Dangerous “shear” earthquakes pose a threat to California

Some scientists say a certain type of earthquake that can cause particularly severe tremors is more common than previously thought, carrying potentially significant risks to communities across California, including those in the path of the notorious San Andreas Fault.

Scientists have increasingly noticed how a fault ruptures during an earthquake can be faster than the speed of another type of damaging seismic wave, theoretically generating sonic boom-level energy.

These shock waves — which originate during “super-shear” earthquakes — can exacerbate how badly the ground shakes from side to side and top to bottom along the affected fault zone, scientists at the University of Southern California, Caltech, and the University of Illinois Urbana-Champaign wrote in a recent opinion piece for the journal Seismological Research Letters.

Although not everyone agrees that supershear earthquakes are inherently more destructive than other types, the potential impacts are enormous and need to be taken into account in seismic predictions, scientists stress.

“It is imperative that we update these scenarios to account for superrupture,” the scientists wrote in the opinion piece. “We must also reassess the vulnerability of infrastructure and update relevant building code provisions – especially for critical facilities.”

Earthquakes are the result of one block of land moving away from another. This action begins at the epicenter and moves in one or more directions down the fault, sometimes hundreds of miles in a matter of minutes. On a fault like the San Andreas, the earthquake moves like a train traveling at full speed on the tracks.

Traditionally, fault rupture has been thought to move more slowly than another type of damaging seismic wave, known as a shear wave. In general, shear waves cause a great deal of the destructive shaking force in an earthquake.

But in the past few decades, scientists have increasingly noticed how faster a fault's rupture during an earthquake can be in some cases.

“This generates something similar to a sonic boom, when the plane exceeds the speed of sound,” said Ahmed Ataf Al-Banna, lead author of the opinion piece and a professor of geosciences and civil engineering at the University of Southern California.

However, not everyone agrees on the implications.

“I don’t think we know yet whether supershear ruptures are truly more destructive,” said Brad Aagaard, a research geophysicist at the USGS, who was not involved in the opinion piece.

“The opinion piece calls for more effort to be done to try to understand where supershear ruptures are likely to occur and what the implications are,” Aagaard said.

The idea that rupture in a fault could move faster than a seismic shear wave was theorized as early as the 1970s, said Al-Banna, who is set to become director of California's statewide earthquake center next year.

But by the turn of the 20th century, there was only one earthquake globally that some scientists suspected was a supershear event, the 1979 6.4-magnitude Imperial Valley earthquake.

Only in the late 1990s did the idea of ​​supershear earthquakes begin to be taken seriously. Laboratory experiments at the California Institute of Technology have shown that this is physically possible. In the last quarter century, an increasing number of earthquakes of magnitude 7 or greater have been identified as supershear, and observations have been aided by the installation of monitoring instruments around the world.

The key breakthrough occurred during the 7.9-magnitude Alaska earthquake in 2002, where a sensor station happened to be close enough to the ruptured Denali fault to record data that confirmed what scientists theoretically expected from a supershear earthquake, Albanna said.

Since then, scientists have identified more earthquakes as such events.

“We're getting better data, and now we're seeing it,” said seismologist Lucy Jones, a research associate at Caltech.

In the past 15 years alone, 14 out of 39 large earthquakes have shown supershear ruptures, the opinion piece said.

Among them are the 7.7-magnitude earthquake that struck Myanmar in March, which killed more than 3,700 people; As well as the twin earthquakes of magnitude 7.8 and 7.5 that killed more than 58,000 people across Türkiye and Syria in 2023; And the 7.5-magnitude earthquake and tsunami in 2018 in Indonesia, which killed at least 2,000 people.

Scientists also now consider the 7.6 magnitude earthquake that struck Türkiye in 1999 to be a supershear earthquake. This earthquake killed at least 17 thousand people.

“We are discovering them more frequently than before,” Al-Banna said.

A look back at the massive 1906 earthquake that destroyed much of San Francisco suggests a great shear event may also have occurred, the opinion piece said, citing a 2008 report in the Bulletin of the Seismological Society of America.

In California, supershear earthquakes are expected to occur on the straightest faults — where one block of earth slides over another — such as 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 above the San Andreas Fault. Among them are Coachella, Indio, Cathedral City, Palm Springs, Desert Hot Springs, Banning, Yucaipa, Highland, San Bernardino, Wrightwood, Palmdale, Gorman, Fraser Park, San Juan Bautista, Palo Alto, Portola Valley, Woodside, San Bruno, South San Francisco, Pacifica, Daly. City and Bodega Bay.

But there is no evidence that supershear earthquakes occur on reverse thrust faults, which are those in which a block of earth at the top of a 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 occur on other strike-slip faults in California that are shorter and more regular, such as the Newport-Inglewood, San Jacinto and Elsinore in Southern California, Aagaard said.

In Northern California, the Hayward and Calaveras fault can creep without triggering an earthquake, and they are less straight, so they are considered less likely to produce supershear ruptures, Aagaard said.

One point made by the opinion piece's authors is to urge updated scenarios that imagine the kind of death and destruction that could affect California in a major earthquake.

“It's a call to action,” Al-Banna said. “What are these super events going to do, knowing what we know about their physics and the intensity of their shaking and so on, what are they going to do to public facilities and buildings?”

Jones said she also believes the state's minimum building standards should be reconsidered, but for a different reason.

She doesn't think the answer is to incorporate a detailed model of how supershear earthquakes work into minimum building standards, “because there's too much variability in where they appear.”

But she objects to how minimum building standards are set. In general, minimum building standards are designed to protect buildings from reasonable average vibration, not maximum.

The new building codes are designed to ensure only “life safety,” requiring buildings to be designed so that people can theoretically crawl to safety during an earthquake. But even in such cases, those buildings can be damaged so badly that they will need to be demolished.

“They're trying to make the buildings as weak as possible without them falling over,” Jones said.

“This whole thing of supershear — as a seismologist, it's really intellectually fascinating. And as a building code advocate, that's another reason why … I'm trying to do this [a new building] “Being as weak as possible without falling is probably not the best public policy,” Jones said.

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