Spatio-temporal Change of Interplate Coupling in the Northeastern Japan Subduction Zone

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  • SUWA Yoko
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • MIURA Satoshi
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • HASEGAWA Akira
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • SATO Toshiya
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • TACHIBANA Kenji
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University

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Other Title
  • 東北日本沈み込み帯におけるプレート境界の固着状況
  • トウホク ニホン シズミ コミ タイ ニ オケル プレート キョウカイ ノ コチャク ジョウキョウ

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Abstract

GEONET, the nation-wide network of GPS stations conducted by the Geophysical Survey Institute of Japan, has made it possible to investigate the crustal deformation of the Japan Islands in unprecedented detail. Because of relatively poor accuracy in vertical displacements, however, previous studies estimated model parameters by weighting horizontal components largely, and sometimes encountered the inconsistency between theoretical and observed vertical displacements. Using the Precise Point Positioning technique, we improved the reliability in vertical component of displacement obtained by GPS observations, and estimated spatial distribution of interplate coupling during 1997-2001 from 3-dimensional velocity field including this vertical component.<br>The results show strong coupling along the plate boundary off Tokachi and off Miyagi, corresponding to the previously reported locations of asperities. Our result indicates that these asperities are cohered in the interseismic period. On the other hand, relatively weak coupling is estimated for the plate boundary off Sanriku probably due to the post-seismic slip after the 1989, 1992, and 1994 Sanriku-oki earthquakes. However, a close look at the timeseries of site coordinates reveals a systematic increase in westward velocity at the beginning of 1999 for the area around northern Iwate and Aomori prefectures. This suggests that the interplate coupling in this area has been recovering since 1999.<br>The aseismic front has been interpreted as the lower limit of the locked zone on the plate boundary, and is estimated to be located at a depth of 50-60km in northeastern Japan. The back-slip distribution derived from three-component velocity data in this study, however, indicates that the back-slip region extends to a depth of about 100km, much deeper than that predicted based solely on horizontal displacement data.

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