Persistent effects of low-velocity fault zones on earthquake rupture after multiple earthquake cycles

Benjamin Idini, & Jean-Paul Ampuero

Published August 14, 2018, SCEC Contribution #8574, 2018 SCEC Annual Meeting Poster #196

Faults are usually embedded in a damaged zone characterized in field observations by distributed fractures and micro-cracks and in seismological and geodetic observations by reduced wave velocities relative to the wall rock. Recent dynamic rupture simulations show that the presence of a damaged zone around a fault can induce complex rupture modes that depart from a classical crack under a simple slip-weakening friction law (i.e. pulse-like rupture or super-shear rupture speed under relatively low background stresses). However, the efficiency of the mechanism promoting such complexity is strongly dependent on the prescribed initial level of stress. Here we investigate the effects of damaged fault zones on the promotion of rupture complexity throughout multiple earthquake cycles, in which the stress at the onset of rupture is a consequence of self-organization along the cycles. We consider a simple tabular model where a fault is bisecting a homogeneous low-rigidity layer (the damaged zone) embedded in an intact medium (the wall rock). The model is described by two parameters: damage level, D = 1 – G_d/G, and damage zone thickness, h. We conduct simulations based on the conventional Dieterich-Ruina rate-and-state friction and the quasi-dynamic approximation. Our numerical results show the development of complex rupture patterns at a critical damage-zone thickness defined between two end members in which h is large or small compared to the fault size. The complex rupture behavior involves secondary fronts emerging from the main rupture front and propagating in the opposite direction. Moreover, it involves shorter rise-times, flatter slip profiles, and lower stress drops than in intact homogeneous media, the first two being signatures of pulse-like rupture. The back-propagating pulses tend to nucleate and arrest at localized stress heterogeneities left by previous ruptures. To test the hypothesis that damage zones are a viable mechanism for Rapid Tremor Reversals (RTR) observed during Episodic Tremor and Slip in Cascadia and Japan, we model slow slip by introducing a linear velocity-strengthening. Results show that our model is capable of reproducing RTRs qualitatively, and motivate further work to examine how damage zones quantitatively affect tremor migration patterns.

Citation
Idini, B., & Ampuero, J. (2018, 08). Persistent effects of low-velocity fault zones on earthquake rupture after multiple earthquake cycles. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)