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[J08-P-10] Heterogeneous structure in the incoming Philippine Sea plate along the Nankai Trough
Rupture of large-thrust earthquakes along the Nankai Trough is known by always initiating from off the Kii Peninsula. The segmentation boundary between the 1944 Tonankai (Mw=8.1) and the 1946 Nankai (Mw=8.4) earthquake rupture is locate off the Kii Peninsula. Activity of the nonvolcanic deep low-frequency tremors and very low-frequency earthquakes observed around the down-dip limit of the coseismic rupture zone of the last Tonankai and Nankai earthquakes is not homogeneous, and the belt-like tremor zone is divided into several segments bounded by gaps [Obara, 2010]. Largest gap is recognized around the Kii channel between the Shikoku Island and Kii Peninsula.
Our recent integrated result of first-arrival tomography based on the 2012 and 2014 wide-angle OBS data shows dramatic along-trough variation in P-wave velocity just beneath the basement of the incoming Philippine Sea plate. Velocity change can be recognized south off the Cape Muroto and the Shima Peninsula just seaward of the trough axis. Such dramatic velocity change corresponds with the structural change in the configuration of the basement reflection in the time-migrated section. Similar along-trough structural variation can be recognized in the central Shikoku Basin far south from the trough axis [Nishizawa et al., 2011]. Heterogeneous structure along the trough may be related to the formation of the incoming Philippine Sea plate, because the structural boundary may correspond with the plate age of about 20-21.5Ma proposed by Okino [2015] based on magnetic lineation. This structural characteristic is thought to continue northwards to the subducting Philippine Sea plate beneath the southwest Japan, and may cause the segmentation of an earthquake rupture, and heterogeneous activity of low-frequency earthquake phenomena.
This study is part of ‘Research project for compound disaster mitigation on the great, earthquakes and tsunamis around the Nankai Trough region' funded by MEXT, Japan.
Our recent integrated result of first-arrival tomography based on the 2012 and 2014 wide-angle OBS data shows dramatic along-trough variation in P-wave velocity just beneath the basement of the incoming Philippine Sea plate. Velocity change can be recognized south off the Cape Muroto and the Shima Peninsula just seaward of the trough axis. Such dramatic velocity change corresponds with the structural change in the configuration of the basement reflection in the time-migrated section. Similar along-trough structural variation can be recognized in the central Shikoku Basin far south from the trough axis [Nishizawa et al., 2011]. Heterogeneous structure along the trough may be related to the formation of the incoming Philippine Sea plate, because the structural boundary may correspond with the plate age of about 20-21.5Ma proposed by Okino [2015] based on magnetic lineation. This structural characteristic is thought to continue northwards to the subducting Philippine Sea plate beneath the southwest Japan, and may cause the segmentation of an earthquake rupture, and heterogeneous activity of low-frequency earthquake phenomena.
This study is part of ‘Research project for compound disaster mitigation on the great, earthquakes and tsunamis around the Nankai Trough region' funded by MEXT, Japan.