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.