Over the past decade, researchers have been revisiting what we think we know about the origins of large earthquakes. A theory put forth by scientists Ross Stein of Temblor, Inc. and Peter Bird of the University of California, Los Angeles, is transforming the way we understand seismic activity on continental transform faults. Their groundbreaking hypothesis suggests that branch faults, previously overlooked in earthquake prediction models, could be the starting point of the next major earthquake—a prediction that has gained more traction over the past 10 years.
These branch faults, which act like “on-ramps” to larger, more mature faults, could initiate ruptures leading to catastrophic earthquakes. The research, published in 2024, draws on data from several of the largest earthquakes since 2000, including the devastating 7.8 magnitude earthquake in Türkiye in 2023 and earlier events in Alaska, Tibet, and New Zealand. If proven correct, this hypothesis could revolutionize the way we monitor faults and prepare for future seismic events.
A Decade of Evidence: Major Quakes Beginning on Branch Faults
Branch faults are smaller, less well-known faults that split off from main continental transform faults, such as the San Andreas Fault in California or the North Anatolian Fault in Türkiye. According to Stein and Bird’s research, these seemingly insignificant faults have initiated some of the largest continental earthquakes in recent memory. For instance, the 2023 Pazarcık earthquake in Türkiye, with a magnitude of 7.8, began on such a branch fault.
Looking back, other significant earthquakes share this pattern. The 2001 Kokoxili earthquake in northern Tibet, the 2002 Denali earthquake in Alaska, and the 2008 Wenchuan earthquake in China all started on branch faults. These quakes, like others that followed, highlight a concerning pattern that researchers believe may help predict where the next big earthquake will originate.
“What struck us was the recurrence of this phenomenon,” said Stein. “The idea that a massive earthquake could start on a ‘wannabe’ fault—one that you’d never consider important—is reshaping our understanding of earthquake origins.”
Transforming Earthquake Monitoring and Prediction
Over the last 10 years, Stein and Bird’s work has prompted seismic monitoring agencies to reconsider how they track earthquake-prone regions. Traditionally, monitoring efforts have focused on the main, most visible faults. But the researchers argue that branch faults—those previously deemed insignificant—deserve attention as well.
“Main faults like the San Andreas get all the attention because they are the ones that ultimately slip, causing the massive quakes,” Stein explained. “But if we are right, we need to broaden our focus to include these smaller faults that might act as the on-ramps for larger seismic events.”
This shift could fundamentally alter earthquake early warning systems, making them more comprehensive by factoring in the branch faults that might trigger the rupture of the main fault. While the hypothesis is still under scrutiny, the regularity of large earthquakes on branch faults over the past decade has spurred new research, testing the validity of this claim.
The Physics Behind Branch Fault Ruptures
One of the key insights Stein and Bird have offered is a deeper understanding of how these smaller faults might act as the starting points for large earthquakes. When earthquakes occur, the intense friction and pressure generated by fault movement can rapidly heat trapped fluids in the rock. This abrupt heating can lead to a slippery, lubricated fault zone, creating what Stein called a “rupture superhighway” that can accelerate the movement along the fault.
Additionally, some branch faults rupture at what is known as supershear velocity, meaning that the rupture spreads faster than the seismic waves themselves. When this happens, the rupture can hit the main fault at an incredibly high velocity, potentially triggering a much larger earthquake than would have occurred otherwise.
“If the branch fault triggers a supershear rupture, it could slam into the main fault with enough force to generate an earthquake of catastrophic proportions,” Stein explained. “We’ve seen this pattern in the past, and it’s something we need to keep an eye on in the future.”
Not All Earthquakes Follow the Same Pattern
While Stein and Bird’s hypothesis has been compelling, it’s important to note that not all large earthquakes begin on branch faults. For example, the 1990 Luzon earthquake in the Philippines and the 2013 Balochistan earthquake in Pakistan both registered at magnitudes of 7.7, but neither appeared to originate from branch faults.
This variability highlights the complexity of earthquake prediction. Even though the pattern holds for some of the most significant quakes, not all seismic activity follows the same path. This variability makes ongoing research essential in determining which earthquakes will follow this branch fault model and which will not.
Implications for the Next Decade
As we look toward the future, Stein and Bird’s hypothesis remains a subject of intense study. The researchers believe their theory is “testable” within 10 years, with magnitude 7.8 or larger earthquakes occurring on continental transform faults every two to five years on average. With this timeline in mind, scientists around the world are eagerly watching seismic activity to determine if more evidence supports the idea that branch faults could ignite future major earthquakes.
The 2023 earthquake in Türkiye has already served as a real-world test case, giving some initial validation to their claims. And with thousands of kilometers of continental transform faults crisscrossing the globe, including the San Andreas Fault in California and New Zealand’s Alpine Fault, the world waits to see where the next big earthquake will strike—and whether it, too, will begin on a branch fault.
In the coming years, the insights gained from this research could revolutionize earthquake preparedness and response, offering the potential to save lives and mitigate damage in earthquake-prone areas.
