Lines in this cover show lineaments or boundaries of largelineaments interpreted from gravity and magnetic data. Theselineaments are thought to reflect structures in the crust that may berelated to earthquake generation or control of earthquake location.The broad Paducah gravity lineament trends southeast across the SW cornerof the map area. Hildenbrand and others (1996) have interpreted that thelineament represents numerous upper-crustal mafic intrustions.Within and west of the map area, small earthquakes are aligned withthe NE edge of the lineament and their abundance decreases abruptlyNE across the edge.The Commerce geophysical lineament trends NE across the map area.Langenheim and Hildenbrand (1997) attribute the lineament to a mafic dikeswarm of unknown age. Topographic lineaments, Quaternary offsetson mapped faults, igneous intrusions, and a few earthquakes alignalong the lineament.The south-central magnetic lineament trends NW across the center ofthe map area and extends far beyond the map area in both directions.Hildenbrand and others (1996) interpret the lineament as a 40 km wide zoneof high susceptibility intrusions of intermediate composition andprobable Pc age. Heigold and Kolata (1993)suggest that the lineament mayrepresent Precambrian Proterozoic terrane boundary.Seismicity south of thelineament (Reelfoot Rift) is greater than seismicity north ofthe boundary (Wabash Valley), but this boundary is located furtherto the south and trends more northerly than the south- centralmagnetic lineament. Present evidence implies that this lineament isthe least likely to be causally related to modern seismicity.