Back in the 1970's oil prospectors discovered a large fault called the Hosgri,  which is three miles from PG&E's Diablo Canyon nuclear plant on the Central Coast.

Now there is concern about a recently discovered fault called the shoreline. It runs a scant seven hundred yards from the reactors at Diablo Canyon.

JEANNE HARDABECK: "The shoreline fault kind of runs off the Hosgri fault like an exit ramp on a freeway.

Jeanne Hardabeck is the scientist with the U.S Geological Survey who discovered the fault.  She says that the Shoreline itself could produce up to a 6.5 quake. But Hardabeck explained that when you have two fault lines near each other, it's possible they could rupture together.

JEANNE HARDABECK: "If  an earthquake were to continue on to the Hosgri fault for some distance then that could increase the magnitude of that earthquake."

PG&E says it is studying the issue and that even with the uncertainty caused by the shoreline fault, the plant remains safe.