LARGEST LIKELY EARTHQUAKE, CENTRAL AND EASTERN UNITED STATES

Most seismic-hazard assessments require an estimate of M(max), the moment magnitude (M) of the biggest earthquake thought possible in a specified region. M(max) affects the USGS national seismic-hazard maps, which impact building codes, and also hazard assessments for critical structures, such as nuclear power plants. In most of the central and eastern United States (CEUS), long intervals between large historical earthquakes preclude observation of M(max). One way to estimate CEUS M(max) is to include global geologic analogs of the CEUS to increase the likelihood that their combined historical earthquake records capture M(max). This strategy is reasonable because geologic controls on the size of a CEUS earthquake rupture, and therefore on CEUS M(max), remain enigmatic. Thus, no evidence argues against the assumption that probably CEUS M(max) is regionally constant and the same as in the analogs. The global analog dataset indicates that the CEUS can be divided into two large regions with different M(max): a central craton, and an extended margin of Phanerozoic rifts, passive margins, and orogens that rim the central craton on the east and south. Two histograms of M of the largest earthquakes in the CEUS and its analogs contain 17 values from craton analogs and 30 values from the extended margin and its analogs. Both histograms have tall peaks at M 6.6-6.7, whose values come from CEUS geologic analogs in diverse continents and plate settings. The peaks suggest that M(max) is unlikely to be smaller than M 6.6-6.7 anywhere in the CEUS. The high-M tails of the histograms indicate that probably M(max) is larger — outliers are 7.5 in cratons and 7.8 in extended margins. Overall, M(max) for the craton is taken as M 6.6-7.2, with a preferred value of M 7.0, and for the extended margin as M 7.1-7.7, with a preferred value of M 7.5. These values are used in the June, 2007 draft of the updated USGS national seismic-hazard maps.