California Earthquake Hazards: New Studies Reveal Critical Insights

Two separate studies published in scientific journals have provided new information about the geological conditions affecting earthquake hazards in California. One study focuses on stress accumulation on fault systems in Southern California, while another reveals previously unknown tectonic complexity beneath Northern California.

Southern California Fault Stress Study

Overview

A study published in the Journal of Geophysical Research: Solid Earth has estimated that tectonic stress on multiple segments of the San Andreas and San Jacinto fault systems in Southern California is at or above the highest levels recorded in the past 1,000 years.

The study was conducted by researchers from the University of Bern, the University of Hawaii at Manoa, Northern Arizona University, the United States Geological Survey (USGS), and the University of California, San Diego.

Methodology

Researchers built a physics-based computer model simulating stress accumulation and release along the southern San Andreas and San Jacinto fault systems. The model was informed by a 1,000-year earthquake history reconstructed from geological evidence, including radiocarbon dating and tree-ring records. The simulation tracked stress buildup, release, and propagation over time to estimate current stress levels.

Key Findings

"The system is in a critically loaded state." — Lead author Liliane Burkhard

• Stress levels on the Mojave South segment of the San Andreas fault were estimated at 2.8 megapascals.
• Stress levels on the San Jacinto Bernardino segment were estimated at 3.6 megapascals, exceeding its previous peak from 50 years ago.
• In the last 1,000 years, the faults have produced at least 36 earthquakes of magnitude 6.4 or greater.
• The last major earthquake on these faults was a magnitude 7.9 event in 1857 on the San Andreas fault, which did not propagate through Cajon Pass.
• The faults have not produced a major earthquake in the Los Angeles area in over 160 years.

The "Earthquake Gate" at Cajon Pass

The study identifies the Cajon Pass, located approximately 50 miles northeast of Los Angeles, as a critical junction where the San Andreas and San Jacinto fault systems meet. This junction is described as an "earthquake gate" that can either stop a rupture or allow it to transfer between fault systems.

The study indicates that earthquakes passed through Cajon Pass in the past when stress levels on both sides were similarly elevated. Researchers noted that current stress levels on both fault systems approach levels associated with conditions that could allow a rupture to cross the junction and involve both faults simultaneously.

Historical records indicate that the Wrightwood earthquake of approximately 1812, estimated at magnitude 7.5, is suspected to have crossed the Cajon Pass.

Implications and Statements

Lead author Liliane Burkhard stated that the system is in a "critically loaded state" with more than 160 years since the last major rupture. She emphasized that the study is not a prediction of when an earthquake will occur.

The researchers stated that the findings are intended to inform seismic hazard assessments, infrastructure planning, emergency preparedness, and building codes. Researchers recommended that emergency planners prepare for joint ruptures involving both fault systems as a realistic scenario.

Northern California Tectonic Complexity Study

Overview

A separate study published in Science on January 15, 2026, has identified a more complex tectonic configuration beneath the Mendocino Triple Junction off the coast of Northern California than previously understood.

The study was conducted by researchers from the USGS, the University of California, Davis, and the University of Colorado Boulder.

Methodology

Researchers analyzed data from low-frequency earthquakes recorded by seismometers across the Pacific Northwest. These subtle tremors are not felt by humans but provide information on tectonic plate movement. The team validated their findings using tidal-sensitivity models, which confirmed how underlying rocks respond to daily tidal stresses.

Key Findings

"Observing surface features alone is insufficient for understanding the underlying configuration." — David Shelly, USGS Geologic Hazards Center

The study revealed the presence of five distinct moving tectonic pieces beneath the Mendocino Triple Junction, where the Pacific Plate, North American Plate, and Gorda Plate were previously thought to interact. Specific discoveries include:

• A section of the North American Plate has broken off and is sinking with the Gorda Plate in a process called subduction.
• The Pioneer Fragment, a remnant of the ancient Farallon Plate, is being dragged by the Pacific Plate beneath the North American Plate.
• The subducting surface where the Gorda Plate is pushed beneath the North American Plate is at a shallower depth than previously assumed.

Implications

The findings alter the previously assumed location of the plate boundary. Researchers noted that the revised model is supported by observations from a 7.2-magnitude earthquake in California in 1992, which originated at a shallower depth than contemporary models predicted.

David Shelly of the USGS Geologic Hazards Center stated that observing surface features alone is insufficient for understanding the underlying configuration. Geophysicist Amanda Thomas of UC Davis stated that understanding underlying tectonic processes is essential for predicting seismic hazards. Tectonic geodesist Kathryn Materna of the University of Colorado Boulder noted that the findings deviate from the assumption that faults follow the leading edge of a subducting slab.