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

Abstract

AbstractThe 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 (Qp−1) structure is clearly imaged for the region beneath Shikoku, the Kii peninsula, and eastern Kyushu at depths down to approximately 50 km. At depths of 5 to 35 km, the seismic attenuation structure changes at the Median tectonic line and other geological boundaries beneath Shikoku and the southwestern Kii peninsula. High‐Qp 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‐Qp zone bounded by low‐Qp zones is located from the land surface to the plate interface of the subducting plate. The high‐Qp zone and low‐Qp zones correspond to high‐Vp and low‐Vp zones of previous study, respectively. The boundaries of the high‐ and low‐Qp 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.