One fault near Diablo Canyon links to another

November 30, 2015

An earthquake fault that neighbors Diablo Canyon nuclear power plant links to a second, larger fault, scientists have discovered. [SF Chronicle]

The Hosgri fault was discovered three miles offshore of Diablo Canyon during construction of the power plant in 1971. After the discovery, the plant’s design was retrofitted to withstand a magnitude 7.5 earthquake.

Seafloor mapping has now revealed the Hosgri connects to the San Gregorio fault, which is located to the north. When combined, the two faults total 250 miles in length, or more than double the length of the Hosgri.

Generally, the longer a fault is, the more potential there is for powerful shaking. Ground shaking from a quake linking multiple faults could be 20 percent larger than movement on the Hosgri alone.

It remains unclear whether there are breaks in the Hosgri-San Gregorio structure, which could limit a full rupture during an earthquake, said Sam Johnson, a research geologist with the U.S. Geological Survey. The San Andreas fault, for instance, is believed to have distinct north and south sections that would not shake simultaneously, Johnson said.

PG&E spokesman Blair Jones said experts have evaluated a possible rupture linking the Hosgri and other faults, including the San Gregorio and northern San Andreas faults. Scientists found the prospect of that occurring very unlikely, but the plant could still shut down properly, Jones said.

In 2008, the USGS discover another fault in the area, which was dubbed the Shoreline fault. The Shoreline is smaller than the Hosgri fault, but it was discovered less than one mile from the power plant.

Earlier this year, the Nuclear Regulatory Commission directed Diablo Canyon to conduct in-dept analysis of earthquake risks by June 2017. Diablo Canyon is among the nuclear power plants in the United States that is most vulnerable to ground shaking, the NRC found.