The competitive effects on slip pulse properties of low - velocity fault zones and strong 1 velocity - weakening friction 2

The damaged rock around mature faults forms a zone of low seismic wave 24 velocities, which can perturb important earthquake rupture properties. In particular, waves 25 reflected within the fault zone structure can induce short-duration slip pulses, an apparently 26 predominant earthquake rupture mode. Another known mechanism to generate slip pulses is 27 strong velocity-weakening friction. Here we study the competing effects of these two 28 mechanisms on the properties of slip pulses using dynamic rupture simulations. We find that 29 even under strong velocity-weakening friction, the fault zone can significantly affect rupture 30 properties such as rise time and slip rate. In addition, we find that the stress perturbation induced 31 by groups of reflected waves generate rupture speed oscillations in fault zones of relatively low 32 velocity reductions. We provide a theoretical relation between oscillation period and fault zone 33 properties. The fault zone can enhance high-frequency ground motions and cause overestimation 34 of moment release if ignored. 35 36 37 38 39 40 41 42 43 44 45 46 HUANG AND AMPUERO: FAULT ZONE AND VELOCITY WEAKENING