From our friends at UC Berkeley: Seismologists have proven that the Calaveras and Hayward faults are linked,  meaning that a rupture on one fault could trigger a rupture on the other. This could produce considerably larger quakes than once thought. Researchers have mapped the land at the southern end of the Hayward Fault and found that the creep continued 15 km beyond to merge with the Calaveras Fault, which was thought to be independent.

Here’s the original article from the UC Berkeley News Center. Thanks to Bill Godwin for bringing this to our attention!