Inter-plate Coupling Along the Nankai Trough and Southeastward Motion Along Southern Part of Kyushu

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  • NISHIMURA Sou
    Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University
  • ANDO Masataka
    Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University
  • MIYAZAKI Shin'ichi
    Space Geodesy Laboratory, Geography and Crustal Dynamics Research Center, Geographical Survey Institute

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Other Title
  • 南海トラフ沿いのプレート間カップリングと九州南部の南東向き運動について
  • ナンカイ トラフ ゾイ ノ プレート カン カップリング ト キュウシュウ ナンブ ノ ナントウ ムキ ウンドウ ニ ツイテ

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Abstract

Inter-plate coupling on the convergent plate boundary along the Nankai Trough is analyzed on the basis of GPS measurement data of horizontal displacements in southwest Japan for a period between March 1996 and April 1998. During this period, five moderate-size earthquakes occurred in the study area: M6.6 event offshore Tanegashima in October 1996, two M6.6 events in Hyuganada in October and December 1996, and two M6.1 events in Kagoshima in March and May 1997. Co-seismic displacements associated with these earthquakes are first removed from the GPS displacements. Then, a back-slip model is applied to estimate slips on the coupling interface. High ratios of back-slip relative to plate motion (approximately 100%) are obtained along the eastern part of the Nankai Trough. On the contrary, offshore Kyushu, the ratios are negative, suggesting southeastward motion of the southern part of Kyushu relative to the Philippine Sea plate. We regard these negative values of the inter-plate coupling as a result of the post-seismic deformation associated with two Hyuganada earthquakes in 1996. The stress relaxation model of the asthenosphere is tested using a 3-D finite element method, and it is found that the effect of the relaxation is too small to explain the observed displacements during the post-seismic period. A feasible model adopted to explain the displacements is the after-slip, which possibly occurred on the focal region or its neighboring extension on the plate boundary. The post-seismic slips in Tanegashima and Hyuganada region thus obtained amount to 145 and 208mm, respectively. Generally, the after-slip can explain the observed post-seismic displacements in Kyushu. However, in the western part of Kyushu, the eastward displacement residuals still remain. To elucidate these displacements, different mechanisms such as a back-arc spreading of the Okinawa Trough, or an internal deformation of the continental plate, should be considered.

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