Seismic and Episodic Slip Characteristics of Frictional-Viscous
Subduction Megathrust Shear Zones
Abstract
The deep roots of subduction megathrusts exhibit aseismic slow slip
events, commonly accompanied by tectonic tremor. Observations from
exhumed rocks suggest this region of the subduction interface is a shear
zone with frictional lenses embedded in a viscous matrix. Here we use
numerical models to explore the transient slip characteristics of
finite-width frictional-viscous megathrust shear zones. Our model
utilizes an invariant, continuum-based, regularized form of rate- and
state-dependent friction (RSF) and simulates earthquakes along
spontaneously evolving faults embedded in a 2D heterogeneous continuum.
The setup includes two elastic plates bounding a viscoelastoplastic
shear zone (subduction interface melange) with inclusions (clasts) of
varying distributions and viscosity contrasts with respect to the
surrounding weaker matrix. The entire shear zone exhibits the same
velocity-weakening RSF parameters, but the lower viscosity matrix has
the capacity to switch between RSF and viscous creep as a function of
local stress state. Results show that for a range of matrix viscosities
near the frictional-viscous transition, viscous damping and stress
heterogeneity in these shear zones both 1) sets the ‘speed limit’ for
earthquake ruptures that nucleate in clasts such that they propagate at
slow velocities; and 2) permits the transmission of slow slip from clast
to clast, allowing slow ruptures to propagate substantial distances over
the model domain. For reasonable input parameters, modeled events have
moment-duration statistics, stress drops, and rupture propagation rates
that match natural slow slip events. These results provide new insights
into how geologic observations from ancient analogs of the slow slip
source may scale up to match geophysical constraints on modern slow slip
phenomena.