A devastating and violent type of earthquake once thought to be rare is actually much more common than previously thought, according to researchers at UCLA.
They’re called supershear earthquakes and they have the potential to be more destructive and cause more shaking than other earthquakes of the same magnitude.
Supershear quakes are a subtype of strike-slip earthquakes. A strike-slip quake happens when the edges of two tectonic plates rub sideways against each other.
Supershear earthquakes are caused when faults beneath the surface rupture faster than the waves responsible for shaking can move through rock.
Lingsen Meng is an associate professor of geophysics at UCLA and was the lead researcher on the study.
He compares a supershear earthquake to a sonic boom, albeit much more devastating.
“The supersonic aircraft flies at a speed faster than sound waves,” Meng said. “When they fly, they have to break the sound barrier, which is causing enormous amount of noise or vibrations — that’s why they’re banned in the United States. A supershear earthquake is similar; they try to propagate faster than the seismic waves. They have to break certain barriers too.”
Because the faults rupture quicker than the waves, that energy is “corralled” and then released violently, researchers say.
“So that means more energy has been released in a shorter timeframe. So the energy is more concentrated, it gives you stronger shaking, and the stronger shaking is responsible for damaging buildings and other things,” Meng said.
The researchers at UCLA say at least one large earthquake that happened in Southern California was a supershear earthquake. It happened in 1979 in the Imperial Valley, just south of the U.S.-Mexico border. It caused extensive damage to irrigation systems and injured dozens, although no deaths were reported.
Meng and his fellow researchers looked at all 6.7+ magnitude strike-slips, searching for evidence of supershear earthquakes. What they found is that of the applicable 87 quakes since the year 2000, 12 were supershear.
They were previously believed to only account for about 6% of all strike-slip quakes. With these new findings, researchers found that those violent supershear quakes happened twice as often, accounting for 14% of all strike-slip earthquakes.
“This is, you know, still rare, but not that rare,” Meng said. “It was pretty surprising to the community.”
Meng said he wasn’t particularly shocked by the results of the study, attributing the new data to the proliferation of better technology.
It’s theorized that the Great 1906 San Francisco Earthquake that may have killed more than 3,000 was a supershear quake, although it predates any scientific monitoring that could confirm that hunch. The quake lasted less than a minute, but fires burned through the city for several days, wrecking what was then the tenth-biggest city in America.
“Of course, we don’t have the seismometer or data in the modern sense at that time to really make a conclusive argument,” Meng added. “But it is absolutely possible.”
The findings suggest that disaster planners should take into account whether or not a nearby fault is capable of producing a supershear quake and plan accordingly. Because they have the potential to produce more devastating shaking, disaster planners and government agencies should consider the worst case scenario when they plan how to respond to future quakes.
The research also found that these violent supershear earthquakes happen beneath the ocean just as frequently as they do on land and they are most likely to take place along strike-slip faults, including the San Andreas Fault, which extends about 750 miles across California.
The quakes tend to be less violent along faults that curve and more violent along faults that are straight. The San Andreas Fault, which runs beneath cities where millions of Californians live, is mostly straight. A major quake along that fault could be devastating.
As for the likelihood of a “big one,” Meng said we should always be planning for the possibility of a devastating quake and a supershear could possibly be the culprit.
“I think we should absolutely consider the possibility of supershear quake, on the San Andreas Fault,” Meng said. “We haven’t had a ‘big one’ in Southern California for 150 years. The scientific community is still kind of imagining what the ‘big one’ should be like. And I think taking into account this supershear possibility is definitely essential.”
Meng adds that the “big one” could come anywhere from 30 years to 100 years from now or as soon as tomorrow.
And while the timeframe is anyone’s guess, the location for the next “big one” is almost assuredly going to happen along the San Andreas Fault.
“We can pretty much determine it’s going to be this Mojave segment, very close to the city of Los Angeles. So we should be definitely prepared for that.”