Transient Slow Slip Characteristics of Frictional-Viscous Sudduction Megathrust Shear Zones

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 demonstrate a mechanism in which stress heterogeneity in these shear zones both (a) sets the speed limit for earthquake ruptures that nucleate in clasts such that they propagate at slow velocities; and (b) 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 overlap with some 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. Plain Language Summary Subduction megathrusts represent the largest and most hazardous seismogenic faults on Earth and exhibit a wide range of earthquake slip patterns. An especially perplexing form of slip on subduction megathrusts are slow earthquakes, which are slip events that release similar amounts of energy as regular earthquakes, but do so over months to years, rather than seconds. These events most commonly occur at deeper levels of the subduction megathrust where rocks are thought to transition from brittle and strong, with deformation dominated by fracture and cracking, to smoother, continuous, and weak-with deformation dominated by flow. In this work, we use numerical models to explore the seismic slip characteristics of megathrust faults that are mixtures of weak and strong materials. We simulate a wide megathrust fault zone with embedded weak and strong sections, and we systematically vary the strength contrasts between, and relative proportions of, weak to strong material. Our results suggest that three regimes of slip behaviors can be defined as a function of these strength contrasts and proportions of weak-to-strong materials: an aseismic regime with no earthquake slip, a slow-slip dominated regime, and a regular earthquake-dominated regime. These results help to reconcile some of the features that geologists find in rock outcrops brought to the surface from deep subduction environments, with the modern-day geophysical record of subduction zone earthquakes and surface deformation patterns.

Automated summary

In this study, numerical models were used to explore the seismic slip characteristics of frictional-viscous megathrust shear zones, revealing the importance of stress heterogeneity in controlling earthquake propagation speed and slow slip transmission distance.