Qualitative correlation of seismic flux and free-air gravity with crustal structure of the midcontinent of the United States
Kohsmann, James J.
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Qualitative interpretation of recently completed maps of seismic flux and the free-air gravity field of the midcontinent suggests a correlation between seismicity and heterogeneous crustal structure. The maps compiled for this study suggest that the regional, horizontal compressive stresses associated with movement of the North American plate are strongly modified in the presence of zones of weakness or discontinuities and mass concentrations. The available data suggest that predominantly vertical stresses acting within the lower crust or upper mantle are transmitted by creep to the upper crust, where the stress concentrates at weaknesses or discontinuities, and is subsequently relieved by unstable creep or brittle failure. The model of seismicity consists of a system of blocks within the upper crust, capable of independent movement, undergoing local, isostatic adjustment in response to regional, isostatic or isobaric adjustment.