AN UPPER BOUND ON TECTONIC STRAIN IN THE CENTRAL U.S. FROM CONTINUOUS GPS MEASUREMENTS

CALAIS E., Purdue University, Department of Earth and Atmospheric Sciences, West Lafayette, IN, MATTIOLI, G., University of Arkansas, Department of Geology, Fayetteville, AR, DEMETS, C. University of Wisconsin, Department of Geology and Geophysics, Madison, WI, NOCQUET, J.M., CNRS, Geosciences Azur, Valbonne, France, STEIN, S., Northwestern University, Department of Geological Sciences, Evanston, IL, NEWMAN, A., Georgia Institute of Technology, School of Earth and Atmospheric Sciences, Atlanta, GA, and RYDELEK, P., Center for Earthquake Research, University of Memphis, Memphis, TN, ecalais@purdue.edu.

Analyses of the continuous GPS data collected in the New Madrid seismic zone performed by three independent groups using different analysis software and processing strategies (Gamit, Gipsy, combined solution) show no statistically significant residual velocities to a rigid plate behavior, with an average weighted misfit of 1.4 mm/yr at the 95% confidence level. Shortening between sites RLAP and NWCC is not significant at the 95% confidence level and likely reflects an unexplained offset in time series between mid-2001 and early 2002. Assuming that characteristic earthquakes repeat regularly in the NMSZ, a 1.4 mm/yr upper bound leads to a minimum repeat time of about 600-1,500 years, broadly consistent earlier estimates based on the paleoseismic history assuming magnitude 7 earthquakes with 1-2 m of coseismic slip. The same analyses, using 156 GPS sites distributed throughout the central and eastern U.S., find no spatially coherent deviation from rigid behavior in the far field of the NMSZ either, except for effects due to glacial isostatic adjustment, with an average weighted misfit to a rigid plate model of 1.4 mm/yr (95% confidence) as well. Although intraplate earthquakes indicate that tectonic stresses within plate interiors accumulate on faults and are released during large infrequent events, deviations from rigid behavior in the central U.S. - and several other major plates - are below the current resolution of GPS measurements and do not reflect this cycle - at least not on a time scale of a decade or less. Longer observation spans and further improvement of geodetic techniques are needed to understand where, why, and how much strain concentrates in plate interiors.