Detection of Interplate Coupling Condition and Shallow Slow Slip Events along the Nankai Trough Subduction Zone by Sensitive GNSS-A Seafloor Geodetic Observation

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  • GNSS-A海底地殻変動観測による南海トラフ海底下のプレート間固着の検出およびその高感度化に基づく浅部スロースリップイベントの発見
  • GNSS-A カイテイ チカク ヘンドウ カンソク ニ ヨル ナンカイ トラフ カイテイ カ ノ プレート カン コチャク ノ ケンシュツ オヨビ ソノ コウカンドカ ニ モトズク センブ スロースリップイベント ノ ハッケン

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<p>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.</p>

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