The relationship between seismicity and faults observed on seismic reflection profiles from the New Madrid Seismic Zone (NMSZ) in the central Mississippi Valley has been intensively studied for the past 15 years. However, comparable studies relating reflector sequences and earthquakes in the southern Illinois Basin, located northeast of the NMSZ, have not been undertaken. This study investigates the possible relationship between the source parameters of the November 9, 1968, magnitude (mbLg) 5.5 earthquake (a NNE-trending, previously interpreted west-dipping reverse fault at 21.2 ± 5.4 km depth) in southern Illinois, and a zone of moderately dipping reflectors in crystalline (?) basement observed on a nearby high-quality seismic reflection profile. The 1968 event was the twentieth century's largest magnitude earthquake in the southern Illinois region. The zone of dipping basement reflectors is part of a broad prominent sequence, in which reflectors are subhorizontal or inclined with a strong west-dipping component, that appears beneath the Wabash Valley Fault System and extends to the west beneath the Illinois Basin where it steepens and plunges deeper into the crust over the 1968 hypocenter. More than one interpretation of the dipping reflector zone is admissible, including intrusion of igneous sills or thrust faults or both within a localized shear zone. The dipping reflector zone cannot be traced from the basement into the overlying Phanerozoic sedimentary section or associated directly with any particular previously mapped fault. If a tectonic interpretation is correct, the correlation between the 1968 reverse fault event and the reflector zone may mean that such quakes are nucleating along a blind compressional structure in the crystalline basement of southern Illinois, possibly analogous to the recent destructive southern California earthquakes.