How San Onofre Nuclear Plant Operators Respond to Earthquakes

ROSEMEAD, Calif., July 6, 2019 — Last night at 8:19 p.m., the U.S. Geological Survey recorded a 7.1 earthquake in the vicinity of Ridgecrest, Calif., located in Kern County (the epicenter was about 165 miles from the plant). The shaking could be felt throughout Southern California, including some minor ground movement at San Onofre nuclear plant. The quake was larger than the 6.4 temblor recorded July 4 in the same vicinity, which was also felt at the plant. Inspections after both events found no issues with any plant equipment or systems.

Following any measurable seismic activity on site, highly trained San Onofre plant operators initiate procedures that include verifying the peak ground acceleration (PGA or “g”) recorded by seismic stations near the plant. Nuclear plant seismic ratings and responses are tied to PGA rather than magnitude. As approved by the Nuclear Regulatory Commission, Units 2 and 3 at San Onofre (including the spent fuel pools) were built to withstand a peak ground acceleration of 0.67g (g refers to the force of gravity). The dry spent fuel storage systems were constructed to withstand 1.5g, more than double the nuclear plant design. California building codes require buildings in the local vicinity to be designed to withstand a PGA of 0.38g.

The table below compares the plant's seismic ratings with the two most recent earthquakes:

Seismic Event Seismic Rating -
Units 2,3
Seismic Rating -
Dry Storage Systems
Seismic Activity Recorded
Near San Onofre Plant
July 4 .67g 1.5g .006g
July 5 .67g 1.5g .015g


For further comparison to other large earthquakes in Southern California and recordings at San Onofre, on June 28, 1992, the Landers Earthquake (magnitude 7.3 and 90 miles away) produced a PGA of 0.038g. On Jan. 17, 1994, the Northridge Earthquake (magnitude 6.7 and 77 miles away) produced a PGA of 0.025g.

Recent Experience

Immediately after a seismic event that meets the criteria, plant personnel make notifications to state and federal agencies, and then conduct visual inspections of various areas throughout the plant, including the spent fuel pools. For the dry storage systems, the Holtec UMAX and AREVA systems are designed with very tight tolerances that prevent the canisters from moving more than about a quarter-inch laterally inside their cavity enclosure containers.

Inspections following both recent earthquakes found no issues and normal operations continued.

Media Contact: John Dobken, (626) 302-2255

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