The latest research by geoscientists at the University of Hawaii at Manoa shows that the San Andreas Fault, which runs about 750 miles (about 1,200 kilometers) through California, and the San Jacinto Fault to the south are currently in a state of "critical stress", and the overall pressure level has risen to the highest point in the past thousand years, which significantly increases the possibility of strong earthquakes on the West Coast of the United States.
The research team used physics-based numerical models and combined with a thousand years of historical earthquake data to reconstruct and evaluate the stress accumulation conditions of the above two major fault systems and their intersection area, Cajon Pass. Liliane Burkhard, the first author of the paper and a researcher at the Institute of Geophysics and Planetology at the University of Hawaii, said that the current stress levels of multiple faults "have reached or exceeded the highest values observed in the past thousand years," and the regional system may have the conditions for a large rupture that runs through the two major faults of San Andreas and San Jacinto.
The study proposed the concept of "earthquake gate" to describe the key hub role played by Cajon Gap between the two major faults. In some cases, when one fault has significantly higher stresses than another, Cajon Gap can act like a "safety valve," blocking large-scale ruptures from propagating between the two faults, thus limiting the size and scope of a single seismic event. However, when the stress levels of the two faults are highly close and increase simultaneously when the rupture occurs, the Cajon Gap may "open the floodgates", allowing the rupture to span the two major faults and evolve into a joint earthquake event covering a larger area.
Burkhard pointed out that current calculations show that the San Andreas Fault in southern California has not experienced a large rupture of similar magnitude in nearly 160 years since the 1857 "Fort Tejon earthquake." During this period, there was a lack of sufficient stress relief, which put the system "in a critical loading state." Models show that under this background of high pressure that has not been released for a long time, if the "gates" of the Cajon Gap are opened, the two major faults of the San Andreas and San Jacinto may rupture simultaneously in a single event, and densely populated areas such as Los Angeles and San Bernardino will face severe earthquake damage risks.
The research also reiterates the basic mechanism of plate tectonics: In the San Andreas Fault Zone, the Pacific and North American plates slide relative to each other along the fault at a rate of about 1 to 2 inches per year. When the plates are locally "stuck" at the contact surface, stress will continue to accumulate at these "stuck points" until at a certain moment the fracture surface suddenly slips, releasing huge energy and propagating to the surface in the form of seismic waves. This is what people feel as earthquakes.
The article compares the differences in seismic activity in different tectonic environments. For example, in the Milun Fault Zone extending in eastern Taiwan, there is continuous "creep" between the Eurasian plate and the Philippine Sea plate. Local residents can feel hundreds of earthquakes every year, most of which have magnitudes between 3.0 and 5.0. Larger earthquakes are relatively rare. Based on his own experience living in northern Taiwan, the author pointed out that such frequent but small-scale vibrations mean that regional stress is regularly released, thereby reducing the possibility of a very large "super earthquake".
In contrast, some sections of the San Andreas Fault have not experienced large-scale ruptures for a long time, and pressure continues to accumulate deep down. From the perspective of the scientific community, a "big earthquake" is no longer a question of "if it will happen", but a question of "when it will happen." The researchers emphasize that their work is not a prediction of specific earthquake times, but a rigorous quantitative model to provide a clearer scenario reference for earthquake risk assessment, infrastructure planning and emergency preparedness for millions of people.
Burkhard said physics-based simulations provide an important tool for understanding stress coupling relationships between different fault segments, helping to clarify the role of key nodes like Cajon Gap in determining the size and path of ruptures. She noted that when fault systems as a whole are "critically loaded," emergency management and infrastructure agencies need to take seriously a variety of potential scenarios, and everything from building codes to urban resilience planning should incorporate the possibility of such large combined ruptures.
The research results have been published in the Journal of Geophysical Research: Solid Earth, and the relevant information is released by the University of Hawaii. The research team believes that although existing science is still unable to accurately predict the time and location of earthquakes, by continuously improving models and accumulating observation data, humans can be "one step ahead" in risk recognition and disaster prevention and control, providing more evidence-based decision-making support for cities on the West Coast of the United States and even in seismic zones around the world.