Dynamic spontaneous earthquake rupture models have proven themselves to be valuable tools to investigate the physics of earthquakes and to help predict ground motion. These numerical models start from basic assumptions about material structure, frictional behavior, and fault geometry, and calculate the spatiotemporal evolution of fault slip (and often the resultant near-source ground motion). Such dynamic models typically use either laboratory-derived friction laws or realistically complex fault geometry, but not both. The researchers propose to bring dynamic earthquake modeling an important step forward by combining these two separate tracks: they will use laboratory-derived friction laws to model spontaneous rupture propagation and slip on faults with realistically complex geometry. They expect to obtain first-order effects in rupture propagation, slip, and ground motion that will differ from previous modeling efforts, leading to both a better understanding of the earthquake process and better predictions of faulting behavior and ground motion. Dynamic earthquake models have historically followed two tracks: 1) investigations of the effect of frictional parameterization and stress pattern on simple planar faults, and 2) investigations of the effects of fault geometry on the earthquake process, using simple frictional parameterizations. the PIs will combine these two tracks to produce a new generation of dynamic earthquake models. Data from laboratory experiments at high slip rates and theoretical models imply that at the high slip rates observed during earthquakes, the typical rate-and-state frictional formulation must be modified to incorporate a greater degree of weakening over a larger length scale. Additionally, research on faults with complex, asymmetrical geometry shows that temporal variation of normal stress, which is inevitable on non-planar faults, can have a significant effect on rupture dynamics. To correctly model both these aspects of faulting behavior, they will develop a modern frictional parameterization and use it to model the behavior of geometrically complex faults, such as systems with stepovers and branches. The new 3D finite element method that will incorporate a new, realistic method for off-fault stress relaxation, which is necessary to avoid pathological stress buildup on such fault systems.
These important ingredients of earthquake physics have never before been combined in a single modeling method, and the result of such a combination will be a state-of-the art tool to model the physics of earthquakes. The researchers will address important questions about the behavior of faults at stepovers and branches, including determining if there are general rules for how to predict rupture path at branches, and the ability of rupture to span stepovers.
The proposed research will have important implications for both earthquake science and the broader scientific and educational community. A key use of the modeling method will be to gain insight into the potential size of earthquakes on geometrically complex fault systems, such as those in the Los Angeles region. Many fault systems are bounded by geometrical features such as fault gaps and changes in segment orientation; the proposed numerical models will help determine the circumstances under which earthquake rupture may propagate across these segment boundaries, and generate larger earthquakes with larger ground motion. In addition, the proposed research will lead to better estimates of the slip distribution and rupture front evolution, which also strongly affect ground motion. The resulting improved earthquake source models can help in the probabilistic assessment of earthquake size and ground motion pattern, with subsequent potential impacts on seismic hazard and building code and design.
