Seismicity of the central United States

The combination of moderate seismic activity, sparsity of seismograph stations and relatively low density of population makes it difficult to assign quantitative seismicity values to most of the central United States. In the New Madrid seismic zone, where the level of seismic activity is higher and the number of seismograph stations is more adequate, one can delineate the active fault zone and determine a magnitude-recurrence relation. These capabilities will be extended to other seismic zones as arrays of seismographs are installed for recording microearthquakes, which will give information on fault delineation, focal mechanism, and magnitude frequency. Only after such information is available will we be able to make positive statements relating seismic activity to specific geologic features. The seismicity data suggest that earthquakes that occur outside the recognized seismic zones or major structural features will have a maximum body-wave magnitude m (sub b) of 5.5 and that this maximum value will occur only infrequently. Experience shows that if these relatively minor earthquakes are only a few kilometres deep they may have an epicentral intensity at least as large as VII (observed for an earthquake of m (sub b) = 3.8), but their magnitude and area of perceptibility will be small. With the exception of the New Madrid seismic zone and possibly the Wabash Valley seismic zone, a conservatively reasonable value for the maximum body-wave magnitude to be expected in the major seismic and structural zones of the central United States is 6.5. For the New Madrid seismic zone an earthquake of body-wave magnitude equivalent to that of a great earthquake (m (sub b) = 7.5) can be expected, on the basis of what has already been experienced in 1811-1812. Because of low anelastic attenuation, the earthquakes in the central United States are felt and cause damage over much wider areas than earthquakes of comparable magnitude in the western United States. Further consequences are that the ground shaking has a longer duration and that the ground-motion spectrum shifts at the larger distances to lower frequencies, which results in relatively low ground acceleration for relatively large ground displacements and a greater effect on high-rise than low-rise structures.