The Effect of Viscoelastic Fault Zone Rheology on the Interseismic Growth of Fault Shear Stress Patches

Abstract:

Seismological studies of recent well-monitored mega-earthquakes have revealed more high-frequency radiation in deeper portion of plate interface in subduction zones which suggests that the degree of stress heterogeneity may increase along the seismogenic zone towards its down-dip limit. These stress heterogeneities likely have great influence on the rupture behavior of earthquake faults, but we have limited understanding on the origins and controls of these heterogeneities. In order to investigate the potential interseismic processes governing the development of fault stress heterogeneity, we conducted numerical modeling to explore how shear stress anomalies on fault interfaces (shear stress patches) evolve spatiotemporally under the influence of viscoelastic rheology assigned to fault zones.

Two-dimensional viscoelastic models consisting of a fault zone and host rock were sheared to simulate shear stress accumulation on faults due to tectonic loading. Results show that magnitudes of shear stress patches evolve not only temporally, but also spatially, especially when the stress anomaly is created by a geometrical irregularity (asperity) along the interface of an elastic host rock and viscoelastic fault zone. Such shear stress anomalies diffuse spatially so that the spatial dimension of the shear stress patch appears to grow over time. Models with varying fault zone viscoelastic properties or varying fault zone viscosity show that such spatial diffusion of shear stress is enhanced by increasing the contribution of the viscous behavior. The absolute rate at which shear stress patches grow spatially is generally not influenced by the size of the shear stress patch. Therefore shear stress patches with smaller dimensions will appear to grow quicker, in the relative sense, compared to larger stress patches. These results suggest that the minimum dimensions of shear stress patches that can exist along a fault could be governed by the effective viscosity of the fault zone. Therefore patterns of accumulated shear stress could vary along faults when viscous properties are heterogeneous, for instance due to depth or material heterogeneity, which has implications on how earthquake rupture behavior could vary along faults.