The partial knowledge about the internal structures of the Earth and incomplete records of past earthquakes are significant hurdles associated with forecasting earthquakes. Earthquake scientists rely heavily on indirect means to derive information about material properties, stresses and mechanical processes that are vital for developing reliable earthquake forecasts. Computational models provide a window into the Earth below enabling us to study the physical processes that control earthquakes. Models bridge the gap between small scale laboratory experiments and nature so that processes on multiple length and time scales can be simulated. Predictive numerical models are essential to evaluate how current seismic patterns can reveal seismic hazard associated with future events. To advance current capabilities in simulating earthquakes and assessing hazards, a numerical framework is developed, capable of modelling earthquake ruptures. The model incorporates a sufficient subset of physics to realistically simulate seismic cycles, arbitrarily oriented strike-slip fault systems and material properties.
This thesis presents numerical investigations on earthquake rupture at faults separating dissimilar materials (also known as bimaterial interfaces). Three main topics are studied in this thesis. Firstly we detailed how a numerical representation of a fault system can be loaded to a pre-earthquake stress state. The specific effects of bimaterial interfaces on loading are highlighted. Secondly we investigated the influence of stress heterogeneities along a bimaterial interface on dynamic rupture propagation. Conditions for the onset of the transition from subsonic to intersonic rupture growth (supershear transition) were explored. Lastly fault systems with discontinuities were examined for the effect of damage on step-over jumps, where the damage introduces a bimaterial interface at the step-over zone.
Dynamic simulations of homogeneous and bimaterial fault rupture were modelled using different loading approaches. We demonstrated that a numerical method of quasi-static loading is capable of immediately loading bimaterial interfaces to rupture. It is a computationally inexpensive approach to tectonic loading and is capable of loading a fault to pre-defined stress levels. After the loading phase the numerical model initiates dynamic rupture, where the importance of material contrast in determining the rupture characteristics of earthquake faults was studied. With this knowledge at hand, the contribution of the tectonic loading mechanism on rupture growth was investigated.
Significant heterogeneous stresses along bimaterial interfaces due to experimental or model setup in both recent laboratory experiments and numerical models were found and quantified. Stresses, partially induced by model arrangement, were shown to affect the onset of the supershear transition, transition length and rupture speed, mode and directivity, in both uniaxial compression tests and dynamic rupture experiments with bimaterial interfaces. Using numerical simulations we showed that normal and tangential stresses at the fault are distorted by the different mechanical properties of the materials abutting the fault. This distortion leads to altered supershear transition lengths, higher rupture potencies and it amplifies the preference for rupture in the direction of slip of the more compliant material. We demonstrated methods to decrease stress distortion in laboratory experiments by using larger specimen samples and in numerical models by using periodic boundary conditions. With the ability to model distortion-free boundary-loaded bimaterial interfaces we were able to study dynamic rupture growth along the fault and the underlying mechanism of the supershear transition in bimaterial interfaces. The supershear transition is enhanced in the direction of slip of the less compliant material (the negative direction) due to the bimaterial effect whereby a decrease in normal stress in front of the crack tip supports yielding ahead of the rupture. In the direction of slip of the more compliant material (the positive direction), an increase in normal stress ahead of the rupture tip delays or prevents the supershear transition. We demonstrated that the material contrast and the parameter S (Andrews, 1976) control whether the supershear transition is smooth or follows the Burridge-Andrews mechanism.
In a final step we extended our model to study interaction in a two-fault system. The devastation inflicted by recent earthquakes demonstrates the danger of under-predicting the size of earthquakes. Earthquakes may rupture fault-sections larger than previously observed, making it essential to develop predictive rupture models. We demonstrated that weakened (damaged) fault-zones and bimaterial interfaces are shown to greatly increase the risk of cascading ruptures and triggered seismicity. However, inter-seismic deformation and energy dissipation can suppress rupture propagation and form earthquake barriers. Numerical models are presented that determine whether a rupture on a segmented fault could grow into a devastating, multi-segment earthquake. By assessing fault stability, identifying rupture barriers and foreseeing multi-segment earthquakes, tools are provided to improve earthquake prediction and risk mitigation.