FINDING FAULT AT THE NORTH ANNA POWER STATION: HISTORY AND PERSPECTIVE

The 2011 Virginia earthquake produced ground accelerations at the North Anna Nuclear Power Station, located 21 km from the epicenter, that exceeded the station’s design value by a factor of two and brought renewed scrutiny to the site and the original geologic investigations.

At the North Anna site, four approximately circular holes were excavated for the reactor foundations (although only two reactors were constructed). Each hole was about 15-25 m deep, 40 m in diameter, and the total distance between the excavations was ~300 m. In 1970, three academic geologists visited the site and recognized a fault cutting through the excavated bedrock. The fault, an ENE-striking structure dips 40˚-50˚ N, and cuts through all four excavations. In the 1973 Final Safety Analysis Report there is no mention of faults at the site. A later supplemental report does note the occurrence of a “minor shear zone” that “while technically a fault… is not considered in any way to represent a fundamental geologic structure of the region”. The report concludes, and was supported by a number of independent geologists, that the fault was a minor normal fault which reactivated a Paleozoic reverse fault and that the last slip event was likely during the Mesozoic.

Based on existing reports, maps, and photos it is clear that the fault under the North Anna Power Station is a minor structural feature in the Piedmont but was unfavorably oriented to have been reactivated by the stress geometry responsible for the 2011 earthquake. The abundant gouge and kinematic features associated with the fault are consistent with brittle reverse faulting in the presence of abundant fluids. Faulting at the site is likely far younger than was supposed in the 1970s.