Earthquake Rupture Through a Step‐Over Fault System: An Exploratory Numerical Study of the Leech River Fault, Southern Vancouver Island

The Leech River fault (LRF) zone located on southern Vancouver Island is a major regional seismic source. We investigate potential interactions between earthquake ruptures on the LRF and the neighboring Southern Whidbey Island fault (SWIF), which can be interpreted as a step‐over fault system. Using a linear slip‐weakening frictional law, we perform 3‐D finite‐element simulations to study rupture jumping scenarios from the LRF (source fault) to the SWIF (receiver fault), focusing on the influences of the offset distance, fault initial stress level, and fault burial depth. We find a smaller offset distance, a higher initial stress level on either fault, or a shallower fault burial depth will promote rupture jumping. Jumping scenarios can be interpreted as the response of the receiver fault to stress perturbations radiated from the source fault rupture. We demonstrate that the final rupture jumping scenario depends on various parameters, which can be collectively quantified by two keystone variables, the time‐averaged over‐stressed zone (where shear stress exceeds static frictional strength on the receiver fault) sizeR e‾ and the receiver fault initial stress level. Specifically, a smaller offset distance, a higher initial shear stress level, or a shallower burial depth will lead to a largerR e‾ . The seismic moment on the receiver fault increases with increasingR e‾ . WhenR e‾ reaches the threshold dependent on the receiver fault initial stress level, the rupture becomes breakaway.