[J06-4] The spectrum of fault-zone deformation processes (from slow slip to earthquake) IV

[J06-4-03] Down-dip variations in a subducting low-velocity zone linked to episodic tremor and slip

^○Aitaro Kato¹, Mitsuhiro Toya², Takuto Maeda¹, Kazushige Obara¹, Tetsuya Takeda³, Koshun Yamaoka⁴ (1.Earthquake Research Institute, the University of Tokyo, 2.Hanshin Consultants Co., Ltd., 3.National Research Institute for Earth Science and Disaster Resilience, 4.Nagoya University)

Fluids are thought to play an important role in controlling episodic tremor and slow slip (ETS) in subduction zones. Therefore, constraining the along-dip distribution of fluids is necessary to better understand source mechanism of ETS, and particularly the role played by fluids in ETS generation. Here, we report clear observations of coherent ScSp phases with a dense seismic array in western Shikoku, Japan, where ETS has been most active over the past decade. Using numerical simulations of elastic-wave propagation to reproduce the observed ScSp phases, we demonstrate that, relative to shallow depths, either the Vp/Vs ratio or the thickness of a low-velocity zone (LVZ) within the subducting oceanic crust increases beneath the mantle wedge corner where ETS has been observed. In addition, amplitudes of receiver functions associated with the LVZ increase at deeper depths, which support the down-dip variations of structural elements. These depth dependences of the structural elements provide us with new evidence that either high-pressurized fluid is confined or that a wide ductile shear zone, lubricated by fluid, develops in the subducting oceanic crust at ETS source depths.