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Most Californians are familiar with earthquakes. But researchers say the state faces an overlooked threat: "supershear" earthquakes that move so fast they outrun their own seismic waves.

In an published in Seismological Research Letters, scientists at the USC Dornsife College of Letters, Arts and Sciences warn that these quakes produce more violent shaking over larger areas than typical earthquakes, and California must update its hazard planning and building codes to reflect the heightened risk of damage.

"While California is no more likely to have supershear earthquakes than other, similar regions with large fault systems like the San Andreas, the threat has gone unnoticed for too long," said Yehuda Ben-Zion, professor of earth sciences and director of the Statewide California Earthquake Center (SCEC), based at USC Dornsife. "The frequency of these supershear ruptures has been greatly underappreciated."

Scientists compare supershear earthquakes to sonic booms. Just as a jet breaking the sound barrier creates an explosive shock in the air, a supershear rupture generates shock fronts in the ground when it outpaces seismic shear waves, said Ahmed Elbanna, professor of earth sciences and director-designate of SCEC. "It breaks the shear wave speed barrier in the rocks and produces destructive waves that are stronger than what's generated by a normal earthquake," he said.

That added force can hit communities hard. Supershear quakes spread strong shaking farther and deliver what Elbanna calls a "double strike"鈥攁n initial jolt from the shock front followed by the trailing waves.

Worldwide, about one-third of large strike-slip earthquakes are supershear. That matters in California, where many faults near large metropolitan areas are strike-slip and capable of magnitude 7 or higher temblors.

"We cannot say exactly when and where the next earthquake will be and which one will be supershear," Ben-Zion said, "but we can say with certainty that over the next few decades, we will have multiple magnitude 7 earthquakes in California.

"They are coming, whether we are prepared or not," he added.

The authors warn that current design standards don't fully account for a supershear quake's extra punch. Buildings and infrastructure are generally engineered for the strongest shaking perpendicular to faults, but supershear quakes direct their energy along the fault line itself.

"Critical structures should be built to this higher standard, and so far, they are not," Ben-Zion said.

To prepare, the team calls for denser monitoring near major faults, advanced computer simulations of supershear scenarios, and stronger building codes.

"This is a collaborative effort where everybody has to chip in," Elbanna said. "And I think here at USC and SCEC, with their reputation in the community, this is the right time and right place to get this effort started."

Elbanna and Ben-Zion co-authored the opinion piece with researchers from Caltech and the University of Illinois Urbana-Champaign.