ViscoSim Earthquake Simulator

Synthetic seismicity simulations have been explored by the Southern California Earthquake Center (SCEC) Earthquake Simulators Group in order to guide long‐term forecasting efforts related to the Unified California Earthquake Rupture Forecast (Tullis et al. , 2012a). In this study I describe the viscoelastic earthquake simulator (ViscoSim) of Pollitz, 2009. Recapitulating to a large extent material previously presented by Pollitz (2009, 2011) I describe its implementation of synthetic ruptures and how it differs from other simulators being used by the group. ### Generation of Synthetic Slip Events Model faults are assumed to reside in an elastic crust that overlies a viscoelastic crust and mantle with a Maxwell viscoelastic rheology. The evolving physical variable is the Coulomb failure function, which is a linear combination of the shear stress and normal stress resolved upon an a priori slip direction along the considered fault surface (Simpson and Reasenberg, 1994). Following Ben‐Zion et al. (2003), a static stress threshold σ s determines the initiation of a slip event. During an event, stress on an initially failing patch drops by an amount Δ σ (a stress‐reduction parameter assigned to a given patch) to the arrest stress level σ a (more precisely, the slip is that which would reduce the stress to σ a in the absence of slip on other patches). After a set of slip events so determined, stress is transferred from slipped patches to neighboring patches, which may fail first at the static stress threshold σ s and subsequently (during the same event) at the dynamic stress threshold σ d . It is useful to define a dynamic overshoot coefficient D =Δ σ /( σ s − σ d ), where Δ σ = σ s − σ a (i.e., the reduction in stress on one initially failing patch). The parameter D characterizes how far the stress in each subevent …