Earthquake rupture through a step-over fault system: An exploratory
numerical study of the Leech River Fault, southern Vancouver Island
Abstract
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
3D 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)
size $\overline{R_e}$ 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 larger
$\overline{R_e}$. The seismic moment on the receiver
fault increases with increasing $\overline{R_e}$.
When $\overline{R_e}$ reaches the threshold
dependent on the receiver fault initial stress level, the rupture
becomes break-away.