SCEC Award Number 21084 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Investigating the influence of dip angle in rupture propagation along branch fault systems using dynamic models
Investigator(s)
Name Organization
Roby Douilly University of California, Riverside
Other Participants 1) Evan Marschall
SCEC Priorities 1d, 4a, 2e SCEC Groups FARM, SAFS, CXM
Report Due Date 03/15/2022 Date Report Submitted 03/12/2022
Project Abstract
 An important consideration in assessing seismic hazards is determining what is likely to happen when an earthquake rupture encounters a geometric complexity such as a branch fault. Previous studies show parameters such as branching angle, stress-orientation and stress heterogeneity as key factors in the self-determined rupture path on branch faults. However all these branched fault studies either assumed a 2D case or vertical faults and didn’t examine how a dipping fault might influence the outcome of the rupture. Considering that many natural faults have some dipping component, it is necessary to investigate how a change in dipping angle would change the rupture propagation relative to an assumed vertical fault geometry. In this study, we conduct a 3D parameter study consisting of a main a secondary planar segments to investigate the effects of dipping angle on rupture propagation. For each geometry we consider several stressing angles and derive the shear and normal stresses on each faults. We consider a slip-weakening friction law and run scenarios for rupture nucleated on the main and branch faults. The results show that varying the dipping angle of a branch fault system does have significant impact on rupture propagation. For rupture nucleated on the branch fault for higher stressing angles, the rupture is more likely to jump to the main fault when the branch fault has a shallow dipping angle. These results highlight the importance of implementing accurate dipping angles when assessing likely rupture scenarios in complex fault systems.
Intellectual Merit Previous rupture dynamic studies have highlighted parameters such as branching angle, stress-orientation and stress heterogeneity that should be considered when modeling potential rupture scenarios. However, the results from this study indicate that dip angle should be a parameter that should be taken into account when modeling potential rupture scenarios on branch fault systems. The dip angle on any fault in a complex system can change the dynamics of the system. Our work will advance the science of fault dynamics in regions of geometrical complexity, especially fault branches. The results obtained in this project can be applicable to regions with significant fault complexity, such as in California.
Broader Impacts This work has broader impacts on the assessment of seismic hazard along branch fault systems. Branch faults are a type of geometric complexity that are often referred to as “earthquake gates”, because they can either terminate rupture or allow it to keep going. Understanding how dipping angle can prevent or facilitate throughgoing rupture across branch faults is vital to the estimation of potential earthquake size in Southern California for example. This study indicates that varying the dip angle can change whether this gate is “open” or “closed”. Furthermore, fault dips are often poorly constrained with high uncertainties in dip angle. Therefore it could be important for societal hazard to test rupture scenarios within a range of dip angles for areas near branch fault systems in Southern California.
Exemplary Figure Figure 4 from the report can be best exemplifies the significance of the results.

Figure 4: Screenshots of slip at different time step on two different geometries (geometry 3 for the upper panels and geometry 1 for the lower panels) for Ψ = 40°. In both cases the main fault is vertical but the branch fault is vertical in the lower panels and has a 55° dip in the upper panels. For both scenarios the rupture is nucleated on the main fault but the rupture only propagates on the branch fault when the branch fault is dipping at 55°.
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