Geodetic Observations From the Region Surrounding the M 5.2 Mt. Carmel, Illinois Earthquake

We present new results from a GPS geodetic network in the southern Illinois Basin, including a post-seismic survey in the aftermath of the M5.2 Mt. Carmel Illinois earthquake. A 56-station regional network at ~30 km spacing in SW Indiana, S Illinois and W Kentucky is augmented by continuous data from IGS, GAMA, and CORS GPS stations. We also present results from a densified, 35-station GPS network (~10 km spacing) in the Fluorspar district of southernmost Illinois, located near near a complex transitional zone between the Wabash Valley and New Madrid seismic zones. The region is traversed by steep-angled, basement-penetrating faults and characterized by moderate (M ~3 to 5.5) earthquakes. The most recent of these events was the April 18, 2008 M 5.2 event, located close to the New Harmony Fault at ~14 km deep near Mt. Carmel, Illinois. We combine data from the regional network with GPS observations from five stations near the epicenter of the April 2008 earthquake. Predictions based on the depth and mechanism of the earthquake suggest horizontal coseismic motions of < 1mm at most network stations. Observed data suggest small, but marginally significant displacements as compared to block motions in the area. Results from the regional network show highly improved position and velocity estimates of these campaign sites relative to previous campaign measurements, with station velocities suggesting systematic northwestward motion of about 0.5-0.7 mm/yr with respect to the Stable North American Reference Frame. We then investigate strain patterns using models to explain tectonic deformation within the Wabash Valley. We use an elastic block modeling approach, supplemented by continuum-based methods to explain variable strain between GPS stations. Block models which assume boundaries along the Cottonwood Grove-Rough Creek Graben (CGRCG) and the WVFS indicate marginal block velocities with possible strike- slip motion along the WVFS, and E-W motions along the CGRCG. We also present results from eight years of GPS observations (2000-2008) from the dense Shawnee network, which appear to be consistent with the regional strain models from the regional network. We also explore geodynamic models that integrate seismicity and crustal deformation in the region.