SCEC Award Number 20136 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Modeling the 2019 Ridgecrest earthquake sequence with fault geometry that matches both surface rupture and seismicity
Investigator(s)
Name Organization
David Oglesby University of California, Riverside Abhijit Ghosh University of California, Riverside Christodoulos Kyriakopoulos University of Memphis
Other Participants Jordan Cortez, Graduate Student, UC Riverside
Kuntal Chaudhuri, Graduate Student, UC Riverside
Baoning Wu, Graduate Student, UC Riverside
SCEC Priorities 1d, 3a, 2e SCEC Groups FARM
Report Due Date 03/15/2021 Date Report Submitted 03/14/2021
Project Abstract
 The 2019 Ridgecrest earthquake sequence was remarkable in a number of ways. The earlier M6.4 Searles Valley earthquake nucleated on a buried right-lateral fault segment and propagated around a perpendicular fault intersection to a surface-outcropping left-lateral segment, but it did not propagate coseismically to the intersecting fault of the subsequent M7.1 Ridgecrest mainshock. We use the 3D finite element method to explore the physical reasons for this curious rupture path. We use simple constant-traction assumptions to explore how initial stress, hypocenter location, and the depth of burial of the initial right-lateral segment may have influenced rupture propagation. The results suggest that only a narrow range of fault stresses, fault burial depths, and hypocenter locations would result in the observed rupture path in this earthquake. We also model the rupture propagation and slip on the subsequent M7.1 event under two conditions: 1) using both a constant-traction assumption, and 2) using the final stress transferred from the M6.4 event as a stress perturbation onto the M7.1 fault. We find that both the constant-traction and stress-transfer model produce heterogeneous slip that to first order matches the inferred slip on this event, including higher slip to the NW of the fault intersection than to the SE. The stress-transfer model has less slip in the area in which it overlaps with the buried nucleating right-lateral segment of the M6.4 event. The results may have implications for interactions between faults in Southern California and beyond.
Intellectual Merit Our simple models investigate the physical origin of the overall rupture path in the M6.4 earthquake—in particular, the propagation of the rupture from a buried right-lateral fault across a right angle to a left-lateral fault, all without immediately triggering the M7.1 fault. The present work can explain the rupture propagation pattern via constrained ranges in the stress level, depth of burial of the nucleating segment, and nucleation location, which is compelling evidence that the rupture propagation is explainable without resorting to the fine tuning of many unconstrained parameters and physical processes. The precise ranges of shear stress, fault burial depth, and nucleation location in this study should not necessarily be taken quantitatively at face value, as their precise numerical values are surely related to the many assumptions required in this (and every) numerical model. Rather, the conclusion is that there exist ranges in these parameters that allow the observed rupture path, and that there are relatively straightforward physical explanations behind these ranges. The examination of models that do not fit the observations allows a better understanding of the physical processes. We also show that a complex, heterogeneous slip model for the M7.1 earthquake does not require complex, heterogeneous physical parameters; a first-order match to the observed slip pattern can result simply from the fault geometrical complexity, even in the absence of heterogeneous fault traction. Incorporating stress transfer from the prior M6.4 earthquake can further modify this slip pattern.
The results imply that the rupture path of the M6.4 earthquake was not a foregone conclusion—small changes in initial or boundary conditions could have led to very different rupture propagation patterns. This result may hold true for earthquakes in general, rendering the prediction of rupture propagation in future earthquakes uncertain, and requiring the modeling of different sets of input parameters to bracket possible faulting behavior. Significantly, the current results depend on the nucleation of the earthquake on an unmapped buried fault segment. Thus, earthquake models that nucleate only on previously-mapped faults may underestimate the range of possible faulting behaviors. To model higher-order observables such as fault slip amplitude and ground motion requires more detailed information, but it is helpful to start with simple models, and add complexity as one attempts to model more detail.
Broader Impacts This project has had an important impact in the education and training of a UCR graduate student, Jordan Cortez, who is from a background traditionally under-represented in the sciences. It was his first exposure to earthquake modeling, and he used this project as a way to learn about basic earthquake physics as well as the technique of 3D dynamic finite element earthquake modeling. He set up and conducted all the fault models in this project. It also provided him with his first first-author publication, which has recently been printed in Geophysical Research Letters.
Exemplary Figure Figure 3. a) Shear and Normal stress increment on M7.1 fault from slip on the preferred M6.4 model. Note the shear stress shadow on the M7.1 in the area in which it overlaps the buried nucleating right-lateral segment of the M6.4 earthquake.
b) Final slip magnitude for homogeneous (top) and heterogeneous (with stress increment from the modeled M6.4 event; bottom) models for the M7.1 Ridgecrest event. Both models display significant slip heterogeneity due to the non-planar M7.1 fault structure; the heterogeneous stress model displays smaller slip where it is shadowed by the M6.4 event.
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