On the Relationship Between the Seismogenic Zone and the ETS Zone

Abstract:

Despite breathtaking developments in the study of episodic tremor and slip (ETS) and related phenonena over the past 15 years, the relation between the seismogenic zone and the ETS zone is still enigmatic. The spatial gap between the two zones observed in some places and the global rarity of ETS-type phenomena challenge the notion that ETS is a universal transitional behaviour as faults change from seismic to aseismic with increasing depth or for other reasons. We address this issue by examining Frictional-Viscous Transitions (FVTs) along subduction faults. Our primary hypothesis is that serpentine (antigorite) saturation of the most seaward portion of the mantle wedge and silica deposition further seaward result in very low permeability and very high pore fluid pressure. The fluid pressure is higher in warm-slab subduction zones because of greater fluid supply from slab dehydration at this depth. Through thermal modelling using heat flow data as constraints, we found that for warm-slab subduction zones Cascadia, Nankai, and Mexico, there are two FVTs, with the first one terminating the seismogenic zone. Downdip of this first FVT, the fault exhibts a semi-frictional behaviour, which may facilitate long-term slow slip, and/or viscous behaviour. In the high-fluid-pressure zone around the mantle wedge corner, the fault returns to the friction mode, but with slip behaviour affected by the presence of hydrous minerals and high fluid pressure. We propose this is the ETS zone. Farther downdip, the second FVT occurs and terminates the ETS zone. This double-FVT mechanism also explains the presence of a “gap” along the San Andreas Fault between the seismogenic zone and the tremor zone. In a way, the mechanism is akin to the “jelly-sandwich” rheology model of the continental lithosphere, but the onset of the lower slice of bread is due to a decrease in frictional strength as opposed to an increase in viscous strength. For most cold-slab subduction zones such as Japan Trench, the shallower FVT is absent, such that frictional behaviour extends to deeper than the mantle wedge corner, retarding ETS. However, two FVTs are present in the cold-slab Hikurangi subduction zone because of the greater frictional strength of its megathrust than in most other subduction zones, so that ETS type phenomena can occur.