[SSS12-P17] Trans-dimensional imaging or random inhomogeneities at northern Ryukyu arc with a non-diagonal covariance matrix
Preliminary analysis with a part of observed data imaged weak random inhomogeneity at most of the study area. These weak inhomogeneities commonly have steep spectral gradient. One of strongly inhomogeneous areas was imaged beneath the Tokara islands. Spectral gradient in this area is gentle as with those beneath the Quaternary volcanoes at northeast Japan (Takahashi et al. 2009). Other strongly inhomogeneous areas were found in the forearc side at 10-30 km depth. These anomalies show linear distributions parallel to the Ryukyu trench. Distances from the trench axis are about 80km and 110 km. Vertical extents of these anomalies have large uncertainties due to insufficient data, but it seems to be certain that the former one locates at low seismicity area of regular earthquakes. Correlation with shallow low frequency tremors (e.g., Yamashita et al. 2015) is not clear, then this strong inhomogeneity probably represents structural anomalies of subducting plate rather than those at the plate boundary.
Acknowledgement: This study is supported by “Research Project for Compound Disaster Mitigation on the Great Earthquakes and Tsunamis around the Nankai Trough Region” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. We are grateful for Nishinoomote-city, Minamitane-town and Toshima-village for their kind corporation for onshore seismic observations.