The Diablo Canyon nuclear power plant and earthquake faults have been uneasy neighbors for decades. Even before the twin reactors produced a single watt of electricity, the plant had to be retrofitted after a submerged fault was discovered 3 miles offshore during construction.

That cleft in the earth, known as the Hosgri fault, has long been considered the greatest seismic threat to a plant that stands within a virtual web of faults. But new questions are being raised by sophisticated seafloor mapping that has found that the Hosgri links to a second, larger crack farther north, the San Gregorio fault.

In general, the longer the fault, the stronger its potential shaking power. Together, the two faults create a strand about 250 miles long, more than double the length of the Hosgri alone.

Earlier this year, the Nuclear Regulatory Commission directed Diablo Canyon to conduct additional, in-depth analysis on earthquake risks by June 2017, part of a broad review of seismic threats following Japan's Fukushima Dai-ichi disaster. The agency found that among commercial U.S. nuclear plants, the California plant is among those that face the highest hazard when potential strong ground shaking is evaluated against the plant design.