LONG-LIVED, WIDESPREAD DEFORMATION IN THE CENTRAL U.S.: CONSTRAINTS FROM HIGH-RESOLUTION REFLECTION DATA

Large magnitude earthquakes within continental plate interiors are puzzling events that seem to defy plate tectonics theory. Indeed the behavior and the processes controlling continental intraplate seismicity are still poorly understood. The Central U.S. hosts one of the most active intraplate seismic areas in the world, the New Madrid seismic zone (NMSZ). Here present seismicity is localized along four main active faults that generate ~200 small (M<4) earthquakes a year. Prehistoric and historic data, however, show that large earthquakes (M>7) have occurred along these faults every ~500 yr for at least the past ~2 kyr. The periodic release of strain associated with large events is expected to result in pervasive crustal deformation. Instead, 20 yrs of geodetic (GPS) monitoring of the NMSZ show that this area is deforming at a rate of less than 0.2 mm/yr, and subsurface imaging shows a puzzling lack of deformation in the post Late-Cretaceous sediments beneath the subdued topography of the Mississippi Embayment. One of the possible explanations to this apparent paradox is that deformation rates vary with time, with the implication that seismicity in the NMSZ could be very young and episodic.

Understanding how the long-term deformation is distributed in space and time in the Central U.S. is a critical but challenging task, particularly in the Mississippi Embayment, as faults are buried under a blanket of sediments and don’t have a clear morphological expression. High-resolution seismic reflection imaging has been an invaluable tool in identifying buried faults and estimating their slip rates. From two decades of seismic reflection surveying in the Mississippi Embayment, evidence is emerging that a substantial amount of Quaternary deformation is accommodated along a network of faults distributed throughout the Embayment, and not just within the NMSZ, suggesting that the present seismicity does not reflect the deformation pattern in the long term. In addition, most of these faults are characterized by a long-lived tectonic history, in some cases dating back to the Late Cretaceous. Whatever mechanism (or mechanisms?) is responsible for the activation of these faults, it has to have been active for a long time and across a large portion of the North American continental interior, and not just in the NMSZ.