Peter Lobner

The U.S. Geologic Society (USGS) National Seismic Hazard Model for the conterminous United States was updated in 2014 to account for new methods, input models, and data necessary for assessing the seismic ground shaking hazard from natural (tectonic) earthquakes. The National Seismic Hazard Maps are derived from seismic hazard curves calculated on a grid of sites across the U.S. that describe the annual frequency of exceeding a set of ground motions. Data and maps from the 2014 U.S. Geological Survey National Seismic Hazard Mapping Project are available for download at the following link:

http://earthquake.usgs.gov/hazards/products/conterminous/

The 2014 maps show higher seismicity in the Eastern U.S. than predicted in previous models. This reflects the significance of the 23 August 2011 magnitude 5.8 earthquake that occurred in Mineral, VA, about 11 miles from the North Anna nuclear power plant. That earthquake was felt as far north as Rhode Island, New York City and Martha’s Vineyard, Mass. The North Anna plant responded well and safely shutdown following the earthquake, which exceeded the plant’s seismic design basis.

The seismic hazard from “potentially induced” earthquakes (I.e., earthquakes that can be associated with man-made activities) was intentionally not considered because there was not a consensus on how to properly treat these earthquakes in a seismic hazard analysis.

The USGS issued a new report on 23 April 2015 examining the sensitivity of the seismic hazard from induced seismicity to five parts of the hazard model: (1) the earthquake catalog, (2) earthquake rates, (3) earthquake locations, (4) earthquake Mmax (maximum magnitude), and (5) earthquake ground motions. In the report, the USGS describes alternative input models for each of the five parts that represent differences in scientific opinions on induced seismicity characteristics.

You can download this interim report for free at the following link:

http://pubs.usgs.gov/of/2015/1070/

The USGS plans to issue a final model after further consideration of the reliability and scientific acceptability of each alternative input model. This matter could have important implications for industries, such as hydraulic fracking and geologic carbon dioxide sequestration, that may contribute to induced seismicity.