September 30, 2009

Speaker: Dr. M. Beatrice Magnani

Title: New contributions to seismic hazard studies in the Central U.S.

Abstract: Plate tectonic theory elegantly explains seismicity at plate boundaries based on the fundamental assumption of rigidity of major plates. The New Madrid Seismic Zone (NMSZ), located in the Central U.S., is one of the most notorious deviations from plate rigidity. This area has been the location of some of the largest earthquakes ever recorded in the US and today the NMSZ exhibits a very high level of background seismicity, suggesting ongoing deformation. In the same area newly published geodetic data show minimal vectors, which corroborate the subdued topography of the Mississippi Embayment and the lack of substantial deformation in the post Late-Cretaceous sediments.

In the summer of 2008 we acquired 300 km of high-resolution seismic marine reflection data along the Mississippi River from Caruthersville, Missouri to Helena, Arkansas. The profile images a large portion of the Mississippi Embayment outside the area of present seismicity and shows that the deformation is accommodated along discrete zones of faulting and folding separated by largely undeformed sections. In particular the data identify the existence of three areas of deformation that involve the sedimentary cover from the Paleozoic strata to the Mississippi River Quaternary alluvium.

Two of these areas lie outside the NMSZ, suggesting that at least during the Quaternary, separate portions of the Mississippi Embayment have been deformed, possibly at different times. The results of this study contribute toward reconciling the lack of cumulative deformation in the seismically active zone, suggesting that the long-term seismic activity in the embayment is not limited to the NMSZ but extends over a broader region than previously suspected. The temporal and spatial pattern of deformation and the processes that control the tectonics of this portion of the continental interior are still unanswered questions.