Two years before an immense coastal earthquake plunged Japan into a nuclear crisis, a geologic fault was discovered about a half—mile (800 meters) from a California seaside reactor - alarming regulators who say not enough has been done to gauge the threat to America’s most populous state.

The situation of the Diablo Canyon plant is not unique. Across the U.S., a spider’s web of faults in the Earth’s crust raises questions about earthquakes and safety at aging nuclear plants, amplified by horrific images from Japan, where nuclear reactors were crippled by a tsunami caused by a 9—magnitude quake.

The Indian Point Energy Center, for example, lies near a fault line 35 miles (56 kilometers) north of Manhattan; on Wednesday, New York Gov. Andrew Cuomo ordered a safety review at the plant.

But none of the questions are more pressing than in quake—prone California, where about 10 powerful shakers = stronger than magnitude 7 - have hit since 1900.

At issue at Diablo Canyon is not what is known, but what is not.

Preliminary research at the site, which sits on a wave—washed bluff above the Pacific, found its twin reactors could withstand a potential earthquake generated by the recently identified Shoreline Fault, just off the coast.

But that hasn’t satisfied California regulators. Since late 2008, when the undersea crack was identified, they have pressed plant owner Pacific Gas & Electric Co. to conduct sophisticated, independently reviewed studies that they say are needed to fully assess the danger at a site within 200 miles of Los Angeles.

The recently discovered fault is close to, and might intersect with, another bigger crack 3 miles (4.8 kilometers) offshore, and the fear is the two faults could begin shaking in tandem, creating a larger quake than either fault would be capable of producing on its own.

“We don’t yet have a firm idea of the hazard posed by the Shoreline Fault,” says Thomas Brocher, director of the Earthquake Science Center at the U.S. Geological Survey in California, who led the team that discovered the fault.

State Sen. Sam Blakeslee, a Republican who holds a doctorate in earthquake studies, wants PG&E to pull back an application to extend the plant’s operating license for 20 years until more is known.

“Aging nuclear power plants and large, active fault systems should not be in close proximity. This isn’t exactly rocket science,” Blakeslee says. Because the Shoreline Fault is so close to the Diablo Canyon plant it “can produce shaking far in excess of what’s expected.”

The U.S. Nuclear Regulatory Commission and PG&E say the plant is safe and built to withstand a magnitude 7.5 earthquake, the maximum considered possible for the site. Damage from a Japan—like tsunami is unlikely, because the reactors sit on an 85—foot (26—meter) cliff above the ocean and fault structure in the area differs from the Pacific Rim.

Critics around the United States say the government has moved too slowly to assess possible threats from earthquakes.

NRC spokeswoman Lara Uselding said she did not know of a single case in which a U.S. reactor was damaged by a quake. But this does not dispel concerns that may be unavoidable because the study of earthquakes remains an imprecise science. They cannot be predicted, and the damage - as witnessed in Japan - can be catastrophic.

The dangers of earthquakes have been raised repeatedly by opponents of nuclear energy. The Perry nuclear plant, east of Cleveland, lies within 40 miles of two faults; in 1986, a year before the plant opened, a 5.0 earthquake shook the area, but didn’t damage the plant, said Todd Schneider, a FirstEnergy spokesman. There have since been less severe quakes.

A citizens group filed suit after the quake, trying to block the plant from opening. They argued that an earthquake greater than the plant was built to withstand was likely to occur in the future; U.S. Supreme Court Justice Antonin Scalia turned down their request.

The plant’s design includes piping with shock absorbers intended to prevent breakage in a quake. “Before the plants are even built, there’s research done by seismologists and geologists to determine what the maximum earthquake could be,” Schneider said. “The plants are designed beyond that.”

Indian Point, too, is safe and built to withstand earthquakes, says a spokesman for owner Entergy Nuclear. But earlier this week, Democratic Rep. Nita Lowey urged the NRC to look closely at the earthquake preparedness there. A 2008 analysis of earthquake activity around New York City found that many small faults that were believed to be inactive could contribute to a major temblor, and that a line of seismic activity comes within two miles (three kilometers) of the plant on the Hudson River.

Another fault line near Indian Point was already known, so the findings suggest Indian Point is at an intersection of faults. The environmental group Riverkeeper says seismic studies used to assess safety are decades out of date.

Major earthquakes are rare in the southeast United States, although the region is crossed by the New Madrid fault in the west and a fault near Charleston, South Carolina in the east. University of Georgia geologist Jim Wright said although the plate sitting under the southeast is stable it’s also rigid, meaning the jolt from an earthquake would carry farther than in a region where the earth’s crust has been fragmented by seismic activity.

The Atlanta—based Southern Electric Co. has reviewed seismic activity in the area that could impact the Georgia site where it has two operating reactors and hopes to build two more. Among the largest known regional earthquakes was an 1886 earthquake that struck Charleston, South Carolina, about 85 miles (137 kilometers) from the Plant Vogtle site, according to the company’s regulatory filings.

To this day, geologists are divided on exactly which faults caused the earthquake. Southern Co. spokesman Beth Thomas said the company’s reactors comply with federal requirements that they be able to safely withstand the strongest earthquake that could be expected in a 10,000—year period. Thomas said the company has not seen anything in Japan to make it alter its current operations.

The Tennessee Valley Authority’s Browns Ferry plant, in Alabama, has boiling water reactors similar in design to the malfunctioning reactors in Japan. That plant was designed to withstand a 6.0—magnitude earthquake based on its proximity to the New Madrid fault, TVA spokesman Duncan Mansfield said.

The TVA’s Watts Bar nuclear plant and its Sequoyah plant in Tennessee are designed to withstand a 5.8—magnitude quake based on an 1897 tremor in Virginia, Mansfield said. None of the TVA’s reactors are seen as being vulnerable to tsunamis since they are so far inland.

Arkansas’ only nuclear plant is located about 150 miles (241 kilometers) away from the New Madrid fault zone, which produced a series of large quakes in 1811 and 1812, including several over magnitude 7. The shaking was so strong that it reportedly caused the Mississippi River to flow backward and could be felt as far away as New England. Arkansas Nuclear One officials said the plant is designed to withstand natural disasters including quakes, has an emergency plan in place, and routinely trains for the worst—case scenario.

Using increasing sensitive technology, scientists are constantly identifying new faults in the country, sometimes after earthquakes are detected. In Southern California alone, there are an estimated 10,000 earthquakes a year, though most of them are too small to be noticed by residents.

The state’s senators, Barbara Boxer and Dianne Feinstein, on Wednesday sent a letter to Nuclear Regulatory Commission Chairman Gregory Jaczko, asking that the agency “perform a thorough inspection” of the plants at Diablo Canyon and San Onofre.

A 30—foot (9—meter) concrete seawall surrounds San Onofre, built along the beach in northern San Diego County, where officials say it’s strong enough to withstand major quakes and any potential tsunami.

Diablo Canyon, whose reactors began operating in the mid—1980s, has a long history of seismic issues. The discovery of the offshore Hosgri Fault in 1971, after the plant’s construction permits were issued, forced a major, costly redesign.

Brocher, the USGS scientist, said scientists do not know how fast the adjacent sides of the Shoreline Fault are sliding, a key measurement to determine potential danger. A higher rate of slippage leads to increased pressure - and a greater chance for an earthquake.

With the two faults in proximity “the uncertainty is ... to what extent they might interact,” says Barbara Byron, a senior nuclear policy adviser for the California Energy Commission. Since 2008, the commission has urged the plant to conduct three—dimensional mapping of the Shoreline Fault, using technology employed in oil exploration.

Funding has been approved for the study. In testimony to the NRC last year, she called the plant’s seismic data “incomplete ... outdated” and urged a review of its evacuation plans.

Uselding, the NRC spokeswoman, said preliminary reviews found that it’s unlikely an earthquake would take place directly under Diablo Canyon, but that potential shaking could cause minor damage to buried piping and conduits.

Diablo Canyon has an extensive seismic monitoring system, ready to detect any shifts in the area. “Potential impacts of the Shoreline Fault fall within all safety margins,” company spokesman Kory Raftery said.

To University of Southern California professor Naj Meshkati, an expert on earthquakes and nuclear power plants, the risk is not the massive plant structures but the reliability of backup systems that failed in the Japanese tsunami.

While such a large quake and killer wave is unlikely in California, the plants face similar dangers in backup equipment.

“If someone says this cannot happen here, they should really ... take a very hard look at some of their assumptions,” Meshkati said.

Keywords: Diablo Canyon