SCEC Award Number 17251 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title The Effects of Separation and Dip Angles on Jumping Rupture on Thrust Faults
Investigator(s)
Name Organization
Julian Lozos California State University, Northridge
Other Participants Paul Peshette (CSUN MSc student)
SCEC Priorities 1e, 2e, 1d SCEC Groups FARM, CS, Seismology
Report Due Date 04/30/2020 Date Report Submitted 11/03/2020
Project Abstract
 We present a dynamic rupture modeling parameter study that attempts determine which geometric and stress conditions promote jumping rupture between parallel-trace thrust faults. We use the 3D finite element method to model rupture on pairs of parallel-trace thrust faults which either dip toward or away from each other. We vary stress drop and fault strength ratio to determine which conditions produce jumping rupture at different dip angles and different minimum distance between faults. We find that geometry plays an essential role in determining whether or not rupture will jump to a neighboring thrust fault. Rupture is more likely to jump between faults dipping toward one another at steeper angles, and the behavior tapers down to no rupture jump in shallow dip cases. Our variations of stress parameters emphasize these toward-orientation results. Rupture jump in faults dipping away from one another is complicated by variations of stress conditions, but the most prominent consistency is that for mid-dip angle faults rupture rarely jumps. If initial stress conditions are such that they are already close to failure the possibility of a long-distance jump increases. Our models call attention to specific geometric and stress conditions where the dynamic rupture front is most important to potential for jumping rupture. However, our models also highlight the importance of near-field stress changes due to slip. According to our modeling, the potential for rupture to jump is strongly dependent on both dip angle and orientation of faults.
Intellectual Merit This research contributes to physics-based understanding of multi-fault rupture, which is crucial for determining the largest possible ruptures that may affect a given fault system or region. In particular, this fills in a research gap both for simple geometry-based rules about jumping rupture for dip-slip faults specifically (most existing comparable studies are for strike-slip faults), and for faults with completely overlapping and parallel surface traces.
We expect that this could be particularly helpful for calculating rupture hazard in places with many parallel thrust faults, such as the Los Angeles basin and the Transverse ranges. While site-specific studies are always best for hazard assessment, understanding the physics of rupture on different simplified geometries can help set some basic ground rules in hazard calculations: for example, maximum stepover distances or bend angles treated as inhibiting rupture propagation on strike-slip faults in the current UCERF model. Similar "rules" for dip-slip faults will be useful for improving baseline hazard assessments and understanding possible maximum events or rupture paths even in the absence of site-specific studies.
Broader Impacts From an educational standpoint:
This project was the centerpiece of CSUN student Paul Peshette's Masters thesis. Paul learned dynamic rupture modeling techniques and scientific reasoning/writing through this project. He conducted all of the modeling in the project, and wrote the draft of the manuscript, which has now been published in BSSA. Paul's multiple SCEC posters and his publication from this project better prepared him for working as a professional in the geosciences.
Additionally - the results of this project brought up several new possible questions for investigation in future studies, which will be good opportunities for future students of mine to learn about the physics of earthquakes, about dynamic rupture modeling, and about scientific research and writing.

From a societal standpoint:
To my knowledge, this is the first published geometrical parameter study on thrust faults with completely overlapping/parallel traces. This is a type of geometry that exists in the real world, but most jumping rupture studies in the past have focused on faults with along-strike offset between them. The results of this study could be useful for establishing some general rules about jumping rupture in hazard calculations, as well as some motivation and baseline for setting up more site-specific modeling studies of real-world faults.
Exemplary Figure Figure 2.
Jumping rupture results for V-faults (left) and A-faults (right), with Δ𝜏 = 10 MPa and fault strength S = 1.5. Individual model results are marked with a symbol relating to type of behavior. Models with no jump are marked by dark colored circles, triggered slip by lighter color triangles, and full renucleations by lightest color squares. We color the regions between boundaries to make differences across results graphs more apparent.

(Figure credits: Paul Peshette and Julian Lozos.)