The University of Bern originally published this article on June 8, 2026. Edits by EarthSky.
California faults under record stress, study finds
Researchers recently modeled 1,000 years of earthquake history along the San Andreas and San Jacinto faults in Southern California. Their finding? Stresses in the crust are higher today than at any time in the last millennium.
Plus, their model indicated that a critical fault junction near Los Angeles could decide how big the next major earthquake will be.
The international research team published its peer-reviewed research on June 3, 2026, in the Journal of Geophysical Research: Solid Earth.
Is California overdue a major earthquake?
Earthquakes usually occur along fracture zones in the Earth’s crust, where large tectonic plates slide past one another and become locked. Stress builds up over long periods of time and is suddenly released in the form of an earthquake. In Southern California, the San Andreas and San Jacinto faults are among the most significant of these zones. They accommodate the majority of the plate motion in the region.
Where the two fault systems approach each other northeast of Los Angeles lies the Cajon Pass. This is a tectonically complex junction where a rupture on one fault could potentially cross onto the other.
The last major earthquake to affect the wider Los Angeles region was the Fort Tejon earthquake of 1857, with a magnitude of 7.9. And since then, tectonic stress along the fault segments has built up continuously. This prolonged quiet period has long concerned researchers, given the potential for a large future rupture.
Modeling 1,000 years of the California faults
For this new study, the researchers modeled 1,000 years of earthquake history along the southern San Andreas and San Jacinto fault systems. They did so by constructing a physics-based earthquake cycle model, and then feeding this with a 1,000-year earthquake record reconstructed from geological evidence such as radiocarbon dating, tree-ring anomalies and historical documentation of ground ruptures. They hoped this model would allow them to estimate the present-day stress loading at Cajon Pass.
Study lead Liliane Burkhard of the University of Bern said:
The model tracks how each earthquake changes stress on neighboring fault segments, how stress accumulates during the quiet intervals between events and how the deeper layers of the crust slowly relax following large ruptures. This simulation allows us to understand how stresses in the fault system build up over centuries.
By running the earthquake history of Southern California as a simulation, we can estimate the extent to which the fault system is already under stress today.
And the results show that tectonic stresses in the region have reached – and in some cases exceeded – the highest levels of the last millennium.
The “earthquake gate” could be the deciding factor
A key finding of the study is that the Cajon Pass can act as a so-called “earthquake gate”. That is, a junction that controls whether large ruptures remain confined to a single fault, or cross both fault systems.
Historical examples of both behaviors exist. The Fort Tejon earthquake of 1857 terminated at Cajon Pass and did not involve the San Jacinto fault, while the Wrightwood earthquake of 1812 ruptured through the junction and across both systems in a single through-going event.
Burkhard explained:
The earthquake gate concept captures something important about how fault junctions work. Cajon Pass doesn’t simply block or channel earthquakes: It responds to stress conditions, and those conditions change over centuries.
Is a joint rupture likely?
The study also shows that the decisive factor is not only how much stress has built up on a single fault, but how aligned the stresses on the two fault systems are. When the stress on both faults rises in concert over time, toward similarly high levels, conditions favor a large joint rupture crossing both systems. When stress levels evolve out of step with each other, ruptures are more likely to terminate at the junction rather than propagate further.
Currently, modeled stress has reached 3.6 MPa on the San Jacinto-Bernardino section, exceeding the highest value seen anywhere in the 1,000-year simulation. On the neighboring Mojave South section of the San Andreas fault, it is 2.8 MPa. Both segments are therefore highly and relatively similarly stressed, placing the system in a configuration that historically has preceded joint ruptures.
Burkhard said:
So not only is it concerning that the stresses are reaching historic highs, but also that the relative stress conditions between the two fault systems are approaching the range we associate with major ruptures crossing both faults simultaneously – and that is a scenario with much larger consequences for the region.
California faults pose increased risk in densely populated regions
A joint rupture of the San Andreas fault and the San Jacinto fault that crosses the Cajon Pass would be a much more severe event than one that is limited to a single fault. The affected region includes some of the most densely populated, infrastructure-critical corridors in the U.S., including the greater Los Angeles area, San Bernardino, Riverside and the Coachella Valley. Major highways, railroads and energy infrastructure run through the Cajon Pass itself.
Burkhard said:
The question of when and how the next major earthquake will occur in this region is one of the most pressing problems in applied geoscience. Our results provide a clearer, physics-based picture of the current stress state of the fault system, and the framework we developed is not just applicable to California, but also for other complex fault junctions worldwide.
However, Burkhard emphasized:
The study is not a prediction of when an earthquake will occur. What we can say is that the system is critically stressed 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 hazard assessment, infrastructure planning and emergency preparedness.
Bottom line: A new model of 1,000 years of earthquake history along two California faults has revealed that these faults are under record levels of stress.
Read more: Big cities are sinking in the US. Is yours one?
The post California faults under record stress, study finds first appeared on EarthSky.
from EarthSky https://ift.tt/bvSVtYe
The University of Bern originally published this article on June 8, 2026. Edits by EarthSky.
California faults under record stress, study finds
Researchers recently modeled 1,000 years of earthquake history along the San Andreas and San Jacinto faults in Southern California. Their finding? Stresses in the crust are higher today than at any time in the last millennium.
Plus, their model indicated that a critical fault junction near Los Angeles could decide how big the next major earthquake will be.
The international research team published its peer-reviewed research on June 3, 2026, in the Journal of Geophysical Research: Solid Earth.
Is California overdue a major earthquake?
Earthquakes usually occur along fracture zones in the Earth’s crust, where large tectonic plates slide past one another and become locked. Stress builds up over long periods of time and is suddenly released in the form of an earthquake. In Southern California, the San Andreas and San Jacinto faults are among the most significant of these zones. They accommodate the majority of the plate motion in the region.
Where the two fault systems approach each other northeast of Los Angeles lies the Cajon Pass. This is a tectonically complex junction where a rupture on one fault could potentially cross onto the other.
The last major earthquake to affect the wider Los Angeles region was the Fort Tejon earthquake of 1857, with a magnitude of 7.9. And since then, tectonic stress along the fault segments has built up continuously. This prolonged quiet period has long concerned researchers, given the potential for a large future rupture.
Modeling 1,000 years of the California faults
For this new study, the researchers modeled 1,000 years of earthquake history along the southern San Andreas and San Jacinto fault systems. They did so by constructing a physics-based earthquake cycle model, and then feeding this with a 1,000-year earthquake record reconstructed from geological evidence such as radiocarbon dating, tree-ring anomalies and historical documentation of ground ruptures. They hoped this model would allow them to estimate the present-day stress loading at Cajon Pass.
Study lead Liliane Burkhard of the University of Bern said:
The model tracks how each earthquake changes stress on neighboring fault segments, how stress accumulates during the quiet intervals between events and how the deeper layers of the crust slowly relax following large ruptures. This simulation allows us to understand how stresses in the fault system build up over centuries.
By running the earthquake history of Southern California as a simulation, we can estimate the extent to which the fault system is already under stress today.
And the results show that tectonic stresses in the region have reached – and in some cases exceeded – the highest levels of the last millennium.
The “earthquake gate” could be the deciding factor
A key finding of the study is that the Cajon Pass can act as a so-called “earthquake gate”. That is, a junction that controls whether large ruptures remain confined to a single fault, or cross both fault systems.
Historical examples of both behaviors exist. The Fort Tejon earthquake of 1857 terminated at Cajon Pass and did not involve the San Jacinto fault, while the Wrightwood earthquake of 1812 ruptured through the junction and across both systems in a single through-going event.
Burkhard explained:
The earthquake gate concept captures something important about how fault junctions work. Cajon Pass doesn’t simply block or channel earthquakes: It responds to stress conditions, and those conditions change over centuries.
Is a joint rupture likely?
The study also shows that the decisive factor is not only how much stress has built up on a single fault, but how aligned the stresses on the two fault systems are. When the stress on both faults rises in concert over time, toward similarly high levels, conditions favor a large joint rupture crossing both systems. When stress levels evolve out of step with each other, ruptures are more likely to terminate at the junction rather than propagate further.
Currently, modeled stress has reached 3.6 MPa on the San Jacinto-Bernardino section, exceeding the highest value seen anywhere in the 1,000-year simulation. On the neighboring Mojave South section of the San Andreas fault, it is 2.8 MPa. Both segments are therefore highly and relatively similarly stressed, placing the system in a configuration that historically has preceded joint ruptures.
Burkhard said:
So not only is it concerning that the stresses are reaching historic highs, but also that the relative stress conditions between the two fault systems are approaching the range we associate with major ruptures crossing both faults simultaneously – and that is a scenario with much larger consequences for the region.
California faults pose increased risk in densely populated regions
A joint rupture of the San Andreas fault and the San Jacinto fault that crosses the Cajon Pass would be a much more severe event than one that is limited to a single fault. The affected region includes some of the most densely populated, infrastructure-critical corridors in the U.S., including the greater Los Angeles area, San Bernardino, Riverside and the Coachella Valley. Major highways, railroads and energy infrastructure run through the Cajon Pass itself.
Burkhard said:
The question of when and how the next major earthquake will occur in this region is one of the most pressing problems in applied geoscience. Our results provide a clearer, physics-based picture of the current stress state of the fault system, and the framework we developed is not just applicable to California, but also for other complex fault junctions worldwide.
However, Burkhard emphasized:
The study is not a prediction of when an earthquake will occur. What we can say is that the system is critically stressed 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 hazard assessment, infrastructure planning and emergency preparedness.
Bottom line: A new model of 1,000 years of earthquake history along two California faults has revealed that these faults are under record levels of stress.
Read more: Big cities are sinking in the US. Is yours one?
The post California faults under record stress, study finds first appeared on EarthSky.
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