A seismic fault in the Sierra Nevada, believed to have been quiet for more than 3 million years, is active after all and capable of triggering strong quakes with magnitudes of 6 or even 7, scientists say.

The Kern Canyon Fault, stretching for nearly 90 miles from north to south above the San Joaquin Valley east of Bakersfield, cuts beneath a major flood control dam on the Kern River.

For a half-dozen years those who oversee the 57-year-old Isabella flood control dam above Bakersfield, as well as California Institute of Technology geologists, have been studying the fault closely.

"It came as a surprise to see that a long-inactive fault can produce significant quakes," said geologist Elisabeth Nadin of Caltech, who has hiked the sparsely populated rugged terrain and mapped where evidence showed the fault ruptured violently at least 3,300 years ago.

Geologists working for the Army Corps of Engineers have also studied the fault's potential for rupturing and are surveying the dam to determine whether it needs strengthening against future large quakes.

The fault emerged some 86 million years ago when the immense granite mass of the Sierra was uplifting, said Nadin, who has found the evidence of past violence in the rocks around it.