EarthquakesCalaveras-Hayward fault link means potentially much more powerful quakes
Seismologists have proven that the Hayward Fault is essentially a branch of the Calaveras Fault that runs east of San Jose, which means that both could rupture together, resulting in a significantly more destructive earthquake 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,” said one researcher. “People have been looking for evidence of this for a long time, but only now do we have the data to prove it.”
UC Berkeley seismologists have proven that the Hayward Fault is essentially a branch of the Calaveras Fault that runs east of San Jose, which means that both could rupture together, resulting in a significantly more destructive earthquake 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,” said lead researcher Estelle Chaussard, a postdoctoral fellow in the Berkeley Seismological Laboratory. “People have been looking for evidence of this for a long time, but only now do we have the data to prove it.”
The 70-kilometer-long Hayward Fault is already known as one of the most dangerous in the country because it runs through large population areas from its northern limit on San Pablo Bay at Richmond to its southern end south of Fremont.
A UC-Berkeley release reports that in an update of seismic hazards last month, the U.S. Geological Survey estimated a 14.3 percent likelihood of a magnitude 6.7 or greater earthquake on the Hayward Fault in the next thirty years, and a 7.4 percent chance on the Calaveras Fault.
These 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.
“A rupture from Richmond to Gilroy would produce about a 7.3 magnitude quake, but it would be even greater if the rupture extended south to Hollister, where the Calaveras Fault meets the San Andreas Fault,” Chaussard said.
Chaussard and her colleagues, including Roland Bürgmann, a UC Berkeley professor of earth and planetary science, report their findings in a paper accepted today (April 2) by the journal Geophysical Research Letters.
Creep connects two faults
Chaussard said there has always been ambiguity about whether the two faults are connected. The Hayward Fault ends just short of the Calaveras Fault, which runs about 123 kilometers from north of Danville south to Hollister in the Salinas Valley.
