We analyze intraplate crustal deformation based on newly acquired geodetic data from a 56-station regional GPS network located in the Wabash Valley Seismic Zone (WVSZ) of southern Indiana and Illinois and western Kentucky. The region is permeated by high-angle, basement-penetrating, faults that traverse relatively undeformed Paleozoic or Mesozoic rocks. The region has experienced moderate (M ~ 3 to 5.5) seismic events in recent history, possibly related to the New Madrid seismicity zone, located ~200 km to the southwest of the WVSZ, and the site of three M > 7 earthquakes in 1811-1812. The regional network is augmented by a densified, 35-station GPS network (~10 km spacing) in the Fluorspar district of southernmost Illinois.
We combine the newly acquired data in 2007 with data from five previous campaigns from the period 1997 to 2003. In addition, we use data acquired from nearby continuous GPS stations of the NOAA/CORS and GAMA networks to densify our geodetic grid. Our GPS processing results show highly improved position and velocity estimates of these campaign sites relative to previous campaign measurements. Results show a systematic N to NW motion of about 0.5 – 0.7 mm/yr with respect to the Stable North American Reference Frame (SNARF V. 1.0); similar results are obtained with an independent realization of the reference frame using a subset of the IGS and other continuous stations located within the stable North American plate.
We then analyze deformation using on two approaches: inversion for a continuous strain field and an elastic block modeling approach. Strain patterns indicate that deformation within southern Indiana is dominated by extension, while southern Illinois is dominated by NE-SW compression and NE-SW extension. Areas near the Wabash Valley Fault (WVFS) system show marginal extension on the NNW-SSE axis and ENE-WSW compression. Block models incorporating boundaries along the Cottonwood Grove-Rough Creek Graben and the WVFS indicate marginal block velocities with possible strike-slip motion along the WVFS. Deformation in western Kentucky is dominated by ENE-WSW extension and NNW-SSE compression.
We also examine models that may help explain seismicity in the region. Our initial results suggest that elevated seismicity and strain in the WVSZ could be considered part of a prolonged aftershock sequence triggered by viscous relaxation in the lower crust long after the New Madrid earthquakes. Another possible explanation of regional seismicity could be stressing due to glacial unloading, as suggested by the correlation between GIA models and our GPS velocity field.