[J06-3] The spectrum of fault-zone deformation processes (from slow slip to earthquake) III

[J06-3-06] Rheologically controlled spatial separation of the megathrust seismogenic zone and the zone of Episodic Tremor and Slip

^○Kelin Wang¹, Xiang Gao² (1.School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada, 2.Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China)

invited

Episodic Tremor and Slip (ETS) is the most peculiar type of slow slip event. While slow slips of various durations and magnitudes occur at all tectonic environments, ETS is the most abundant in subduction zones of young and warm subducting slabs such as Cascadia, Nankai, and Mexico. Adding to the peculiarity is a tendency to be confined to a narrow depth range around the tip of the mantle wedge. If the ETS zone is assumed to represent a transition from seismic to aseismic behavior, its spatial separation from the seismogenic zone makes it even more peculiar. Here we propose a thermo-rheological model to explain all these peculiarities, especially the separation of the two zones. The model consists of the following key components. (1) The dehydrating slab supplies ample fluids at shallow depths. (2) The tip portion of the mantle wedge is fully serpentinized, and slab-derived fluids precipitates large amounts of silica further updip. (3) Resultant permeability reduction around the wedge tip leads to locally very high pore-fluid pressure (Pf). (4) The seismogenic frictional behavior of the megathrust is terminated at a depth much shallower than the wedge tip. (5) But the very high-Pf around the mantle wedge tip gives rise to another, isolated friction zone responsible for ETS. (6) Separating the seismogenic and ETS zones is a segment of semi-frictional or viscous behavior that can produce long-term slow slip events. Using numerical calculation, we show that warm slabs provide favorable conditions for components (1), (2), and (4), leading to components (3), (5), and (6). We also show that under special geological circumstances, similar conditions may occur and promote ETS-like phenomena even without a warm slab, such as in Hikurangi, or even not in a subduction zone, such as along the San Andreas Fault. The new model reconciles a wide range of seemingly disparate observations and defines a new framework for the study of slip behaviour and seismogenesis of major faults.