Journal of the Geodetic Society of Japan
Online ISSN : 2185-517X
Print ISSN : 0038-0830
ISSN-L : 0038-0830
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Volume 67
Displaying 1-5 of 5 articles from this issue
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MEMORIAL PAPER FOR THE TSUBOI PRIZE
  • Detection of Interplate Coupling Condition and Shallow Slow Slip Events along the Nankai Trough Subduction Zone by Sensitive GNSS-A Seafloor Geodetic Observation
    Yusuke Yokota
    2021 Volume 67 Pages 1-17
    Published: 2021
    Released on J-STAGE: August 18, 2021
    DOIhttps://doi.org/10.11366/sokuchi.67.1
    JOURNAL FREE ACCESS
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    Global Navigation Satellite System – Acoustic ranging combination technique （GNSS-A） by the Hydrographic and Oceanographic department, Japan Coast Guard has gained the ability to detect subseafloor interplate coupling condition and shallow slow slip event（SSE） through various technological developments in 2010s. The observation frequency has been improved to about 4 times/ year by introducing a new method for transmitting and receiving acoustic signals much efficiently. By developing a new method of extracting a gradient field of undersea sound speed structure（SSS） from signal travel-time residuals, we succeeded in improving the observation accuracy to about 1-2 cm （empirically） in the best case. The extracted SSS properly reflects the actual ocean field and can be used as oceanographic data in a unique range on the km scale. In the future, by classifying the extracted gradient fields, we may be able to consider the accuracy of each result. Along the Nankai Trough, subseafloor interplate coupling condition has been newly detected from the GNSSA observation result. A coupling condition has spatial unevenness and was complementary to subducting seamounts and shallow very low frequency events（VLF） activity. In addition, shallow SSE signals have been detected, and a temporal correlation with shallow VLF activity was also observed. With the use of open science and new sea-surface platforms in the future, GNSS-A is expected to contribute to the elucidation of more advanced geodetic phenomena and to be widely applied.

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TECHNICAL REPORT
  • Calibration of Instrumental Offsets of Absolute Gravimeters FG5 #109 and #241 by Intercomparison at Tokyo and Mount Fuji
    Yuichi Imanishi, Ryuichi Nishiyama, Ryo Honda, Ryo Honda
    2021 Volume 67 Pages 18-28
    Published: 2021
    Released on J-STAGE: August 19, 2021
    DOIhttps://doi.org/10.11366/sokuchi.67.18
    JOURNAL FREE ACCESS
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    In order to precisely calibrate the instrumental drift of superconducting gravimeters with absolute gravity measurements, instrumental offsets of the absolute gravimeters used for the measurements must be known to the precision on the order of 0.1 μGal. In this study, absolute gravimeters FG5 #109 and #241 owned by Earthquake Research Institute, The University of Tokyo were compared for precisely calibrating relative offsets of the instruments. Two experiments at Tokyo and Mount Fuji gave consistent results, though the uncertainties in the experiment at Tokyo were large. The experiment at Mount Fuji achieved calibration precision of 0.33 μGal. Frequencies of the Rubidium clocks used with the absolute gravimeters were also measured against the Hydrogen maser clock at Mizusawa VLBI Observatory, National Astronomical Observatory of Japan.

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