Is There a Connection Between Seismicity and Deformation in the New Madrid and Wabash Valley Seismic zones?

We compare geodetic and geophysical data for two spatially connected intraplate seismic zones in the central U.S.: the Wabash Valley Seismic Zone (WVSZ) of southern Indiana and Illinois and the New Madrid Seismic Zone (NMSZ) of the Mississippi Valley. In both cases, regional seismic and potential field data provide evidence for high-angle, basement-penetrating, faults that define narrow, elongate Precambrian grabens that lie beneath relatively undeformed Paleozoic or Mesozoic rocks. While only the NMSZ has experienced large-magnitude earthquakes in the historical record, both areas have a Quaternary history including numerous moderate to large- magnitude events. They are separated by an enigmatic tectonic zone characterized by basement uplift, major Precambrian strike-slip and normal fault zones, and Mesozoic and Cenozoic magmatism. We examine data from a 56-site campaign GPS geodetic network in the southern Illinois Basin to infer present-day deformation in the WVSZ. We combine newly acquired data in 2007 with that from five previous GPS campaigns from the period 1997-2002. Results for the 1997-2002 period provide little evidence for statistically significant velocities for individual sites in the southern Illinois Basin. Average strains for the entire network, however, show marginally significant strains, with an orientation rotated 45° from the overall direction of intraplate stress in the U.S. mid-continent. We are currently processing results from the 2007 campaign, and anticipate that the signal/noise ratio will be considerably improved by this five-year extension of the observation period. In addition, we examine models that test the effect of the 1811-1812 New Madrid earthquakes on the near- and far-field strain and seismicity rates in the region through the processes of instantaneous elastic deformation in the lithosphere and associated postseismic viscoelastic flow in the asthenosphere. Our results indicate that significant changes in strain and seismicity rates in the southern Illinois Basin can persist for several hundred years following the New Madrid earthquakes. The seismicity rate can increase by as much as a factor of seven over the background rate in the near-field, but by a much smaller amount in the far-field. However, the effect on the modeled seismicity rates is highly dependent on the choice of lower-crust viscosity. We also investigate the possibility that the New Madrid earthquakes could modify seismicity or strain in the WVSZ by producing triggered slip on a buried fault in the Illinois Basin region. Our initial results demonstrate that elevated seismicity and strain in the WVSZ could result from aseismic slip triggered by viscous relaxation in the lower crust long after the New Madrid earthquakes.