The June 5, 1993 west-central Minnesota earthquake (mbLg = 4.1) provides an opportunity to re-investigate Minnesota seismicity and its possible relationship to reactivated basement structures. This earthquake occurred within 25 km of the much larger July 9, 1975 west-central Minnesota earthquake (mbLg = 4.6), and could have a similar geologic cause. Modified Mercalli intensity data from the 1993 earthquake indicate a felt area of about 69,500 km2 and a maximum intensity of V in the epicentral area. Detailed intensity data in the epicentral area generally accord with the position of the instrumentally determined epicenter, although a node of enhanced shaking inferred within the main intensity V area implies an epicenter some 5–10 km north or northwest. The correlations of some local perturbations in both the 1993 and 1975 isoseisms with features in the Precambrian bedrock imply that seismic energy may be locally focused by crustal structure. Intensity-depth relationships indicate that the 1993 focal depth is about the same as the 1975 earthquake, which has been independently estimated to be 7.5 km.
The Morris fault, a northeast-striking crustal discontinuity separating two Archean terranes, has been generally thought to be the major seismogenic feature in the region, but neither the 1993 hypocenter inferred by this study nor the 1975 hypocenter agrees with the subsurface trace of the fault as determined by seismic reflection data and magnetic modeling. Alternative sources for the 1975 and 1993 earthquakes are west-northwest-striking, subvertical structures that are interpreted to consist chiefly of fractures that are occupied by mafic dikes. Some of these fractures are also associated with minor faulting, as shown by small offsets in the Morris fault, and the possible disruption of Cretaceous strata. Neither the Morris fault nor the west-northwest structures agrees with a loosely constrained focal mechanism for the 1975 earthquake, but the latter structures are favorably oriented to be reactivated by the east-northeast regional compression estimated for the eastern United States.
Minor basement structures, such as mafic dikes and fractures, may influence the seismicity of continental interiors more than is generally believed, because in many parts of the north-central United States, such structures are not resolvable with existing geological and geophysical data sets.