Three earthquakes of magnitude 7.5 or more have been recorded near these facilities in the past 200 years, according to the Southern California Earthquake Data Center at Caltech.
Nevertheless, Marvin Fertel, president and CEO of the Nuclear Energy Institute declared on the television program Meet the Press on Sunday that these and all other US plants are "designed to withstand the maximum credible earthquake."
He added, "We've done things post-9/11 to make sure ... if you lost all power you could get water to the core and continue to cool it."
What appears to have been the critical factor leading to the Japanese reactors' problems was the tsunami.
The onrush of seawater knocked out the onsite generators that provided the power to water pumps for cooling the immensely hot reactors. Engineers have since been struggling to bring in and install replacement generators to resupply the reactors with cooling water.
Although both Diablo Canyon and San Onofre are on the Pacific coast, where they might be vulnerable to tsunamis, they are sited on headlands about 120 and 80 feet above sea level, respectively.
The tsunami that washed ashore in Japan last week was estimated at 20 to 30 feet.
On the other hand, some East Coast nuclear plants are virtually at sea level, such as Indian Point on the Hudson River north of New York City and Seabrook Station in New Hampshire.
Although tsunamis are much rarer in the Atlantic Ocean because of the different geology relative to the Pacific Rim, they are not unheard of. Lisbon and other towns along the Portuguese coast suffered major damage from a tsunami that followed an estimated magnitude-9.0 quake in 1755.
Also, according to the Maine Geological Survey, a tsunami struck eastern Canada in 1929, killing 25 to 50 people, with wave heights approaching 90 feet at the heads of some long, narrow bays that acted as funnels.
Medical Disaster Plans in Place
The NRC requires that nuclear facilities develop plans for dealing with radiation leaks into the surrounding environment, but gives them considerable latitude in determining the appropriate response to a given scenario.
Robert Rathie, an attorney involved with the independent committee monitoring safety at the Diablo Canyon Nuclear Power Plant, noted that every plant faces different risks based on the local geography.
Flexibility in the emergency plans makes sense, Rathie told MedPage Today in an interview.
Emergency practice sessions, overseen by the federal Nuclear Regulatory Commission, occur at least twice a year at the Diablo Canyon plant, he noted.
But state and local emergency services take on more of the safety planning for the community, with oversight by FEMA.
Key information, such as evacuation routes, for communities near a nuclear power plant can typically be found in the phone book, Rathie noted.