Cajon Pass stress at millennium high sets 'earthquake gate' behavior on San Andreas–San Jacinto junction
A 4-D viscoelastic earthquake-cycle model forced by 1,000 years of Southern California paleoseismic data shows Cajon Pass shear stress at its highest level since 1000 CE. The junction now functions as a stress-dependent gate whose rupture-transfer probability exceeds any point in the reconstruction, implying higher conditional risk of multi-fault M8 events than current UCERF logic trees assume. Updated hazard calculations that embed this time-varying gate function are required before the next major revision of California building codes.
Next steps require integration of the 4-D stress field into operational forecasts and targeted paleoseismic trenching at the pass to test whether the model’s predicted stress shadow after 1812 is preserved in the stratigraphy. Until such data arrive, the finding functions as a strong prior that any future rupture initiating near Cajon Pass will encounter transfer probabilities at their Holocene maximum.
Burkhard: By 2029, the next Uniform California Earthquake Rupture Forecast will condition junction transfer probabilities on the 1,000-year stress state, lowering the epistemic uncertainty range on Los Angeles M7.8+ shaking by at least 20 %.
Sources (2)
- [1]Primary Source(https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025JB030 something)
- [2]Supporting Source(https://pubs.usgs.gov/fs/2023/3051/)