11:00 AM - 11:15 AM
[SSS04-08] Structures of the subducted Philippine Sea plate and the overriding SW Japan arc from reprocessing of seismic wide-angle reflection data in Kii Peninsula, SW Japan
Keywords:plate subduction, Philippine sea plate, wide-angle reflection data, plate boundary, low frequency earthquake, frictional property
The Kii peninsula is located in the eastern part of the well-known seismogenic zone developed 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 last event (1944 Tonankai earthquake (M7.9)). The surface geology of the overriding SW Japan arc is divided to two parts by the E-W trending Median Tectonic Line (MTL), the most prominent tectonic boundary in SW Japan. South of the MTL, Cretaceous-Jurassic accretionary complexes are exposed, whose northernmost unit consists of high P-T metamorphic rocks (the Sanbagawa metamorphic belt (SMB)). The region north of the MTL, on the other hand, is occupied by older accretionary complexes, partly suffered from the Cretaceous magmatic intrusions.
Our seismic data from five dynamite shots were acquired in 2006 along 80-km line almost perpendicular to the Nankai trough. The structure of the SW Japan arc was obtained both from intensive wide-angle reflection analysis and advanced reflection processing by seismic interferometry technique. The former analysis delineated clear structural change in the uppermost crust across the MTL. In the latter processing, we retrieved virtual shot records at 512 receiver points from free-surface backscattered waves by the deconvolution interferometry. The subsequent CRS (Common Reflection Surface)/MDRS (Multi-Dip Reflection Surfaces) methods provided an enhanced image within the island arc, including a northward dipping reflector band just south of the MTL. This reflection band, about 10-15 km thick, includes the SMB, 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.
In the reflection processing, 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. The wide-angle reflection analysis delineated lateral reflectivity change along the plate boundary. 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. Such reflective signature fades out to the north, approaching to stable regime region. The obtained lateral structural change are well correlated with the frictional properties of the plate boundary, probably controlled by the dehydration process within the PHS plate.