11:15 AM - 11:30 AM
[J08-3-04] Imaging of the subducted Philippine Sea plate and the overriding SW Japan arc - Reinterpretation of the wide-angle reflection data in the Kii Peninsula, SW Japan -
Our recent reinterpretation for seismic refraction/wide-angle reflection data in eastern Kii Peninsula, SW Japan, provided new structural information on the subducted Philippine Sea (PHS) plate and overriding the SW Japan arc.
The Kii peninsula is located in the eastern part of the well-known seismogenic zone along the Nankai trough. The plate boundary beneath this peninsula is in the stable or conditionally stable regime except for its southernmost tip (the northwestern end of the rupture area at the 1944 Tonankai earthquake (M7.9)). Our data were acquired in 2006 along 80-km line almost perpendicular to the Nankai trough. To image the structure of overriding the SW Japan arc, we retrieved virtual shot records from free-surface backscattered waves by the deconvolution interferometry. The subsequent CRS (Common Reflection Surface)/MDRS (Multi-Dip Reflection Surfaces) methods delineated a northward dipping reflector band just south of the Median Tectonic Line (MTL). This reflection band, about 10-15 km thick, includes the High-P Sambagawa Metamorphic belt, extending from 2-10 km to 25-35 km depth. The MTL itself is recognized as the uppermost part of this band inclining northward to a depth of nearly 25 km.
The PHS plate is well imaged as northward dipping reflectors in a depth range of 20-35 km beneath the southern half of our profile both by seismic refraction and reflection analyses. A thin (less than 1 km) low velocity (3.5~5km/s) layer is situated at the top of the PHS plate under the southernmost part of the profile, namely the trenchward half of the conditionally stable zone. In the central part of the profile (the landward half of the conditionally stable zone), strong reflectors with 2-3 km/s velocity contrast are distributed in a diffused manner at 30-35 km depths, around which low frequency earthquakes are occurring. The obtained lateral structural change is probably controlled by dehydrated fluids from the subducted PHS plate.
The Kii peninsula is located in the eastern part of the well-known seismogenic zone along the Nankai trough. The plate boundary beneath this peninsula is in the stable or conditionally stable regime except for its southernmost tip (the northwestern end of the rupture area at the 1944 Tonankai earthquake (M7.9)). Our data were acquired in 2006 along 80-km line almost perpendicular to the Nankai trough. To image the structure of overriding the SW Japan arc, we retrieved virtual shot records from free-surface backscattered waves by the deconvolution interferometry. The subsequent CRS (Common Reflection Surface)/MDRS (Multi-Dip Reflection Surfaces) methods delineated a northward dipping reflector band just south of the Median Tectonic Line (MTL). This reflection band, about 10-15 km thick, includes the High-P Sambagawa Metamorphic belt, extending from 2-10 km to 25-35 km depth. The MTL itself is recognized as the uppermost part of this band inclining northward to a depth of nearly 25 km.
The PHS plate is well imaged as northward dipping reflectors in a depth range of 20-35 km beneath the southern half of our profile both by seismic refraction and reflection analyses. A thin (less than 1 km) low velocity (3.5~5km/s) layer is situated at the top of the PHS plate under the southernmost part of the profile, namely the trenchward half of the conditionally stable zone. In the central part of the profile (the landward half of the conditionally stable zone), strong reflectors with 2-3 km/s velocity contrast are distributed in a diffused manner at 30-35 km depths, around which low frequency earthquakes are occurring. The obtained lateral structural change is probably controlled by dehydrated fluids from the subducted PHS plate.