Heterogeneity in the Crust and Uppermost Mantle beneath southwestern Japan Imaged by Coda Envelope Inversion

We have investigated short-wavelength heterogeneous structures in the crust and uppermost mantle beneath the Chugoku and Shikoku regions, southwestern Japan, in which the Philippine Sea plate subducts along the Nankai trough beneath the overriding plate. A large number of observed envelopes of S coda were analyzed by an envelope inversion method [Asano and Hasegawa, 2004] in order to estimate 3-D distribution of scattering coefficients. It is known that the seismic coda is a superposition of seismic waves scattered by short-wavelength heterogeneous structures. Estimated distribution of scattering coefficients provides us information about the short-wavelength heterogeneous structures, which is hardly imaged by travel-time tomography. We analyzed 3553 three-component seismograms recorded at 143 Hi-net stations installed with about 20- to 30-km separation. The target area is covered with 18 areas for inversion grid having 4139 node points with horizontal and vertical separations of 8 and 10 km in the central part, and with horizontal and vertical separations of 16 and 20 km in the surroundings. Scattering coefficient values in each node point are unknown parameters to be estimated. Obtained results show that there exist several zones with large scattering coefficients (LSZs). In the upper crust, LSZs are distributed in northen part of the Chugoku region and eastern part of Shikoku region, which is approximately corresponding to seismically active regions. In the lower potion, most predominant LSZs are widely distributed at depths of about 30 km and deeper beneath the central part of the Chugoku region. These LSZs are thick and extend to other LSZs located near the top of the subducting Philippine Sea slab. Shiomi et al. [2006] found that the depth of continental Moho is relatively deeper in the central part of the Chugoku region than in the surroundings by receiver function analysis; Matsumoto [2006] showed that bottom depth of seismogenic layer is deep and heat-flow value is low in this region. Therefore it seems that the LSZs in this region show distribution of reflective and/or heterogeneous structures in lower part of such cold and thick crust suggested by these previous studies.