Capitalizing on the success of a pilot study conducted in 2008, which tested the feasibility of marine seismic data acquisition along the Mississippi River, University of Memphis and University of Texas researchers are planning two field programs to acquire approximately 600 km of high-resolution, marine seismic reflection and sub-bottom profile data along the Mississippi River. The two campaigns will image with unprecedented resolution the New Madrid Seismic Zone active fault system and a series of inferred faults suspected to be responsible for earthquake induced liquefaction features predating the seismic activity of the New Madrid Seismic Zone. Exploiting the advantages of marine acquisition (time effective, low cost), the new data will identify the location and timing of deformation within the active fault system (which generates more than 200 magnitude 1.5-4.0 earthquakes a year), and outside the New Madrid Seismic Zone, and contribute to the understanding of how long-term deformation is partitioned in time and space among faults within the Mississippi Embayment. This information is critical to solve the apparent paradox raised by the contrasting evidence of small geodetic vectors, a puzzling lack of substantial deformation, and the high level of instrumental and historical seismicity.
One of the pillars of plate tectonics is the assumption of the rigidity of major plates. Indeed GPS studies show that most plate interiors behave rigidly and, as a result, the majority of earthquakes occur along narrow or diffuse belts at plate boundaries. Located in the heart of the North American continent and 2,000 km away from the nearest plate boundary, the New Madrid Seismic Zone is one of the most active intraplate seismic areas in the world, and one of the most notorious deviations from plate rigidity. Paleoseismological observations show that the area experienced a series of catastrophic earthquakes in historic and prehistoric time and evidence is mounting that the long-term seismicity is not limited to the presently active fault system, but that large earthquakes have occurred in other areas within the Mississippi Embayment, for which the responsible fault (or system of faults) are yet to be identified. This study will provide observational constraints on theoretical models proposed to explain the timing, magnitude and location of intraplate seismicity in the Central United States. The results of this project will also have a crucial impact for earthquake hazard assessments by localizing and characterizing seismogenic faults, and improving the quality and usefulness of seismic hazard maps.
Although the theory of plate tectonics revolutionized our understanding of earthquake processes, it does not explain why earthquakes occur in regions distant from plate boundaries, where deformation rates are low and plate motions supply insufficient driving energy. This project investigated the New Madrid seismic zone in the Central US, one of the most active intraplate regions in the North American continent located ~2000 km away from the nearest plate boundary. Here, the high level of present, historical and prehistorical seismicity clashes with small geodetic vectors and a puzzling lack of deformation at the surface and in the subsurface, which suggests that the New Madrid seismic zone might have not been the only fault system active in the region. Indeed, evidence is mounting that Quaternary deformation has been accommodated along structures that are presently aseismic. Using an innovative technique, this study acquired ~950km of marine high-resolution seismic reflection data along the Mississippi river, providing observational constraints on theoretical models proposed to explain the timing and magnitude of continental seismicity in the Central US. In particular the data clearly document Quaternary deformation within the New Madrid seismic zone and at four additional locations well beyond the seismically active region. At these locations the unconsolidated and semi-consolidated sedimentary units appear to be deformed/displaced from the top of the Paleozoic rocks to the Quaternary river alluvium. The amount of deformation at the fault generally increases with age of the displaced units, indicative of a persistent deformation along the fault. Besides documenting Quaternary deformation in the region, the 950 km-long continuous seismic survey shows that deformation is not uniformly distributed, but rather that is focused along distinct structures, which spatially coincide with the margin of the failed Paleozoic Reelfoot Rift and with the southern margin of the Proterozoic Laurentian continent, marked in this region by the Alabama-Oklahoma transform. This spatial correlation indicates a tectonic control possibly imposed by pre-existing structures, which are capable of concentrating strain and of localizing intraplate deformation. Surprisingly, no Quaternary deformation is detected across the buried Paleozoic Ouachita orogenic belt, suggesting that under the same stress field, not all pre-existing structures appear to be equally capable of focusing deformation.
