Seismic attenuation structure of the accretionary prism in the Kumano Basin, Nankai Trough, Japan, revealed by walk-away VSP

The Kumano Basin is a forearc basin located on the landward slope of the Nankai Trough. It lies directly above the large coseismic slip area of the 1944 Tonankai earthquake (M 8.1). A number of seismic surveys have revealed the structural heterogeneity of the accretionary prism relevant to the evolution of the margin including the dynamic behaviors of the megathrust and megasplay faults. In this paper, we report on the seismic attenuation structure of the accretionary prism, which has been poorly known, especially in the seismic frequency band (~ 10 Hz), by analyzing a walk-away vertical seismic profile (WVSP) obtained during IODP Exp. 319 in 2009, as a part of the NanTroSEIZE project. A 16 channel vertical geophone arrays installed in the deep (from 907 to 1,135 m below seafloor) borehole provided seismic waveform records with broad frequency band and enabled us to estimate an attenuation parameter (Q) through calculations of the amplitude fall off rates of the high frequency component of the recorded signal. By taking the spectral ratio between up-coming P-waves from large offset shots and down-going waves from near offset shots, we could estimate Qp of the prism down to ~ 5 km below seafloor. Preliminary estimated value of Qp was 80 ~ 120 in frequency range of 10 to 50 Hz, assuming frequency independent Q. The smaller estimated Qp in Nankai indicates that the seismic attenuation in the accretionary prism is larger than in the onshore part of the NW-Japan (Q ~ 250 at ~ 10 Hz), but Qp is larger in Nankai than in the Costa Rica margin wedge (Q = 25 ~ 75) indicating substantially less attenuation in Nankai than Costa Rica.