Stress accumulation in the seismic cycle and loading at the present time

#19,013

Although the United States has experienced a small amount of earthquake-induced drought in the past century, the good (and bad) news is that all droughts eventually end.

It is not known exactly where and when this collapse will occur, but in 2024 the US Geological Survey unveiled a new seismic hazard map (see below), increasing the proportion of Americans at risk of experiencing a damaging earthquake to nearly 75% (about 240 million people).

In addition, there are approximately a dozen “high-risk” volcanoes in the continental United States (4 in Washington, 4 in Oregon, 3 in California), along with dozens of “less-risk” threats. While earthquake damage is generally local, volcanic eruptions can affect property and residents thousands of miles away. While the next big earthquake could happen in Alaska, Hawaii, South Carolina, Missouri, or the Pacific Northwest (see FEMA: Cascadia Rising 2016), history and Hollywood have conspired to put California at the top of most people’s lists.

Earlier this month, the journal Geophysical Research: Solid Earth published a paper by a team of international researchers who modeled the 1,000-year history of earthquakes along two California fault systems (the southern San Andreas and San Jacinto) to estimate current stress where they intersect at the Cajon Pass.

While this study has received significant – sometimes exaggerated – coverage on social media and YouTube, lead author Dr. Liliane Burkhard from the Department of Space Research and Planetary Science (WP) at the Institute of Physics at the University of Bern emphasized that: “The study is not a prediction of when an earthquake will occur. What we can say is that the system is under extreme stress and that physics-based models like ours give a clearer picture of the range of scenarios we should be prepared for. This information is important for risk assessment, infrastructure planning and emergency preparedness.”

While none of this indicates that a large earthquake is imminent, it does provide a plausible mechanistic description of how a single earthquake can affect two major fault systems, potentially increasing the duration of shaking and the area affected.

First, look at the abstract and summary of the study. Although much of this report is technical, many will want to read it in its entirety.

The Cajon Corridor and the southern San Andreas Fault System: seismic cycle stress accumulation and present-day loading

Lillian M. L. Burkhard, Bridget R. Smith-Counter, Katherine M. Sharer, and David T. Sandwell

First Published: June 3, 2026 https://doi.org/10.1029/2025JB033213 View Digital Object Identifier (DOI) metrics

PDF

a summary

With more than a century having passed since the last major rupture affected the broader Los Angeles area, tectonic stress has steadily built up along the southern San Andreas and San Jacinto fault systems, raising fears of an imminent major earthquake. The Cajon Corridor, located at the intersection of these faults, represents an important location for potential rifting in Southern California. We created new 4D seismic cycle simulations using the 1,000-year old seismic rupture history of the San Andreas Fault System (SAFS) to evaluate spatio-temporal variations in stress. A semi-analytical Fourier transform model was used to calculate the stress due to three-dimensional dislocations in an elastic lamina covering half of the viscoelastic Maxwell space, assuming a complete cosmic reset of the resolved shear stress on the ruptured elements.

The results show higher stress buildup north of the Cajon Pass (∼1.8 MPa/100 yr) due to increased slip rates and lower rates south of the Cajon Pass (∼1.0-1.5 MPa/100 yr). By 2025, the Coulomb stress is estimated to be 2.8 MPa in the South Mojave Sector (MOS), 1.8 MPa in the North San Bernardino Sector (NSB1), and 3.6 MPa in the San Jacinto Bernardino Sector (SJB). The sections accumulate stresses with distinct ranges of pre-event stress interpreted as failure thresholds: 1.2-2.7 MPa for MOS, 0.4-1.6 MPa for NSB1, and 1.2-2.9 MPa for SJB. As the stress disparity between sections SJB and MOS narrows, the faults appear to rupture together, suggesting that stress levels may control how Cajon Pass acts as an earthquake gateway. These results may inform seismic risk assessments by linking stress evolution to fault interactions.

Simple language summary

The San Andreas and San Jacinto Fault in Southern California have not caused a major earthquake near Los Angeles in more than a century. During that period, tectonic stress continued to build along these faults, increasing the likelihood of a major rupture in the future. One major area of ​​concern is the Cajon Corridor, where two fault systems meet and are likely to rupture together.

To investigate this, we used computer simulations of the last 1,000 years of large earthquake activity to estimate how stress builds up on fault segments and affects adjacent segments over time. The model shows that stress has now reached high levels throughout the region and that the two fault systems may interact when their stress levels become similar. This suggests that the Cajon Corridor could act as an “earthquake gateway” that sometimes blocks and other times allows large cracks to propagate between faults. These results improve our understanding of earthquake interactions in Southern California and help improve regional hazard assessments.

(continues…)

Regardless of whether the next “big mistake” is caused by one or two mistakes, its impact can be significant, and it is important to be prepared. Every year since 2009, I have promoted the Shakeout.org Safety Campaign, and I strongly encourage anyone living in any seismically active area to participate.

After the shaking stops, you will have to find ways to deal with the consequences.

While the government will send help, you may find yourself largely alone for days and living in less than comfortable conditions for weeks. For starters — and at a minimum — every household should have a disaster plan, a good first aid kit (and the knowledge to use it), a battery-powered NWS weather radio, and emergency supplies to last at least 72 hours during a disaster.

Although 72 hours is a great start, since I previously lived near the New Madrid Fault, and now live in hurricane country, I wouldn’t feel comfortable stopping there.

Many agencies and organizations in the United States recommend that families maintain an adequate supply of food, water, and emergency supplies for 10 to 14 days, which I consider much more prudent.

For more information about earthquake preparedness, both here in the United States and around the world, you may want to review:

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