Dilatant strengthening as a mechanism for slow slip events

The mechanics of slow slip events (SSE) in subduction zones remain unresolved. We suggest that SSE nucleate in areas of unstable friction under drained conditions, but as slip accelerates dilatancy reduces pore pressure p quenching instability. Competition between dilatant strengthening and thermal pressurization may control whether slip is slow or fast. We model SSE with 2-D elasticity, rate-state friction, and a dilatancy law where porosity phi evolves toward steady state phi(ss) over distance d(c) and phi(ss) = phi(0) + epsilon ln(v/v(0)); v is slip speed. We consider two diffusion models. Membrane diffusion (MD) is approximated by -(p -p(infinity))/t(f) where p and p(infinity) are shear zone and remote pore pressure and t(f) is a characteristic diffusion time. Homogeneous diffusion (HD) accurately models fault-normal flow with diffusivity c(hyd). For MD, linearized analysis defines a boundary epsilon = 1 - a/b between slow and fast slip, where epsilon = f(0)epsilon/beta b(sigma - p(infinity)), f(0), a, and b are friction parameters and beta is compressibility. When epsilon < 1 - a/b slip accelerates to instability for sufficiently large faults, whereas for epsilon > 1 -a/b slip speeds remain quasi-static. For HD, E-p equivalent to epsilon h/(beta(sigma - p(infinity))root v(infinity) / c(hyd)d(c)) defines dilatancy efficiency, where h is shear zone thickness and v(infinity) is plate velocity. SSE are favored by large epsilon h and low effective stress. The ratio E-p to thermal pressurization efficiency scales with 1/(sigma - p infinity), so high p(infinity) favors SSE, consistent with seismic observations. For E-p similar to 10(-3) transient slip rates, repeat times, average slip, and stress drops are comparable to field observations. Model updip propagation speeds are comparable to those observed along-strike. Many simulations exhibit slow phases driven by steady downdip slip and faster phases that relax the accumulated stress. Model SSE accommodate only a fraction of plate motion; the remaining deficit must be accommodated during coseismic or postseismic slip.