A specific type of earthquake that can cause particularly intense shaking is more common than previously believed, some scientists say — carrying potentially profound risk for communities across California, including those in the path of the notorious San Andreas fault.
Scientists have increasingly observed how the rupturing of a fault during an earthquake can be even faster than the speed of another type of damaging seismic wave, theoretically generating energy on the level of a sonic boom.
These shock waves — created during “supershear” earthquakes — can worsen how bad the ground shakes both side to side and up and down along an affected fault area, scientists at USC, Caltech and the University of Illinois Urbana-Champaign wrote in a recent opinion article for the journal Seismological Research Letters.
Although not everyone agrees that supershear earthquakes are inherently more destructive than other types, the potential implications are massive and need to be accounted for in seismic forecasts, the scientists contend.
“It is imperative that we update these scenarios to account for supershear rupture,” scientists wrote in the opinion article. “We must also reassess the vulnerability of infrastructure and update the relevant building code provisions — especially for critical facilities.”
Earthquakes are the result of one block of earth moving away from another. This action starts at an epicenter and moves in one or more directions down a fault, sometimes for hundreds of miles over a matter of minutes. On a fault like the San Andreas, the earthquake moves like a train barreling full speed down the tracks.
Traditionally, it was thought that the rupturing of a fault moved slower than another type of damaging seismic wave, known as a shear wave. Generally speaking, shear waves cause a great deal of the destructive shaking power in an earthquake.
But in the last few decades, scientists have increasingly observed how the rupturing of a fault during an earthquake can be even faster in some cases.
“This generates something similar to the sonic boom, when an aircraft exceeds the speed of sound,” said Ahmed Ettaf Elbanna, the lead author of the opinion piece and a USC professor of earth science and civil engineering.
Not everyone agrees on the implications, however.
“I don’t think we know yet whether supershear ruptures really are more destructive,” said U.S. Geological Survey research geophysicist Brad Aagaard, who wasn’t involved in the opinion article.
“The opinion piece calls out for more effort to really try and understand both where supershear ruptures are likely to occur and the implications,” Aagaard said.
The idea that the rupture on a fault can move even faster than the seismic shear wave was theorized as early as the 1970s, said Elbanna, who is set to become director of the Statewide California Earthquake Center next year.
But by the turn of the century, there had been only one earthquake globally that some scientists suspected was a supershear event — the magnitude 6.4 Imperial Valley earthquake of 1979.
It was only in the late 1990s that the idea of supershear earthquakes began to be taken more seriously. Lab experiments at Caltech showed they were physically possible. And in the last quarter-century, a growing number of earthquakes of magnitude 7 or greater have been determined to be supershear, observations that have been assisted by the installation of monitoring devices around the world.
A key breakthrough came during the magnitude 7.9 earthquake in Alaska in 2002, where a sensor station happened to be close enough to the ruptured Denali fault to record data confirming what scientists would expect from a theoretical supershear earthquake, Elbanna said.
Since then, scientists have identified more earthquakes as such events.
“We’re getting better data, and so now we see this,” said seismologist Lucy Jones, a Caltech research associate.
In just the last 15 years, 14 of 39 large strike-slip earthquakes have exhibited features of supershear ruptures, the opinion article said.
Among them were the magnitude 7.7 earthquake in Myanmar in March, which killed more than 3,700 people; as well as the dual magnitude 7.8 and 7.5 earthquakes that killed more than 58,000 people across Turkey and Syria in 2023; and the magnitude 7.5 earthquake and tsunami of 2018 in Indonesia, which killed at least 2,000 people.
Scientists also now consider a magnitude 7.6 earthquake that struck Turkey in 1999 to be a supershear earthquake. That quake killed at least 17,000 people.
“We are detecting them more frequently than before,” Elbanna said.
A look back at the massive 1906 earthquake that destroyed much of San Francisco suggests a likely supershear event too, the opinion piece said, citing a report published in 2008 in the Bulletin of the Seismological Society of America.
In California, supershear earthquakes would be expected on the straightest of “strike-slip” faults — in which one block of earth slides past another — like the San Andreas and perhaps the Garlock, which runs through the Mojave Desert along the Tehachapi Mountains, Aagaard said.
There are a number of communities directly on top of the San Andreas fault. Among them are Coachella, Indio, Cathedral City, Palm Springs, Desert Hot Springs, Banning, Yucaipa, Highland, San Bernardino, Wrightwood, Palmdale, Gorman, Frazier Park, San Juan Bautista, Palo Alto, Portola Valley, Woodside, San Bruno, South San Francisco, Pacifica, Daly City and Bodega Bay.
But there’s no evidence that supershear earthquakes occur on reverse thrust faults — those in which a block of earth on the upper part of the fault moves up and over a lower block, Aagaard said. Such movement occurred during the 1994 Northridge and 1971 Sylmar earthquakes, he said.
It’s an open question whether supershear earthquakes happen on other strike-slip faults in California that are shorter and more irregular, Aagaard said, such as the Newport-Inglewood, the San Jacinto and the Elsinore in Southern California.
In Northern California, the Hayward and Calaveras faults can creep without producing an earthquake, and are less straight, so they’re considered less likely to have supershear ruptures, Aagaard said.
One point the opinion piece’s authors make is to urge updating scenarios that imagine the kind of death and destruction that could affect California in a large earthquake.
“It’s a call to action,” Elbanna said. “What would these supershear events, knowing what we know about their physics and intensity of shaking and so on, what would they do for the utilities, for the buildings?”
Jones said she also thinks the state’s minimum building standards should be revisited, but for a different reason.
She doesn’t think the answer is incorporating a detailed model of how supershear earthquakes work into minimum building standards, “because there’s way too much variability on where it shows up.”
But she does object to how minimum building standards are set. Generally speaking, minimum building standards are designed to protect buildings from the average shaking that’s plausible, rather than the maximum.
And codes for new construction are designed to ensure only “life safety,” requiring buildings be designed so people can theoretically crawl out to safety during an earthquake. But even in such cases, those buildings could be so severely damaged they’ll need to be torn down.
They’re “trying to make the buildings as weak as they can without falling down,” Jones said.
“This whole thing of supershear — as a seismologist, it’s intellectually really cool. As a building code advocate, it’s one more reason why … trying to make it [a new building] as weak as possible without falling down is maybe not the best public policy,” Jones said.
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