Cajon Pass and the Southern San Andreas Fault System: Earthquake cycle stress accumulation and present-day loading

Liliane Burkhard, Bridget R. Smith-Konter, Katherine M. Scharer, & David T. Sandwell

Under Review March 17, 2026, SCEC Contribution #15025

With over a century since the last major rupture affecting the wider Los Angeles region, tectonic stress has been steadily building along the southern San Andreas and San Jacinto fault systems, raising concerns of an imminent large earthquake. The Cajon Pass, located at the junction of these faults, represents a critical site for potential through-going ruptures in Southern California. We constructed new 4D earthquake cycle simulations using a 1000-year paleoseismic rupture history of the San Andreas Fault System (SAFS) to assess spatial and temporal variations in stress. A semi-analytic Fourier transform model was used to compute stress from 3D dislocations in an elastic plate overlying a Maxwell viscoelastic half-space, assuming a complete coseismic reset of resolved shear stress on ruptured elements. Results show highest stress accumulation north of Cajon Pass (~1.8 MPa/100 yrs) due to greater slip rates, and lower rates south of Cajon Pass (~1.0-1.5 MPa/100 yrs). By 2025, Coulomb stress is estimated at 2.8 MPa on the Mojave South (MOS) segment, 1.8 MPa on the North San Bernardino (NSB1) segment and 3.6 MPa on the San Jacinto Bernardino (SJB) segment. Each segment accumulates stresses with characteristic ranges of pre-event stress which we interpret to be thresholds for failure: 1.2-2.7 MPa for MOS, 0.4-1.6 MPa for NSB1, and 1.2-2.9 MPa for SJB. When the stress disparity between segments SJB and MOS narrows, the faults appear to rupture jointly, suggesting that stress levels may control how Cajon Pass acts as an earthquake gate. These results may inform seismic hazard assessments by linking stress evolution to fault interaction potential.

Key Words
San Andreas Fault, San Jacinto Fault, Cajon Pass, stress accumulation

Citation
Burkhard, L., Smith-Konter, B. R., Scharer, K. M., & Sandwell, D. T. (2026). Cajon Pass and the Southern San Andreas Fault System: Earthquake cycle stress accumulation and present-day loading. Journal of Geophysical Research: Solid Earth, (under review).