If you click on the tracker’s new Earthquake Risk filter, you may be surprised by what you see: 27 reactors clustered primarily in the eastern and southern coastal states, with a handful in the Midwest. “Where,” you might reasonably ask, “are the West Coast reactors?”

The reason for their absence has to do with what we mean here by “earthquake risk”: not the absolute risk of earthquakes occurring, but the risk of earthquakes that the reactor was not designed to withstand—such as the 5.8-magnitude August 2011 quake with its epicenter ten miles from the North Anna plant in Virginia. The West Coast reactors are not on the list because the risk of earthquake damage was appropriately incorporated into their design.

The NRC has known since 1996 that some reactors were at greater earthquake risk than they were designed for, and in 2005 identified the 27 reactors most at risk. But thus far the NRC has not required these known protection shortcomings to be resolved.