Realistic Variability in Seismic Moment and Recurrence Time of Repeating Earthquakes Reproduced in Models with Fractal Heterogeneity in Friction Properties

Observations indicate that even repeating earthquake sequences display significant variability in their moment and recurrence time. Similarly, creeping segments may be accumulating their slip unsteadily, as suggested by surface creep meters and inversions of GPS data. Such behavior may be pointing to the heterogeneity of fault properties within both seismogenic and creeping fault areas. Here, we explore the consequences of heterogeneity in velocity-weakening (VW) patches and/or the surrounding velocity-strengthening (VS) matrix on rate-and-state faults in terms of earthquake nucleation and the resulting slip patterns . Both slow and rapid slip events could occur on such faults, depending on the fractional area, connectivity, strength contrast of VW patches with respect to the surrounding VS region, and the size of the VW patches with respect to the nucleation size. We study fractal-like distributions of frictional properties on the interface, using 3D numerical simulations of a 2D fault embedded in a homogeneous elastic space, with an efficient and rigorous numerical procedure for elastodynamic analysis of dynamic ruptures on slowly loaded interfaces [Lapusta and Liu, JGR, 2009]. We find that 1) The fractal shape of source patches helps us reproduce observed variability, which is further enhanced by the presence of perturbing patches; 2) the triggering processes are affected by changes in the surrounding velocity strengthening region, and 3) smallest-scale heterogeneity creates visible changes to the slip behavior. Our findings can help constrain the level of heterogeneity on natural faults, highlight the role of creep in modulating seismogenic behavior, and indicate the importance of properly upscaling small-scale heterogeneity in fault-slip models.