Anisotropic viscoplasticity explains slow-slip M0-T scaling at convergent plate margins

Abstract

In this study, we quantify the mechanisms that govern two related observations with regard to deep (15–50 km) subduction-zone aseismic slow-slip events (SSEs): (i) the linear scaling relationship between seismic moments (M0) and event durations (T), and (ii) the direction-dependent slow-slip rupture speeds. Geological observations suggest that deep-subduction slow-slip shear zones are anisotropic and viscoplastic; the anisotropy is due to the presence of dip-parallel mafic lineaments created by seamount subduction, whereas the viscoplasticity is due to deformation of mixed brittle mafic and ductile felsic materials. We postulate that a dip-parallel (=slip direction) mafic lineament in an overall felsic slow-slip shear zone acts as a stress guide, which localizes initial slow-slip rupture in the dip direction. Subsequent stress concentration along the dip-parallel edges of the early ruptured lineament leads to along-strike rupture, with the rupture front propagating through the felsic shear zone. The second-phase slip-area expansion maintains a constant dip-parallel rupture-zone length, inherited from the length of the early ruptured mafic lineament. By combining an energy balance equation with a two-phase rupture model outlined above, we obtain the first analytical expression of the observed linear scaling law in the form of M0 = c0T, where c0=4γ1ΔzG2L2μ¯s−μ¯dρgHΔzηecosδ+VFW2LΔzμ¯s2−μ¯d2ρgH2−4γ1GL+Δz−ρΔzLGVFW+va2; the observed value of this empirical constant is between 1011.5 and 1013.5 J/s. In the above expression, L, H, Δz, δ, G, ηe, μ¯s, and μ¯d are length, depth, thickness, dip angle, shear rigidity, effective viscosity, and effective coefficients of static and dynamic friction of the slow-slip shear zone, γ1 is surface-energy density of the initially ruptured mafic lineament, ρ is overriding-plate density, VFW and va are subducting-plate and slow-slip velocities, and g is surface gravity. Our model, based on the assumed shear-zone anisotropy, successfully predicts fast (~100 km/h) dip-parallel rupture along high-viscosity (~1020 Pa s) mafic lineaments and slow (2–10 km/day) strike-parallel rupture through low-viscosity (~1017 Pa s) felsic materials during a deep-subduction slow-slip event.