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Spatiotemporal Trends in Slip Rate and Earthquake Recurrence on the Sierra Madre-Cucamonga Fault Zone, Southern California

Description: 

The north-dipping Sierra Madre-Cucamonga reverse fault system (SMdF-CF) in southern California extends for ~100 km east-west along the southern base of the ~1.5- to 3-km-tall San Gabriel Mountains, ~5 km north of the Pasadena Convention Center. Joint rupture of both the SMdF and CF portions would result in a Mw ≥7.5 earthquake adjacent to the major population centers of the Los Angeles metropolitan region. Despite such a major looming seismic threat, the slip rate and paleoseismic histories of these faults remain under-constrained. Previous studies using geomorphic analyses of mountain front sinuosity and low-temperature thermochronology have demonstrated relatively fast long-term uplift rates on the eastern (CF) portion of the San Gabriel mountain front compared to the central and western (SMdF) portions. However, review of the current literature on Quaternary fault slip rates obtained along the SMdF-CF provides an ambiguous result as to whether or not this gradient in uplift along the San Gabriel mountain front persists today. We present newly determined luminescence ages of alluvial fans and, combined with vertical separation measurements extracted from high-resolution lidar data, calculate the slip rate of the CF over two distinct time intervals within the Holocene. Our results suggest that the CF is indeed slipping significantly faster than the SMdF. Additionally, we present three previously unpublished paleoseismic trenches, one on the eastern SMdF and two on the CF, excavated over the past 30 years in order to assess the potential record of previous throughgoing rupture along the entire fault system. We find that ruptures at the eastern and western ends of the SMdF-CF system, such as the 1971 M_w 6.7 San Fernando-Sylmar earthquake, occur more frequently than ruptures within the central part of the system. Large displacements observed in the MRE of several central SMdF trenches suggest occurrence of a large magnitude event, likely involving rupture of the entire SMdF-CF system. Recurrence of such an event, albeit rare, could have devastating consequences for the Los Angeles metropolitan region.

Session: The Landscape Record of Earthquakes and Faulting - III

Type: Oral

Date: 4/16/2026

Presentation Time: 02:30 PM (local time)

Presenting Author: Caje Kindred Weigandt

Student Presenter: Yes

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