Researchers from the Statewide California Earthquake Center at USC Dornsife are warning that extremely fast and powerful earthquakes could strike California and are calling for tougher building standards and improved fault monitoring.
Most residents of the state are accustomed to the idea of earthquakes, but scientists say a particularly dangerous type has been largely ignored: "supershear" earthquakes that move so quickly they overtake their own seismic waves.
In an opinion article in Seismological Research Letters, experts from the USC Dornsife College of Letters, Arts and Sciences explained that these rare events cause far stronger shaking across a wider region than typical quakes. They argue that California needs to strengthen its hazard planning and update building codes to address the greater potential for destruction.
"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 describe supershear earthquakes as being similar to sonic booms. When a jet surpasses the speed of sound, it produces a shock wave in the air. Likewise, when a supershear rupture travels faster than seismic shear waves, it creates powerful ground shock fronts, 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.
This added energy can cause severe damage. Supershear quakes send intense shaking farther from the epicenter and strike twice, according to Elbanna -- a sharp initial blow from the shock front followed by the trailing waves.
Globally, about one in three major strike-slip earthquakes are supershear. This is especially significant for California, where many faults near large cities are strike-slip and capable of producing magnitude 7 or higher events.
"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.
