Colloquium: Beatrice Magnani

Maria Beatrice Magnani, Associate Research Professor, Center for Earthquake Research and Information (CERI), University of Memphis

Abstract - The processes governing earthquakes in intraplate regions are still poorly understood, mostly because the very occurrence of such earthquakes violates plate tectonic theory, recurrence intervals are long, and seismicity occurs in quiescent areas where structures are buried/unexposed.

In the central US, most of the seismicity is concentrated along the faults of the New Madrid seismic zone, located in the heart of the Mississippi embayment. However, the clash between the high level of present, historical and prehistorical seismicity, the geodetic vectors, and the puzzling lack of deformation at the surface and in the subsurface 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.

If seismicity has been migrating throughout the central US during the Quaternary, then what controls the location of seismicity? Here we present the results of ~950 km marine and land high-resolution seismic reflection data acquired along and near the Mississippi River in the Mississippi embayment between 2008 and 2011. The data clearly document Quaternary deformation within the New Madrid seismic zone and at 3 locations well beyond the seismically active region, with displacement of all the unconsolidated sedimentary units from the top of the Paleozoic sedimentary rocks to the base of the Quaternary river alluvium. Beside documenting Quaternary deformation in the region, perhaps one of the most important results of the ~950 km-long seismic survey is that deformation is not uniformly distributed throughout the profile, but 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 fault. This spatial correlation indicates a tectonic control, possibly imposed by pre-existing structures, which are capable of concentrating strain and stress and localizing intraplate deformation. Surprisingly, no Quaternary deformation is detected across the buried Paleozoic Ouachita orogenic belt, indicating that under the same stress field, not all the pre-existing structures appear to be capable of focusing deformation.