Using dynamic rupture simulations to explore fault segmentation and rupture length on the Sierra Madre Fault Zone

Julian C. Lozos, David Velador, & Jose Tepal

Published September 11, 2022, SCEC Contribution #12588, 2022 SCEC Annual Meeting Poster #180

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, southern California. Based on its length alone, it is capable of producing a ~M7.7 earthquake, but its discontinuous geometry may lead to it rupturing in multiple smaller (~M6-M7) events (such as the M6.6 1971 San Fernando earthquake). Given the SMFZ’s location through the densely-populated San Fernando, San Gabriel, and San Bernardino valleys, 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, which we parameterize here with four major segments (from east to west: the Cucamonga Fault, the eastern and western Sierra Madre Fault, and the San Fernando Fault), to assess plausible rupture behaviors and ground motion distributions for this fault system. We use SCEC CFM fault geometry, SCEC CVM surrounding rheology, and SCEC 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, nonplanar 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 (the Cucamonga Fault to the east and the San Fernando Fault to the west) are limited by stepovers, and do not allow more than a small amount of slip beyond the nucleating segment. Ruptures that nucleate on the eastern or western Sierra Madre Fault always involve complete rupture of at least two fault segments, and sometimes may grow into a wall-to-wall rupture of the whole fault system; this also depends strongly on nucleation location along strike and down dip. The maximum rupture size our simulations produce is M7.47; there are several different slip distributions for this magnitude, however, which also implies several different patterns of maximum ground motion, even from similarly-sized earthquakes on the same faults.

Key Words
sierra madre fault zone, thrust faults, rupture dynamics, dynamic rupture simulations, earthquake simulations, cucamonga fault, san fernando fault, sierra madre fault

Citation
Lozos, J. C., Velador, D., & Tepal, J. (2022, 09). Using dynamic rupture simulations to explore fault segmentation and rupture length on the Sierra Madre Fault Zone. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)