USING INDUSTRY SEISMIC REFLECTION DATA BASES IN ORDER TO CHARACTERIZE A POTENTIAL SEISMIC SOURCE ZONE

Petroleum companies have vast collections of seismic reflection data in their archives, most of which have never been available to academic researchers. This is especially true for mature basin areas like the Illinois basin. For the past several years we have been procuring such data for the Illinois basin in order to study issues related to seismic hazard assessment. Large historic earthquakes and paleo-liquefaction sites imply the presence of active or potentially active seismic zones in the southern part of the basin, north of the New Madrid seismic zone. Although liquefaction features are useful to reconstruct seismic history, they shed little light on what causes these earthquakes. Analysis of earthquake records and of previously published geophysical surveys likewise has not revealed the detailed structure of these seismic sources. We used seismic reflection profiles acquired from petroleum company data bases to analyze the geologic structure of Precambrian basement and Paleozoic sedimentary cover in southern Illinois. Reprocessing or reanalysis of these data reveals that several large folds, notably the Du Quoin Monocline and the Louden Anticline, are cored by high-angle reverse faults in Precambrian crystalline basement. These structures thus are fault-propagation folds, analogous to many Laramide foreland structures of the Colorado Plateau and the Central to Southern Rocky Mountains. Principal displacements took place during the Morrowan and Atokan (Early and Middle Pennsylvanian), with lesser, intermittent movements continuing through Late Pennsylvanian and possibly into Permian time. In some cases, Pennsylvanian reverse faults later were reactivated as normal faults. Some deep-seated Pennsylvanian-age faults in Illinois may still be active, as implied by coincident earthquake epicenters. Focal mechanisms of these quakes suggest that strike-slip is occurring along north-trending faults. Considering the surface lengths of the relevant faults, the maximum moment magnitude for earthquakes nucleating in basement rocks in this area is between 6 and 7.