How little-known fault lines could trigger the next catastrophic earthquake in the US

Major faults like the San Andreas strike fear of producing devastating earthquakes.

But scientists have found that little-known 'branch' faults, minor faults that stem from larger ones, could release high-magnitude quakes that would be just as destructive.

A new analysis discovered that five of the largest earthquakes in the last 25 years, including the magnitude 7.9 Denali earthquake that struck Alaska in 2002, originated in the smaller branches.

The team has sounded the alarm for more monitoring on the overlooked branches as they are found in several US states.

'All these mangy wannabe faults that we pretty much ignore as candidates for big earthquakes actually could be very important,' Ross Stein, earthquake researcher and CEO of Tremblor, an earthquake-risk assessment app, told SciTechDaily .

The 'wannabe faults' are found branching off of major fault lines like the San Andreas, which runs through California, and the Queen Charlotte Fault, which runs offshore from Vancouver Island, Canada to southeast Alaska.

Stein and his co-author, professor emeritus Peter Bird of the University of California , Los Angeles , analyzed records of continental transform earthquakes that were magnitude 7.8 or larger going back 25 years.

Continental transform earthquakes are quakes that occur when two tectonic plates slide past each other.

This movement generates friction, which causes tension to build until the fault - or the boundary between the two plates - ruptures, triggering an earthquake.

But Stein and Bird's analysis revealed that the five largest quakes since 2000 originated on small, often ignored 'branch' faults.

As the name suggests, a branch fault is a minor fault that branches off from a major fault.

In the US, the 2002 magnitude 7.9 Denali earthquake that struck Alaska originated on a branch fault.

This quake hit a sparsely populated region and caused no deaths or injuries, but triggered thousands of landslides.

But the most recent confirmed example of a major quake that originated on a branch fault occurred on February 2023, when Turkey and Syria were rocked by a magnitude 7.8 earthquake that killed at least 3,000 people and injured thousands more.

Based on their analysis, Stein and Bird believe minor faults could play a much larger role in generating big earthquakes than previously thought.

'It just seemed so bizarre that a magnitude 7.8 earthquake could start on a little wannabe fault... it would be nobody's candidate for something important,' Stein said

'So it raised the question: is this just a freak earthquake or is this telling us something about how earthquakes begin?' he added.

The researchers propose that branch faults can act as 'on-ramps' to generate ruptures on 'superhighway' continental transform faults.

Over time, movement of tectonic plates along a fault line smashes and shears rocks within a zone that extends roughly 330 feet around the fault, Stein explained.

This damage creates a type of rock called cataclastite. This fractured, chewed-up rock has a lot of pore space that could be filled with fluids.

If an earthquake were to start on an 'on-ramp' branch fault, the influx of heat generated by the shearing of rocks could 'suddenly heat up those fluids, which then expand, and it could make the fault suddenly very slippery, transforming it into a rupture superhighway,' Stein said.

'What might make these branch fault ruptures effective in triggering something huge is if they rupture at supershear velocity,' he added. Supershear velocity is where the rupture itself is moving faster than the strongest seismic waves.

In this case, the branch rupture 'could come flying into the main fault at high velocity and kick the main fault hard all at once,' Stein said.

But not all branch faults trigger transform fault earthquakes. Stein and Bird's survey showed that roughly 25 percent to 50 percent do. It's still unclear why branch faults might trigger some continental transform fault earthquakes but not others.

The researchers' hypothesis requires further testing. But if their theory proves to be correct, it would suggest that seismologists need to expand their view of which faults are capable of generating devastating earthquakes.

'We need to take in a broader area,' Stein said.

Seismic networks that monitor faults are usually most robust along main faults 'because we often think that earthquakes begin on the fault that ultimately slips,' Stein said.

He suggest that they will be able to test their hypothesis in about 10 years, as magnitude 7.8 or larger earthquakes take place roughly every two to five years on these faults.

The findings could prove useful for adapting earthquake early warning systems, and might change which faults are now instrumented and monitored, Stein said.

Improving earthquake monitoring and warning systems is especially important for people living along the West Coast, who have long held fears of 'The Big One.'

The Big One is a hypothetical, but inevitable, high-magnitude continental transform earthquake that would decimate entire cities.

Experts have estimated that this quake would cause roughly 1,800 deaths, 50,000 injuries and $200 billion in damage, according to the California Earthquake Authority.

The seismic event is predicted to occur somewhere along the San Andreas or the Cascadia Subduction Zone. But if Stein and Bird's theory is correct, it could originate on a nearby branch fault - making it even harder to predict and prepare for.