CONCENTRATED CRUSTAL DEFORMATION AND THE DISTRIBUTION OF INTRAPLATE EARTHQUAKES

The three most active intracratonic seismic zones in eastern North America (Charlevoix, Eastern Tennessee, New Madrid) are located along ancient basement structures, either accretionary fabrics of supercontinent assembly or extensional fabrics of continental breakup; however, seismicity is limited to only parts of the ancient basement structures rather than being distributed along the complete length. The locations of seismic zones coincide with sites of concentrated crustal deformation along the basement structures—impact shattering at Charlevoix, extension in a transpressional transform releasing bend at Eastern Tennessee, and extension in a bend in an intracratonic synrift fault system overprinted by contraction at New Madrid. Concentrated crustal deformation is necessary to localize a major siesmic zone, but is not sufficient in the absence of appropriately oriented fractures. Thus, some zones of concentrated crustal deformation currently may not be seismically active, but may have been in the past or may be in the future, as result of perturbations in the regional stress field. Areally smaller or less active zones of seismicity pose the question of possible relationship to more limited zones of concentrated crustal deformation. Examples include Charleston at an Atlantic rift-transform intersection, Central Virginia along Atlantic extensional faults overprinted on Appalachian contractional structures, Lower St. Lawrence Valley at an Iapetan rift-transform intersection, and Southwestern Quebec along an undocumented structure in the Grenville footwall. Better understanding of the role that concentrated crustal deformation plays in the occurrence of intraplate/intracratonic earthquakes offers better resolution of seismic hazard, as well as a more robust scientific concept of seismic zones. Independent recognition of presently undetected zones of concentrated crustal deformation is an important research goal.