Nucleation of Earthquake Slip on Heterogeneous Interfaces

Abstract:

Understanding how frictional instabilities initiate is an outstanding problem, with implications for earthquake forecasting and seismic hazard. The nucleation process on a uniform frictional interface has a relatively well-understood progression. The slip begins quasi-statically, meanders within a localized zone, reaches a critical length-scale, termed the nucleation size, and thereafter, transitions to a dynamic slip event. However, the process is much more complex and not well understood on heterogeneous interfaces.

To shed light on this process, we study the initiation of slip on interfaces with fractal-like distributions of normal stress. We use 3D numerical simulations of slip on a 2D frictional interface embedded in a homogeneous elastic space, with an efficient and rigorous numerical procedure for elastodynamic analysis of sequences of aseismic slip and dynamic ruptures on slowly loaded interfaces. Using the diverse spectrum of nucleation behaviors observed in these models, our work is directed towards understanding several questions fundamental to earthquake science. Do small and large earthquakes initiate similarly? Are large earthquakes just small earthquakes that ran away? What model features result in foreshock-like events, i.e. smaller events that occur during the nucleation of larger events? How does the simulated critical length scales for instability, averaged over several events, compare with the nucleation size estimate for uniform interfaces?

We find that earthquakes can initiate from scales much smaller than the nucleation size estimates calculated for uniform interfaces with equivalent average properties. As the heterogeneities become stronger, small-scale events appear, complicating the nucleation process. We observe a continuum of behaviors ranging from purely fault-spanning events to persistent foreshock-like events interspersed between mainshock cycles. We also find more variation in the final sizes of events in these heterogeneous conditions. Overall, the broad goal of this work is to understand how local heterogeneities in friction properties translate into large-scale behaviors, both in terms of stability and slip patterns.