Fault zone geometrical complexity and material contrast on earthquake rupture propagation: Observation and modeling

Seismic hazard analysis of continental faults is a difficult task due to many challenges. Continental faults, especially intraplate faults are usually poorly mapped with elusive activity. In addition, they usually exhibit high geometrical complexities such as step-overs and bends. Finally, continental faults can also serve as material interfaces, introducing the bimaterial effect. Fault geometrical complexity and bimaterial contrast often coexist and the study on their joint influence is still insufficient. This thesis is designed to address these challenges using both seismic observations and numerical models. I first explore how to clearly identify the seismogenic potential as well as subsurface geometries of the Leech River fault zone (LRFZ), an intraplate fault system in southern Vancouver Island. My results reveal an ~ 8-10 km wide, NNE-dipping zone of seismicity representing a subsurface structure associated with the LRFZ. Based on seismicity clustering analysis, repeating events analysis and focal mechanism inversion, I find that the LRFZ can be interpreted as an extensional step-over system, which consists of two right-lateral active fault structures: the LRF structure to the north, and a secondary sub-vertical structure to the south. The latter is possibly an extension of the Southern Whidbey Island fault (SWIF).Then, I 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 (L0), fault stress ratio (S0), and fault burial depth (D). A smaller L0, a smaller S0 on either fault, or a shallower D will promote rupture jumping. I show that the final rupture jumping scenario depends on various parameters, which can be collectively represented by two keystone variables, the time-averaged Over Stressed Zone size $\overline{R_e}$ and the receiver fault initial stress level. Specifically, a smaller L0, a smaller S0 on either fault, or a shallower D 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 S0, the rupture becomes break-away.Finally, I conduct 2D finite-element simulations to investigate the joint influence of fault bends and the bimaterial effect on rupture processes. The bimaterial effect promotes rupture propagation in the preferred direction with higher rupture speeds and peak slip rates, and produces larger rupture sizes in partially ruptured scenarios. But it cannot promote a completely arrested rupture evolving into a partially or completely ruptured one. This is attributed to the fact that the bimaterial effect is severely suppressed when the bending angle is large. The rupture is completely arrested when $\Delta G$ (the difference between the static energy release rate and the fracture energy on the kinked segment) is negative. A larger $\Delta G$ produces a larger rupture. Compared to the influence of fault bending, the bimaterial effect can only increase $\Delta G$ by a negligible amount. Therefore, the fault bending geometry is the dominant factor controlling earthquake rupture propagation, while the material contrast is a secondary factor

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