Observations from a new high-precision GPS geodetic network in the southern Illinois basin provide evidence for present-day tectonic strain in the Wabash Valley seismic zone, an area associated with a concentration of historical and instrumentally recorded earthquakes, paleoseismic evidence of repeated, large-magnitude earthquakes, and possible Quaternary faulting. The GPS network consists of 56 sites distributed over a 100,000 km2 area of Illinois, Indiana, and Kentucky. The preliminary results reported here are based on a one-year measurement interval, from 1997–1998, and suggest statistically significant horizontal motions at 28 of the sites surrounding the Wabash Valley seismic zone. The inferred velocities are highly variable, presumably influenced by systematic and random geodetic errors, as well as significant nontectonic deformation sources, such as mine- and solution-related subsidence. Nonetheless, the individual site velocities, as well as a formal inversion for tectonic strain, suggest a systematic pattern of shear strain that may be interpreted either as sinistral shear along the north-northeast-trending Wabash Valley fault system or as dextral shear along the northeast-trending Commerce geophysical lineament. While most of our strain estimates remain statistically indistinguishable from zero, the averaged shear strain for the entire network is estimated at 12.7 ± 6.0 × 10−9 yr−1. The shear strain rate estimated for the area surrounding the Wabash Valley fault system is estimated at 3.6 ± 4.8 × 10−9 yr−1, in a similar direction but at a significantly smaller magnitude than previously measured rates in the New Madrid seismic zone. This observed present-day tectonic strain signal, while close to the level of noise, suggests a systematic pattern that may be tested by future GPS measurements in the region.

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