SCEC Award Number 20045 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Cajon Pass Earthquake Gate Area: Testing the hypothesis that the Glen Helen fault has not ruptured in 2000 years, and measuring the slip-rate of the middle San Jacinto fault strand.
Investigator(s)
Name Organization
Nate Onderdonk California State University, Long Beach Paula Figueiredo North Carolina State University
Other Participants Drake Kerr. CSULB Graduate student
SCEC Priorities 1a, 3a, 2e SCEC Groups Geology, SAFS
Report Due Date 03/15/2021 Date Report Submitted 08/17/2022
Project Abstract
 The purpose of this study is to investigate the relationship between displacement along the northern San Jacinto fault zone and the San Andreas fault in the Cajon Pass area as part of the larger SCEC effort to understand the Cajon Pass Earthquake Gate. Our goal is to determine where slip transfer occurs between the two faults, and to test previous interpretations that the two faults occasionally rupture together. Our methods are lidar-aided field mapping, trenching at specific sites, and dating late Quaternary deposits. In period covered by this report (end of 2019-2021) we excavated another site along the Glen Helen to confirm the lack of recent slip, evaluated a potential slip-rate site on the middle San Jacinto strand, and evaluated consultant trenches that became available to us on the San Jacinto fault in the San Bernardino basin. Numerous setbacks over the past two years limited our success, but we have confirmed that there has been no surface displacement on the Glen Helen fault in the past 2000 years, and our mapping suggests that Holocene displacement extends well into the eastern San Gabriel Mountains and at least some slip transfers to the San Andreas north of Cajon Pass.
Intellectual Merit This project contributes to the primary SCEC goal of understanding how fault systems work. Specifically, we are addressing how separate fault strands interact in terms of strain transfer both between and during earthquake ruptures. The area being investigated has been targeted as a focus location for understanding “earthquake gates”, and what controls when ruptures entering the earthquake gate will continue through, stop, or jump to a separate fault.
Broader Impacts This project has supported the teaching and training of one Masters student, and has contributed to course materials in the form of lectures, labs and field trips at CSULB. In addition, trenching and drone-based lidar surveys conducted in 2019 were done in conjunction with Onderdonk’s Tectonic Geomorphology class. Onderdonk also led two field trips and seminars for visiting Chinese earthquake scientists in December of 2018 and 2019, and one field trip for a joint class from Colorado College and University of Wellington, New Zealand in November 2019. These trips and seminars were largely focused on topics related to this project. Ultimately the outcome of this work will improve public safety and estimates of seismic risk for key infrastructure in the Cajon Pass area.
Exemplary Figure Figure 1. Faults in the Cajon Pass area mapped with key locations mentioned in the text. Red stars indicate Kenwood and Ranger Station sites. White stars indicate other locations mentioned in the text: A= Glen Helen Regional Park, B= Scotland, C = Sycamore Canyon E= Glen Helen Regional Park. Inset map shows location of TetraTech trenches in Rialto that were investigated as part of this work.