Effects of Tidal Modulation in Heterogeneous Models of Slow Slip

Abstract:

Since their discovery, numerous models have been put forward to explain the occurance of slow slip and associated tremor. These models invoke a wide array of causal mechanisms and are all successful in reproducing the first-order behavior of slow-slip events. Discriminating amongst the various proposed models requires looking at second-order effects of slow slip and tremor. Here, we consider the effects of tidal modulation on slow slip in subduction zones. A great deal of observational evidence has established that slow-slip and associated tremor are modulated by the small stress perturbations associated with tides and teleseismic events. Recent modeling studies that have examined the influence of tidal stresses (<10 kPa) have focused either on the effects of tidally induced changes in shear stress, or on changes in shear and normal stress that coincide. However, along the Cascadia margin, the relative phase of the tidally induced fault-normal and shear stresses depends on position along the plate boundary fault, and can vary from being in phase, to completely out of phase. We report on the predictions of models designed to examine the sensitivity of slow-slip in subduction zones to the phase shift γ between tidally induced normal and shear stress perturbations. We consider both simple spring-slider and 1-D elastodynamic models that are designed to mimic the effects of geologic heterogeneity by allowing for variations in the rate-and-state frictional parameters. For a given slow-slip event, spring-slider results indicate that the phase lag γ_v between the peak slip rate and the tidally induced shear stress perturbation depends on both the phase shift γ, and the perturbation amplitude. Models parameterized for Cascadia are capable of producing phase lags γ_v within the range (15^◦ to 30^◦) of those reported by Royer et al. (JGR, 2014). Additionally, our models predict that the correlation between tidally induced shear stress perturbations and resultant slip also depends on the phase shift γ. Our results indicate that the influence of tides, especially the effects of phase differences between tidaly induced shear and normal stresses, must be taken into account when evaluating the physical mechanisms of slow-slip and tremor.