SCEC Award Number 18149 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Investigating Earthquake Gate Stress Evolution of the Cajon Pass
Investigator(s)
Name Organization
Bridget Smith-Konter University of Hawaii at Manoa Katherine Scharer United States Geological Survey David Sandwell University of California, San Diego
Other Participants Liliane Burkhard, Graduate Student
SCEC Priorities 1c, 5c, 1d SCEC Groups SAFS, SDOT, CXM
Report Due Date 03/15/2019 Date Report Submitted 05/08/2019
Project Abstract
 The primary objectives of this project were to conduct a new analysis of earthquake cycle stress accumulation, specifically targeted at the Cajon Pass, using (1) newest paleoseismic rupture estimates, (2) emerging rheological constraints of the southern California lithosphere provided by the CRM/CTM, and (3) a modernized 4-D spatially heterogeneous earthquake cycle model of the San Andreas Fault System (SAFS). While seismogenic stressing rates are largely dependent on fault system slip rates and locking depths, our understanding of how stresses evolve throughout earthquake cycles and whether they are inclined to promote or inhibit earthquake gate (opening) is still developing. Here, we investigate stress accumulation rates for the Cajon Pass in southern California, representing a critical junction for possible past and future through-going ruptures. Using a new 4D viscoelastic earthquake cycle model that incorporates heterogeneous rheological constraints of the southern California lithosphere, we estimate stress rate disparities caused by spatial variations in crustal structure. We find moderate stress accumulation rates for major segments of the Cajon Pass (1.2-2.8 MPa/100yrs). These models are still under development as we explore alternative rupture scenarios, but preliminary results suggest reduced stress accumulation rates (by ~2-5 kPa/yr, or ~10%) south of the Mojave segment, in a region that already hosts lower stress rates due to fault strain accommodated over the multiple paralleling segments. Over 100-200 year earthquake cycle time scales, accumulated stress could be reduced by at least 1 MPa on these segments, making the occurrence of major through-going ruptures less frequent at this earthquake gate junction.
Intellectual Merit A major objective of SCEC5 is to bridge the enduring efforts of several community models through the establishment of the SCEC CRM, a large-scale effort to deliver a provisional rheological description of the lithosphere of southern California based upon a simplified geologic framework. To first order, determining the relationship between strain (or strain rate) and stress requires a fundamental knowledge of material rheology. Geodetic data provided by the Community Geodetic Model (CGM) measure vector surface velocities and strain rates with increasingly high resolution and with broad regional coverage. A physical kinematic model, outfitted with refined fault representations (like those provided by the Community Fault Model, CFM) and governed by informed rheological assumptions, is required to interpret these measurements in a spatially continuous and 3-D manner. Moreover, model estimates of time-dependent earthquake cycle deformation and stress loading rates (contributed to the Community Stress Model, CSM) require a broad understanding of the rheology and structure of the crust and upper mantle. Contributions from a developing Community Thermal Model (CTM), which provides heat flow estimates of the southern California lithosphere, are another essential component. Integrating these community models to better inform the collaborative efforts of the geology, geodesy, seismology, and hazard communities is a critical objective for advancing the science goals of SCEC.
To this end, the primary objective of this project was to extend our 4-D earthquake cycle modeling capabilities to incorporate spatial variations in lithosphere rheology, and in turn, to provide insight into earthquake cycle crustal stress variations at seismogenic depths where earthquake ruptures nucleate.
Two major science priorities of the SCEC5 Stress and Deformation Over Time (SDOT) working group and the Earthquake Gates thematic area are to (1) improve our understanding of fault loading over a range of temporal and spatial scales to (2) test the hypothesis that earthquake gates control the probability of large, multi-segment and multi-fault ruptures. 4-D simulations of stress evolution, or fault loading, are now possible (e.g., Smith-Konter and Sandwell, 2009; Smith-Konter et al., 2014) and provide rare insight into earthquake cycle crustal stress variations at seismogenic depths where earthquake ruptures nucleate. The findings of this work promote further investigations into the relationship of stress rates and crustal rigidity variations within the seismogenic zone and how these together may lower (or elevate) seismic hazards in region like the Cajon Pass. Moreover, this work contributes to the development of a critical SCEC science question, How are faults loaded across temporal and spatial scales? and is designed to explore the hypothesis that earthquake gates control the probability of large, multi-segment and multi-fault ruptures, by conducting numerical studies of deformation and the stress state of the crust and its sensitivity to spatial variations in rheology (Research Priorities P1c-e).
Broader Impacts A component of this SCEC5 funded project emphasized Earth Science education and training, as well as communication of pertinent and accessible earthquake information to the general public. Graduate student L. Burkhard received partial RA funding and travel support from this award. Manoa and Waialae Elementary Schools, Kamehameha School, Waipahu Intermediate School, and Kailua High School have benefited greatly from interactive educational products provided by our team, in conjunction with the research activities supported by this award. We have also utilized the UH Hawaiian Institute of Geophysics visualization center many times over the year to display San Andreas visualizations for classroom and public education activities. Coursework lectures and visualization exposure of these datasets were provided to over 100 UH undergraduate and graduate students enrolled in GG101 Dynamic Earth, GG451 Earthquakes and Crustal Deformation, and GG631 Solid, Wave, and Fluid Mechanics.
Two publications have thus far resulted from activities that support this project (Sandwell and Smith-Konter, 2018; Xu et al., 2018) and we have one additional paper in a very mature state of preparation (Ward et al., 2018) and another manuscript in preparation that we intend to submit for peer review this Fall (Burkhard et al., 2019). Results from this project were also presented at the 2018 SCEC Annual Meeting and AGU Fall Annual Meeting (Burkhard et al., 2018a;b; Ward et al., 2018) and our team also participated in the Cajon Pass EGA and CRM workshops (September 2018, B. Smith-Konter, L. Ward, and L. Burkhard). Our latest code distribution of Maxwell is available on GitHub.
Exemplary Figure Figure 3.
Figure 3. (Left map set) A candidate sequence of stress accumulation and drop as a result of several major SAFS earthquakes impacting the Cajon Pass over the last 400 years. Stress accumulation is calculated for the year just before (peak stress accumulation) and for the year just after (assuming a complete stress drop) these major paleoseismic earthquakes. For the 1694 rupture sequence, moderate stress levels of 1.24 MPa are simulated just before the earthquake at Cajon Pass. Accumulated stress along the SAFS immediately prior to the 1812 earthquake is modeled at ~2.5 MPa, but stress along the San Jacinto Fault is very low (< 0.2 MPa) due to a proposed rupture in 1800 along this section of the fault system. In 1856, accumulated Coulomb stress is 4.5 MPa along the Fort Tejon epicenter of the 1857 earthquake, however at Cajon Pass the accumulated stress is fairly low, at 0.76 MPa, due to the stress release that occurred in 1812. (Right bottom profiles). Coulomb stress accumulation for events in 1694, 1812, 1857, and the present-day (in MPa), extracted along a fault-perpendicular profile across the SAFS at the Cajon Pass. Fault profile location is indicated on the map (outset), showing present-day accumulated Coulomb stress. Values are assessed at the near-surface (0.1 km), 4 km and 10 km depth before and after the ruptures of 1694, 1812, 1857, as well as for the present-day. Lightly-shaded background stress profiles represent the stress before each event, while darker-shaded profiles represent the stress after each event.

Burkhard, L., B. Smith-Konter, L. Ward, K. Scharer, and D.T. Sandwell, Earthquake cycle stress accumulation disparities of the Cajon Pass region, 2018 SCEC Annual Meeting, Palm Springs, CA.
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