Scientists warn California should prepare for destructive ‘supershear’ earthquakes

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 opinion piece 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”—an 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.