Seismic and Transient Slip Characteristics of Heterogeneous Frictional-Viscous Shear Zones

Abstract:

Both shallow and deep sections of the subduction interface in several modern subduction zones exhibit aseismic slow slip events, commonly accompanied by low frequency earthquakes. Observations from exhumed rocks suggest that the subduction interface in these regions is a shear zone in which frictional lenses are embedded in a weaker, distributed viscous matrix. Here we use seismo-mechanical modeling to explore the transient slip characteristics of finite-width frictional-viscous shear zones.

Our model formulation follows Herrendörfer et al. (JGR, 2018), which includes an invariant form of rate- and state-dependent friction (RSF) and simulates earthquakes along spontaneously evolving faults embedded in a 2D continuum. The setup includes two elastic plates bounding a viscoelastoplastic shear zone (subduction interface) with inclusions (clasts) of varying sizes, aspect ratios, densities and viscosity contrasts with respect to the surrounding matrix. The entire shear zone exhibits the same velocity-weakening RSF parameters, but the low viscosity matrix in the shear zone has the capacity to switch between RSF and linear viscous creep as a function of its prescribed viscosity and local stress state.

Results show that for a range of matrix viscosities near the frictional-viscous transition, the viscous component of these heterogeneous shear zones both 1) sets the ‘speed limit’ for earthquake ruptures that nucleate in clasts such that they propagate at velocities similar to observed slow slip events; and 2) simultaneously permits the transmission of slow slip from clast to clast, allowing slow ruptures to propagate substantial distances over the model domain. Modeled events have moment-duration statistics that span much of the slow slip spectrum, with very low frequency earthquakes favored at high clast densities, and longer duration, higher-magnitude events favored at intermediate clast densities. Calculated stress drops for modeled events are in the range 1-100 kPa, consistent with stress drops inferred for slow slip events based on tidal sensitivities. Additionally, the models commonly show secondary slip fronts (some reverse-propagating) that are triggered when active ruptures propagate into regions of concentrated stress on clast margins.