Seismic attenuation structure associated with episodic tremor and slip zone beneath Shikoku and the Kii peninsula, southwestern Japan, in the Nankai subduction zone

The three-dimensional seismic attenuation structure (frequency-independent Q) beneath southwestern Japan was analyzed using t* estimated by applying the S coda wave spectral ratio method to the waveform data from a dense permanent seismic network. The seismic attenuation (Q(p)(-1)) structure is clearly imaged for the region beneath Shikoku, the Kii peninsula, and eastern Kyushu at depths down to approximately 50km. At depths of 5 to 35km, the seismic attenuation structure changes at the Median tectonic line and other geological boundaries beneath Shikoku and the southwestern Kii peninsula. High-Q(p) zones within the lower crust of the overlying plate are found just above the slip regions at the centers of the long-term slow-slip events (SSEs) beneath the Bungo and Kii channels and central Shikoku. Beneath central Shikoku, within the overlying plate, a high-Q(p) zone bounded by low-Q(p) zones is located from the land surface to the plate interface of the subducting plate. The high-Q(p) zone and low-Q(p) zones correspond to high-V-p and low-V-p zones of previous study, respectively. The boundaries of the high- and low-Q(p) zones are consistent with the segment boundaries of tremors (segment boundaries of short-term SSEs). These results indicated that the locations of the long- and short-term SSEs could be limited by the inhomogeneous distribution of the materials and/or condition of the overlying plate, which is formed due to geological and geographical process. The heterogeneity of materials and/or condition within the fore-arc crust possibly makes an effect on inhomogeneous rheological strength distribution on the interface.