The latest research by American seismologists shows that the approximately 1,200-kilometer San Andreas Fault and the San Jacinto Fault next to its southern section are currently in a state of "critical stress", with cumulative pressure reaching the highest level in the past thousand years, significantly increasing the possibility of strong earthquakes on the West Coast of the United States. The research team pointed out that Cajon Pass, located at the intersection of two major faults, may act as an "earthquake gate." Once opened, it will trigger a large-scale rupture event across the two major faults, posing a serious threat to highly populated areas such as Los Angeles and San Bernardino.
The study, led by geoscientists at the University of Hawaii at Manoa, used physical simulations and thousands of years of historical earthquake data to reconstruct the stress evolution of the San Andreas and San Jacinto fault systems and the Cajon Pass region. Liliane Burkhard, lead author of the paper and a researcher at the Institute of Geophysics and Planetology at the University of Hawaii, said that stress levels on many fault segments are now close to or exceeding the highest values recorded in the past thousand years, which means that the area "has the ability to have a large earthquake that cuts through two major faults."
The study proposed the key concept of "seismic gate" to describe the controlling role of the Cajon Pass in chain ruptures of large earthquakes. When the stress levels of the two faults are very different, this "gate" may prevent earthquake ruptures from crossing from one fault to the other; when the stress heights of the two faults are close and both are in an elevated state, the "gate" is more likely to be "opened", allowing an earthquake event to continuously expand along the two faults, forming a strong earthquake covering a wide range. Burkhard pointed out that the overall regional stress is currently at a historical high, and more than 160 years have passed since the last large-scale rupture occurred in this section, and the entire system is already in a "critical loading state."
The San Andreas Fault is the junction of the Pacific Plate and the North American Plate. It is about 750 miles (about 1,200 kilometers) long and the fault plane is buried about 16 kilometers below the surface. It is one of the largest and most representative active faults in North America. In California, smaller faults will merge or branch off this main fault, among which the San Jacinto Fault in Southern California is an important branch. The relative movement rate of the plates is about 1 to 2 inches (about 2.5 to 5 centimeters) per year. If a "stuck" phenomenon occurs at the plate boundary, local stress will continue to accumulate until it is suddenly released at a certain moment, forming a strong earthquake. This is a basic physical process that occurs repeatedly in this area.
In contrast, some fault zones have long-term slow "climbing" and continue to release energy through frequent small earthquakes, making it difficult to accumulate energy on the same scale. For example, the friction between the Eurasian Plate and the Philippine Sea Plate along the Milun Fault Zone in eastern Taiwan can trigger hundreds of felt earthquakes every year. Most earthquakes have magnitudes between 3.0 and 5.0, and larger earthquakes are relatively rare. For residents living in areas with high seismicity, such as Hualien County, although frequent shaking is difficult to get used to, from the perspective of seismic energy release patterns, this state of "frequent earthquakes but not major earthquakes" actually reduces the probability of "super major earthquakes" to a certain extent. However, in some sections of the San Andreas fault, due to the long-term lack of such sustained release, huge energy has quietly accumulated deep down, making the possibility of "big earthquakes" gradually increasing.
Historical records show that a large-scale rupture occurred in the southern section of the San Andreas Fault in 1857, the famous Fort Tejon earthquake, which was one of the largest seismic events in California history. Since then, the relevant area has not experienced a complete fault rupture of similar scale for nearly 160 years. In the consensus of many scientific institutions and seismologists, the so-called "big one" is no longer a question of "will it come", but only a question of "when" it will come.
Still, the team stresses that the current work does not constitute a specific prediction of when the earthquake will occur. Burkhard said that the physical model and long-term series data analysis used this time are intended to provide a more refined quantitative basis for earthquake risk assessment and disaster management, rather than giving an exact time forecast. She noted that such studies are an important part of national and global seismic hazard research, helping governments and city managers understand the actual range of risks faced by millions of people.
Researchers pointed out that the clearer judgment that can be given at present is that the San Andreas and surrounding fault systems are in a state of "critical stress", and the possible rupture scenarios in the future are uncertain, but the scope and intensity of potential damage cannot be underestimated. Through this type of physics-based model, researchers can more clearly depict various possible earthquake scenarios, providing scientific reference for seismic risk assessment, infrastructure construction standards and emergency plan formulation. Relevant research results have been published in the Journal of Geophysical Research: Solid Earth. The paper was written by a team from the University of Hawaii. The study of the "critical fault" state in California is also attracting widespread attention from the international seismological community.