California better brace for a whopper.

• Position: Earthquakes

Seismologists have proven that the Hayward Fault essentially is 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," says researcher Estelle Chaussard, a postdoctoral fellow in the University of California's 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 team used 19 years of satellite data to map ground deformation using interferometric synthetic aperture radar and measure creep along the southern end of the Hayward Fault, and found that the creep did not stop south of Fremont, the presumed southern end of the fault, but continued as far as the Calaveras Fault.

The 70-kilometer-long Hayward Fault already is known as one of the most dangerous in the country because it runs through large population...