Researchers Ross Stein and Peter Bird have proposed a hypothesis that many major continental transform earthquakes begin on branch faults, which they describe as “on-ramps” that lead to ruptures on larger, more mature faults. This theory, if validated, could have significant implications for earthquake monitoring and preparedness, potentially altering how and where seismic networks are deployed.
Stein and Bird’s hypothesis stems from the observation that the five largest continental transform earthquakes since 2000 all began on branch faults rather than on the main fault lines. These earthquakes include the 2001 magnitude 7.8 Kokoxili earthquake in Tibet, the 2002 magnitude 7.9 Denali earthquake in Alaska, the 2008 magnitude 7.9 Wenchuan earthquake in China, the 2016 magnitude 7.8 Kaikōura earthquake in New Zealand, and the 2023 magnitude 7.8 Pazarcık earthquake in Türkiye. The researchers predict that future major continental transform earthquakes may also originate on similar branch faults, challenging the traditional focus on main fault lines alone.
Branch faults, as Stein and Bird argue in their Seismological Research Letters paper, act as precursors to larger ruptures by allowing slips to occur at lower shear stress levels. Over time, continuous slippage on the main transform fault creates a zone of cataclastite—a fractured rock type that facilitates easier slippage. When a branch fault is triggered, it can act like an on-ramp, transferring energy to the main fault. This behavior suggests a need for seismic monitoring to be expanded to include branch faults, which might currently be overlooked as candidates for initiating large earthquakes.
One of the mechanisms that Stein and Bird suggest could make branch faults particularly effective in triggering significant seismic events is the role of fluids within the rock. Cataclastite has a porous structure, allowing it to store fluids. When a branch fault slips, the sudden frictional heating could expand these fluids, making the fault surface more slippery and allowing a rapid transition to a larger rupture. This process could transform a localized slip into a full-blown rupture along the main fault line, essentially turning a branch fault into a rupture superhighway.
Another critical aspect of the researchers’ hypothesis involves the concept of supershear velocity. If a branch fault rupture reaches speeds faster than the strongest seismic waves, the energy transfer to the main fault can be more abrupt and forceful, effectively “kicking” the main fault into motion. Such rapid ruptures can lead to more intense seismic events, as seen in past continental transform earthquakes.
Stein and Bird emphasize that their hypothesis, while promising, still requires further testing. Not all continental transform earthquakes originate on branch faults, and smaller events, such as the 1990 Luzon earthquake in the Philippines and the 2013 Balochistan earthquake in Pakistan, did not show this pattern. This inconsistency highlights the need for more research to understand why some large earthquakes nucleate on branch faults while others do not.
The implications of Stein and Bird’s work are broad. If branch faults are indeed precursors to major earthquakes, the focus of seismic monitoring systems would need to shift, incorporating a wider range of faults that have previously been overlooked. Such changes could lead to improved earthquake early warning systems, potentially saving lives and reducing damage during future seismic events. As Stein noted, “All these mangy wannabe faults that we pretty much ignore as candidates for big earthquakes actually could be very important.”
The researchers anticipate that their hypothesis could be tested within the next decade, given that magnitude 7.8 or larger continental transform earthquakes typically occur every two to five years. Their ongoing analysis could reshape current understandings of seismic risk along the world’s major continental transform faults, such as the San Andreas Fault, the North Anatolia Fault in Türkiye, and New Zealand’s Alpine Fault.