THE U.S. NATIONAL SEISMIC HAZARD MAPS FOR THE CENTRAL AND EASTERN U.S.: SCIENCE AND POLICY ISSUES

The USGS updates the U.S. National Seismic Hazard Maps (NSHMs) about every 6 years to provide input into modern building codes. These maps are based on the “best available science” obtained from discussions at a series of regional and topical workshops. The 2008 NSHMs have undergone an intensive review process and have been accepted as input into 2009 and 2010 building codes. These codes provide the provisions for structural design of buildings and other structures. For the central and eastern U.S. (CEUS), the national maps incorporate seismicity data to characterize random events M 5-7 and paleoseismic data to characterize larger events on the New Madrid Seismic zone (NMSZ), Charleston SC zone, Cheraw fault, and Meers fault. There have been many recent debates about the potential magnitudes, recurrence, and ground motion models applied in the NSHMs. Therefore, the complexity of the model has increased through time as alternative models are considered in the calculations. The 2008 version of the NMSZ considered 5 alternative fault locations, 3 recurrences, 4 magnitudes, and 7 ground-motion models. Similar complex logic trees form the framework of the Charleston and random seismicity models. These logic trees and additional information on the weights are available in the 2008 NSHM documentation. The seismic hazard maps are translated into seismic-design maps based on guidelines provided by the building-code communities. Design maps prior to the 2009 NEHRP maps include deterministic caps while the latest version applies a risk-targeted ground motion that is based on a 1% probability of collapse of a structure in a 50-year period. The design ground motions are similar to ground motions expected from NMSZ earthquakes. The next version of the NSHMs is planned for 2013, and workshops will again be held across the country. In the next update process we will review new data on earthquake magnitudes and earthquake recurrence. In addition, the new Next Generation Attenuation (NGA-East) project will develop ground motion models based on shaking data and computer simulations. The nuclear industry has recently sponsored a new source model for the CEUS that includes new catalogs, methodologies, and geologic compilations. We encourage anyone interested in these models to contribute to the discussions on the map update.