Are splay faults the “gateways” that trigger great continental transform earthquakes?

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The five largest continental transform earthquakes since 2000 have all occurred on tributaries of the main fault, and the two researchers predict that the next big earthquake of this type will also occur on a tributary or branch fault.
Last year’s magnitude 7.8 Pazarchik earthquake in Turkey was one of several large, damaging earthquakes caused by two continental plates sliding horizontally against each other. It occurred on a splay fault, along with the 2001 magnitude 7.8 Kokoshiri earthquake in northern Tibet, the 2002 magnitude 7.9 Denali earthquake in Alaska, the 2008 magnitude 7.9 Wenchuan earthquake in China, and the 2016 magnitude 7.8 Kaikoura earthquake in New Zealand.
Continental transform faults exist around the world for about 25,000 kilometers, including the San Andreas Fault, the Alpine Fault in New Zealand, and the North Anatolian Fault in Turkey.When a fault ruptures, does it start as a spur fault?
Writing in Seismology Research Letters, Ross Stein of Tembler and Peter Bird of the University of California, Los Angeles propose that splay faults act as “gateways” to trigger the rupture of more mature “superhighway” continental transform faults.
For now, Stein and Byrd acknowledge that their prediction is a hypothesis that requires further testing, but since, on average, a magnitude 7.8 or higher earthquake occurs on these faults every two to five years, we probably won’t have to wait too long to find out if their prediction is correct.
“This could be testable in about 10 years,” Stein said.
The discovery could help with the implementation of earthquake early warning systems and could change which faults are currently instrumented and monitored, he added.
The 2023 Turkey earthquake has prompted researchers to consider the question of where and how continental transform megathrust earthquakes occur.
“It seemed really odd that a magnitude 7.8 earthquake would occur in what seemed like a small fault. No one would have thought anything significant was going to happen,” Stein recalls. “So the question arose: Was this just a freak earthquake, or could it tell us something about how earthquakes start?”
The researchers looked at continental transform earthquakes of magnitude 7.8 or greater that have occurred over the past 25 years and found that the five most powerful quakes originated on severance faults. They also identified several other earthquakes of that magnitude that could have originated on severance faults, including the magnitude 7.8 quake that devastated San Francisco in 1906.
But smaller earthquakes, such as the magnitude 7.7 Luzon earthquake in the Philippines in 1990 and the magnitude 7.7 Balochistan earthquake in Pakistan in 2013, were not caused by severance faults, Stein and Bird concluded.
In their paper, the researchers discuss why large earthquakes don’t occur on the main fault itself, and how spur faults trigger ruptures.
Cumulative slip along continental transform faults “fracture and shear the rocks within a 100-meter-wide zone around the fault,” Stein explains, forming a type of rock called cataclastite. The crushed and crumbled rocks rupture at very low shear stress levels, relieving the stress buildup that leads to a major earthquake.
Cataclastites have lots of gaps between the rock particles that could fill with fluid. The researchers say that if an earthquake were to occur at the “mouth” of a spur fault, the sudden heat generated by the shearing of the rocks could “suddenly heat up the fluid, which would expand and make the fault suddenly slippery,” turning the fault into a fracture highway, Stein said.
“The rupture of these branch faults could be effective in triggering something huge if they rupture at supershear rates,” he added.
If the branch fault ruptures at supershear velocities — meaning the rupture itself moves faster than the strongest seismic waves — “it could jump into the main fault at high speed and shake the main fault violently all at once,” Stein said.
Stein and Bird said every part of the proposal needs to be tested in the future because it’s unclear why splay faults trigger some continental transform earthquakes and not others.
Seismic networks that monitor faults are typically strongest along the main faults, “because it’s often thought that earthquakes start on faults that eventually slip,” Stein said.
But if Stein and Bird’s hypothesis is correct, “we need to look at a much larger area,” he said, adding that “these ugly faults that are largely ignored as candidates for big earthquakes may actually be very important.”
Further information: Ross S. Stein et al. “Why do continental transform earthquakes occur on branch faults?” Seismological Research Letters (2024). DOI: 10.1785/0220240175
Courtesy of the Seismological Society of America
Citation: Are splay faults the “gateways” that trigger great transcontinental earthquakes? (September 26, 2024) Retrieved September 26, 2024 from https://phys.org/news/2024-09-faults-ramps-great-continental-earthquakes.html
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