SCEC Award Number 22158 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Using Dynamic Rupture Simulations to Exploring Fault Segmentation and Rupture Length on the Sierra Madre Fault Zone
Investigator(s)
Name Organization
Julian Lozos California State University, Northridge
Other Participants Two CSUN undergraduate students: David Velador-Santos and Jose Tepal
SCEC Priorities 2e, 5b, 4a SCEC Groups FARM, SAFS, GM
Report Due Date 03/15/2023 Date Report Submitted 03/15/2024
Project Abstract
 The Sierra Madre Fault Zone (SMFZ) is a 125 km-long, north to northeast-dipping thrust fault which arcs along the southern edge of the San Gabriel and Santa Susana Mountains in Los Angeles and San Bernardino counties. Based on its length, it is capable of producing a ~M7.7 earthquake; its discontinuous geometry may lead to it rupturing in multiple smaller (~M6-M7) events. Given the SMFZ’s location, even a smaller earthquake on this fault would have major implications for human safety and infrastructure stability. We conducted 3D dynamic rupture simulations on the SMFZ to assess plausible rupture behaviors and ground motion distributions for this fault system. We use SCEC CFM fault geometry, CVM surrounding rheology, and CSM regional stress orientations to ensure that our model setup is grounded in observation and that our results are realistic. We find that the segmented geometry of the SMFZ has a controlling effect on its possible rupture behaviors, with the nucleating segment playing the largest role. Ruptures that nucleate on the ends of the fault system are limited by stepovers, and only allow a small amount of slip beyond the nucleating segment. Ruptures that nucleate on the central SMFZ always involve complete rupture of at least two fault segments, and may grow into a wall-to-wall rupture of the whole system; this also depends strongly on nucleation location. Our maximum simulated magnitude is M7.47; there are several slip distributions for this magnitude, however, which produce different patterns of maximum ground motion, even from similarly-sized earthquakes.
Intellectual Merit This work will help develop a physics-based understanding of controls on multi-segment rupture in thrust faults, with a specific application toward a large fault in a densely-populated area. Not only can the results be helpful for refining hazard calculations on the Sierra Madre Fault Zone specifically, new understanding of the broader physical processes that control the SMFZ’s behavior may also be useful for interpreting rupture behaviors and ground motion patterns on other large thrust faults. Additionally, the Cucamonga Fault, which is the easternmost segment of the SMFZ, approaches the San Andreas and San Jacinto Faults in Cajon Pass. Thus, this study may also help address possible fault behaviors within the Cajon Pass Earthquake Gate Area.
Broader Impacts From a societal standpoint, rupture hazard calculations, as well as ground motion and surface displacement calculations, may be useful for developing municipal-level and personal-level emergency preparedness plans.

From an educational standpoint, this project gave two CSUN undergraduate students their first experiences with earthquake physics and computer modeling, as well as with being involved in a scientific study to begin with. They expressed interest in the Sierra Madre Fault Zone because of its large potential local impacts, and were excited to work on a project with significant local relevance. This experience helped them grow as scientists, and has already helped one of them get a job following graduation. Additionally, CSUN is a Hispanic Serving Institution, and both of the students worked with me on this project are part of that demographic. This project therefore also gave underrepresented students a chance to be active in and excited about scientific research.
Exemplary Figure Figure 2 summarizes the results of our whole suite of simulations.

Caption: Results summary for our 33 Sierra Madre Fault Zone dynamic rupture simulations. The fault surface plots on the left show representative slip distributions for nucleation points on the listed fault segment. The charts on the right represent the fault schematically; each column is a one fault segment, and each row is an individual simulation. The stars mark the along-strike and down-dip position of the hypocenter, and the color coding indicates how much of each segment slipped.