State-of-the-Art for Assessing Earthquake Hazards in the United States. Report 21. Seismic Source Zones of the Eastern United States and Seismic Zoning of the Atlantic Seaboard and Appalachian Regions

摘要

Seismic and geologic data indicate that the eastern United States has particular seismic source zones which can be delineated in terms of maximum expected epicentral intensity. Historic and recorded seismicities demonstrate that significant earthquakes arise only from specific source zones which are associated with major fracture zones that trend northwest, an important exception being a northeast-trending zone extending from the central Mississippi Valley to the Gulf of St. Lawrence. Many fracture zones are related to those cutting the Gulf and North Atlantic ocean basins, and their present development is related to the formation of these basins. Sags or embayments developed over their fracture zones during the Late Cretaceous where they intersected the Cretaceous coast. These embayments appear to have been relatively subsideing, and the major earthquakes occur in their inland portion. Rising uplands adjacent to the Atlantic coast also produce earthquakes as well as some inland extensional fault zones that extend northerly from the northwest-trending fracture zones. Earthquakes do not appear related to movements along large faults as on the West Coast, but rather to adjustments on short segments of faults at structural intersections. Earthquake zonation is accomplished by estimating the maximum credible earthquake for each source zone. Most zones probably have not experienced this during the historic record, but some have, such as Charleston, SC. (Modified Mercalli, MM, Intensity X), and New Madrid, MO. (MM Intensity XII). Zonation in terms of Intensity is less subjective than values of acceleration.