Seismic structure of western end of the Nankai trough seismogenic zone

[1] In 1998 a seismic experiment along a 320-km-long profile using an air gun array and ocean bottom seismographs (OBSs) was collected in the on shore-offshore, forearc region of the western Nankai trough seismogenic zone, southwestern Japan. This area is outside the coseismic rupture area of the 1946 Nankaido earthquake (M-s = 8.2). The main objectives of this paper are to clarify the P wave and the Poisson's ratio profile around the forearc region of southwestern Japan, and estimate differences between this area and the Nankaido coseismic rupture area. The P wave velocity model estimated by two-dimensional (2-D) ray tracing and travel time inversion indicates three units with distinctive velocity patterns. The lens-shaped unit has a maximum thickness of 5 km and velocity of 3.4-4.0 km/s. The underlying unit has velocity of 5.4-5.6 km/s, and its P wave velocity increases on land (V-p = 6.2-6.4 km/s). The relative homogeneous unit with P wave velocity of 6.6-6.9 km/s exists just beneath secondary unit with P velocity of 5.4-5.6 km/s. The angle of subduction is similar to7degrees. We also obtained a Poisson's ratio of the crustal structure of the forearc region. The Poisson's ratio of the southern part of secondary unit is larger than that of the northern part. We conclude that these data indicate clay-rich metamorphic rocks plus oceanic basaltic rocks, rather than high-porosity materials, comprise the majority of the layer. We suggest that the region of high Poisson's ratio probably corresponds to underplating and the metamorphism of the clay-rich sediments and possibly the oceanic basalt just below the decollement.