Study of the inhomogeneous structure in the subduction zone based on the analysis of seismogram envelopes

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The elastic structure near around the subduction zone shows relatively strong inhomogeneities. Focusing on the decay of seismogram envelopes with increasing travel distance in the randomly inhomogeneous structure, we have constructed mathematical models, and estimated the characteristics of randomness in the wedge mantle beneath Tokai, Kanto, and Tohoku area from the practical analysis.(1) Envelope analysis: We proposed an inversion method to estimate the attenuation and randomness from the full seismogram envelope. From the evidence that the envelope broadening is larger at the back arc side of the volcanic front compared with those in the fore arc side, we found short wavelength components of the randomness are larger in the back arc side than those in the fore arc side.(2) Frequency dependent attenuation: From the comparison of the incident and the reflected phases in borehole seismograms, we quantitatively estimated S wave attenuation in the sediment layer. Extending the conventional coda normalization method, we proposed a method for the simultaneous measurement of P and S wave attenuation.(3) Measurement of the ratio of scattering attenuation and intrinsic absorption: Supposing isotropic scattering we calculated the time trace of S wave energy density based on the Monte-Carlo simulation. Comparing with the simulation and the observed data, we quantitatively estimated the ratio of scattering to intrinsic attenuation in the Kanto-Tokai area.(4) Mathematical modeling: We have formulated the multiple isotropic scattering model including P-S conversions based on the energy transportation theory.(5) Field observation: We started field observation at three sites in Tohoku, and found that the envelope shapes clearly depend on their ray paths.