SCEC Award Number 19132 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Cajon Pass Earthquake Gate Area: Measuring the Holocene slip-rate of the Glen Helen strand of the northern San Jacinto fault zone at a newly discovered site in the Cajon Pass area
Investigator(s)
Name Organization
Nate Onderdonk California State University, Long Beach Paula Figueiredo University of Cincinnati
Other Participants Drake Kerr- MS student at CSULB
SCEC Priorities 1a, 3a, 2e SCEC Groups Geology, SAFS, SDOT
Report Due Date 03/15/2020 Date Report Submitted 03/12/2020
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. We are working 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 the first year of this work (2018) we focused on mapping, assembling prior data from consultant reports and previous maps, and identifying possible slip rates sites. In 2019 we focused on a site we found along the Glen Helen fault where we expected to measure a Holocene slip rate. Instead we found evidence in trenches and from surface mapping that the fault has not slipped in at least 2000 years. This surprising result suggests that other fault strands in the area are must be responsible for slip transfer. Luckily, we also identified a new slip rate site along the middle San Jacinto strand, and a possible site along the Tokay Hill fault, both of which will be evaluated in 2020. Our mapping throughout the area 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 current and future slip rate sites (Kenwood and Ranger Station). White stars indicate other locations mentioned in the text: A= Applewhite Campground, B= Scotland, C= Texas Hill, D= Sycamore Canyon Glen, E= Helen Regional Park. Inset map shows location of Cajon Pass with slip-rate data (in mm/yr) from nearby sites on the San Jacinto and San Andreas faults.