front fault farther to the west in southeast Idaho. Paleoseismic studies show that even the most active faults in this region produce such big earthquakes only every few thousand years. Based on the Holocene average, the current level of activity appears to be abnormally high in the central Nevada seismic belt and abnormally low in the intermountain seismic belt. One area of future concern is the Wasatch front traversing the populated areas of Salt Lake City and Provo, where large, prehistoric earthquakes have been documented by paleoseismic techniques along six segments of front-bounding fault, but no large earthquakes have occurred in the historic period (Figure 2.4).

Central United States

Seismic activity decreases markedly in the stable continental interior, east of the Rockies. However, seismic waves propagate more efficiently through the colder, thicker lithosphere that underlies this region than through the hotter crust and upper mantle of the western United States (48). Earthquakes of comparable magnitude can consequently cause damage over larger areas. For instance, a large earthquake in southeastern Missouri on December 16, 1811, generated strong shaking (Modified Mercalli Intensity V or greater) over an area at least five times bigger than in the 1906 San Francisco earthquake, which had a larger magnitude. This was the first in a violent sequence of earthquakes that occurred for the next several months along an abandoned set of Paleozoic extensional faults called the Reelfoot Rift (Figure 3.13). The large magnitudes of these earthquakes, combined with the relatively short return periods for events of similar size found from paleoseismic studies, imply that the central Mississippi Valley has the highest seismic hazard east of the Rocky Mountains, although the level of this hazard remains controversial (Box 3.1).

Other areas of potential seismic hazard in the central United States include eastern Kansas and Nebraska, which has been the site of two M 5 earthquakes in the past 150 years. Liquefaction features indicate that M 6.5 to 7.5 earthquakes with recurrence times of a few thousand years have occurred in the Wabash Valley of southern Indiana and Illinois (49). The Meers fault in southern Oklahoma has generated two large earthquakes (M 7) in the past 3000 years (50), and the Cheraw fault in southeast Colorado has produced M 7 earthquakes in the past 10,000 years (51).

Eastern United States

Most people think of the eastern United States as seismically benign. At present, this region lies near the center of the North American plate, far from active plate boundaries to the east and west. The histori-



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