Teleseismic scattering constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs

Abstract:

Slow earthquakes generally occur on the megathrust fault around the transition between unstable and stable slip regimes and in the vicinity of the mantle wedge corner, where three major structural elements are in contact: the subducting oceanic crust, the overriding forearc crust and the continental mantle. In this region, thermo-petrological models predict significant fluid production from the dehydrating oceanic crust and mantle due to prograde metamorphic reactions, and their consumption by hydrating the mantle wedge. These fluids are expected to affect the dynamic stability of the megathrust fault and enable slow slip by increasing pore-fluid pressure and/or reducing friction in fault gouges. Resolving the fine-scale structure of the deep megathrust fault and the in situ distribution of fluids where slow earthquakes (and tremors) occur is challenging, and most advances have been made using teleseismic scattering techniques (e.g., receiver functions). Here we review the teleseismic structure of six well-studied subduction zones (i.e., Cascadia, southwest Japan, central Mexico, Costa Rica, Alaska, and Hikurangi) that exhibit slow earthquake processes and discuss the evidence of structural and geological controls on the slow earthquake behavior. We also present new evidence using OBS data that the locked zone of Cascadia may be characterized by inferred high pore-fluid pressure, suggesting that the entire megathrust fault may be weak due to fluid overpressure. We conclude that properties of geological materials play a dominant role in controlling the transition from seismic to slow slip behavior, and that near-lithostatic pore-fluid pressures near the megathrust fault may be a necessary but insufficient condition for their occurrence.