The activity region of the aftershocks was positioned in the upper part of the upper crust. Focal mechanisms were estimated using the polarity of the first arrivals. Although many aftershocks had a reverse-fault focal mechanism ...
    Keywords 2019 Yamagata-oki earthquake ; Aftershock observation ; Anchored-buoy system ; Ocean bottom seismometer ; Shallow water seismic observation ; Geography. Anthropology. Recreation ; G ; Geodesy ; QB275-343 ; Geology ; QE1-996.5
    Subject code 551
    Language English
    Publishing date 2022-01-01T00:00:00Z
    Publisher SpringerOpen
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

    Full text online

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    Inter-library loan at ZB MED

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  5. Article ; Online: Lithosphere–asthenosphere boundary beneath the Sea of Japan from transdimensional inversion of S-receiver functions

    Takeshi Akuhara / Kazuo Nakahigashi / Masanao Shinohara / Tomoaki Yamada / Hajime Shiobara / Yusuke Yamashita / Kimihiro Mochizuki / Kenji Uehira

    Earth, Planets and Space, Vol 73, Iss 1, Pp 1-

    2021  Volume 17

    Abstract: Abstract 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 ... ...

    Abstract Abstract 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.
    Keywords Back-arc basin ; Lithosphere–asthenosphere boundary ; Sea of Japan ; S-receiver function ; Transdimensional inversion ; Ocean-bottom seismometer ; Geography. Anthropology. Recreation ; G ; Geodesy ; QB275-343 ; Geology ; QE1-996.5
    Subject code 950
    Language English
    Publishing date 2021-08-01T00:00:00Z
    Publisher SpringerOpen
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

    Full text online

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    Inter-library loan at ZB MED

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