Fault link means larger quakes possible on US west coast

Researchers with University of California at Berkeley have proven that two faults in the Golden State are actually one, and that the two could rupture together in an earthquake more destructive than previously thought.
"The maximum earthquake on a fault is proportional to its length, so by having the two directly connected, we can have a rupture propagating across from one to the other, making a larger quake," lead researcher Estelle Chaussard, of the Berkeley Seismological Laboratory, said in a press release issued Thursday by UC Berkeley.
What the postdoctoral fellow and her colleagues have worked on are the 70-km-long Hayward Fault, known as one of the most dangerous in the United States as it runs through large population areas near San Francisco, and the Calaveras Fault, which runs about 123 km.
Knowing that the Hayward Fault ends just short of the Calaveras Fault, there has always been ambiguity about whether the two are connected, said Chaussard. "People have been looking for evidence of this for a long time, but only now do we have the data to prove it."
The UC Berkeley team used 19 years of data from European Space Agency satellites to map ground deformation and measure creep along the southern end of the Hayward Fault, and found that the creep did not stop at its presumed end, but continued on another 15 km and the trace merged with the trace of the Calaveras Fault.
In addition, seismic data show that micro-earthquakes on these faults 3-5 kilometers underground have also merged, Chaussard said. "With this evidence from surface creep and seismicity, we can argue for a direct junction on the surface and at depth for the two faults."
The researchers also found that the underground portion of the Hayward Fault meets the Calaveras Fault 10 kilometers farther north than where the creeping surface traces of both faults meet. This geometry implies that the Hayward Fault dips at an angle where it meets the Calaveras Fault.
The U.S. Geological Survey (USGS) updated seismic hazards last month and estimated a 14.3 percent likelihood of a magnitude 6.7 or greater earthquake on the Hayward Fault in the next 30 years and a 7.4 percent chance on the Calaveras Fault.
The estimates are based on the assumption that the two faults are independent systems, and that the maximum quake on the Hayward Fault would be between magnitudes 6.9 and 7.0. Given that the Hayward and Calaveras faults are connected, the energy released in a simultaneous rupture could be 2.5 times greater, or a magnitude 7.3 quake.
"It would be even greater," Chaussard said, if the rupture extended south to where the Calaveras Fault meets the San Andreas Fault, which has produced major quakes in the past.