A new study has reconstructed 1,000 years of earthquake activity along Southern California’s San Andreas and San Jacinto fault systems and found that tectonic stress in the region has reached levels not seen in the past millennium. Using a physics-based 4D earthquake cycle model, researchers combined paleoseismic data (from geological evidence, radiocarbon dating, tree-ring anomalies, and historical records) with simulations of stress accumulation and release. The results show that current stress on key fault segments, particularly around the Cajon Pass junction northeast of Los Angeles, equals or exceeds the highest values observed over the entire 1,000-year period. Cajon Pass functions as an “earthquake gate”, a critical junction where ruptures on one fault can sometimes propagate to the other under the right stress conditions, potentially triggering much larger, multi-fault events. Historical patterns indicate that joint ruptures across both fault systems have occurred when stress levels on the adjacent segments (such as the Mojave South section of the San Andreas and the San Jacinto Bernardino section) become similarly high and aligned. Today, those segments are modeled at approximately 2.8 MPa and 3.6 MPa, respectively, placing the system in a configuration historically associated with through-going ruptures. This has significant implications for the densely populated greater Los Angeles region, including San Bernardino, Riverside, and critical infrastructure corridors through Cajon Pass. However, the study does not predict the timing of the next major earthquake—such events cannot currently be forecasted precisely. Instead, it provides a physics-based assessment of accumulated stress and the range of plausible rupture scenarios that could occur. [Burkhard, L. M. 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, 131(6), e2025JB033213. DOI: 10.1029/2025JB033213]