Abstract

Historic earthquakes in Illinois occur on faults that have been intermittently active since Precambrian time. Vertical deflection of the lithosphere continues, but at an abbreviated pace. Evidence for this statement follows: (1) blocks of undeformed and coherent Precambrian lithosphere are outlined by gravity closures; (2) Paleozoic strata are folded and, in most cases, unfaulted over Precambrian blocks that have undergone vertical, differential displacements; (3) gravity closures, which contain negative residuals, overlie crustal blocks that have moved upward relative to blocks that are overlain by gravity closures, which contain positive residuals; and (4) historic earthquakes are not located within closures but are in high gravity gradient areas on the edges of closures or in the lower gradient areas lying between closures.

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