Scientists have long known of faults stretching along the western edge of the Santa Cruz Mountains—faults that are also gradually pushing up the mountains. But new findings unveiled at a December meeting of the American Geophysical Union in San Francisco showed faults could also extend to a section of the valley below known as the Cupertino basin.

"Seismic imaging shows there are likely to be faults extending out into the basin," said Rufus Catchings, a research geophysicist with the U.S. Geological Survey who led a study of the fault zones.

The Santa Clara Valley Water District also partnered in the study, using wells drilled by the Geological Survey to better understand the area's water supply. "It would have been a lot more expensive to drill on our own," said the water district's spokesman, Mike Di Marco. "Half our water comes from the underground basin, and we want to better understand not only the amount, but the quality of the water we have."

In addition to drilling wells, the study also included driving specialized trucks over streets in Cupertino and Sunnyvale, and testing for depth by measuring vibrations.

"The faults are shallower than we expected," Catchings said, with the shallowness indicating fairly recent seismological activity. Studies of earthquakes over the last 30 years, particularly the 1989 Loma Prieta quake, also point to the faults being active.

According to Catchings, the soil in the area is also fairly soft, as the Cupertino basin—a geological dip in the earth's crust—has slowly filled in with the softer sedimentary soil over thousands of years. This soft soil means two things. "Any seismic activity on the San Andreas or Hayward Faults are likely to be amplified greatly," Catchings said, as the basin traps and concentrates shock waves. As a result, seismic activity in the highly populated Cupertino basin is likely to be much more destructive than average.

Di Marco said the Stevens Creek Reservoir in Cupertino is an earthen dam built to withstand major earthquakes.

Even so, a major earthquake is still speculative. Catchings said scientists still have to dig more trenches and further examine the geological record of the Cupertino basin. "There is so much that is unknown," he said. "Generally, stress reaches a breaking point [leading to an earthquake], but we have no idea what it is capable of."