Seismic-hazard assessment in the central United States

Problems with and approaches to seismic-hazard estimation in the midcontinent of the United States are evaluated by using recent data on stress regime, crustal age and structure, and seismicity of other stable continental regions. Evaluating earthquake hazard in the central U.S. is difficult because of the lack of identifiable seismogenic faults and because of the low rate of seismic activity. Furthermore, the recurrence intervals of large earthquakes are poorly known, in part because of the short historical record that spans only a fraction of the repeat times of these quakes. The seismotectonic regime of the central U.S. is dominated by the Reelfoot rift complex and the associated New Madrid, Missouri, seismic zone. However, there are other major tectonic structures in the region such as the Nemaha ridge, the Midcontinent rift system, and the Wichita-Ouachita orogenic belt; earthquakes generating damaging ground motion (approximately magnitude 5.0 or greater) have occurred in the states of Ohio, Illinois, Oklahoma, Texas, Kansas, Nebraska, Kentucky, Alabama, and Arkansas, as well as Missouri. Opinions vary widely about the best way to delineate seismic source zones in such a diffuse and varied seismotectonic environment. Moreover, detailed paleoseismic or neotectonic data that could improve hazard assessments are extremely sparse in the central United States. The Meers fault scarp in southwestern Oklahoma, with its evidence for Holocene displacement and its lack of background seismicity, highlights a new set of assessment problems. Development of site-specific probabilistic hazard curves are further hampered by the lack of strong ground-motion data and high-resolution attenuation data. We address aspects of the overall seismic-hazard assessment problem for which neotectonic information provides constraints. These include a seismic source zonation for the central U.S. and estimates of maximum possible earthquakes for these zones, especially for the New Madrid region.