FAILED RIFTS AND INTRACONTINENTAL EARTHQUAKES: HOW ARE THEY RELATED?

Globally, many large earthquakes within continents are found in failed rifts, leading to speculations of their causative relationship. The best-studied case is the New Madrid seismic zone (NMSZ) in the central U.S. The NMSZ occurs within the Reelfoot Rift (RR), one of three major rifts within the North American craton that failed rather than evolving to continental breakup and seafloor spreading. The rifts formed in similar tectonic settings and followed similar evolutionary paths of extension, magmatism, subsidence, and inversion by later compression, leading to similar width and architecture. However, in historic times the Reelfoot Rift zone is seismically much more active than the Midcontinent Rift (MCR) and Southern Oklahoma Aulacogen (SOA). This difference offers insight into the relative roles of rift structures in hosting intracontinental earthquakes. While most studies seek to explain the seismicity by the failed rifts being reactivated by large-scale stress changes, the much lower seismicity on the MCR and SOA indicates otherwise. The higher RR seismicity indicates that these earthquakes result from local stressing. For example, whereas the NMSZ seismicity has been attributed to stress changes caused by the melting of the Laurentide ice sheet or large-scale mantle flow, these mechanisms should also affect the MCR. Further insight comes from temporal constraints, in that although the earthquakes probably occur by reactivation of favorably oriented faults remaining from rifting, a localized stress source must have recently triggered these particular faults. This is consistent with global observations of episodic, clustered, and migrating intracontinental earthquakes. Thus, although slowly varying plate-wide or regional forces may have a role in the seismicity, the primary triggers must be localized and variable in space and time – perhaps stress transfer from other faults or stress changes from late-Pleistocene erosion - rather than being a direct manifestation of tectonic stresses, which change on timescales of millions of years and usually at plate-wide scales.