Horizontal velocities in the central and eastern United States from GPS surveys during the 1987-1996 interval

The National Geodetic Survey and the Nuclear Regulatory Commission jointly organized GPS surveys in 1987, 1990, 1993, and 1996 to search for crustal deformation in the central and eastern United States (east of longitude 108{degrees}W). We have analyzed the data of these four surveys in combination with VLBI data observed during the 1979-1995 interval and GPS data for 22 additional surveys observed during the 1990-1996 interval. These latter GPS surveys served to establish accurately positioned geodetic marks in various states. Accordingly, we have computed horizontal velocities for 64 GPS sites and 12 VLBI sites relative to a reference frame for which the interior of the North American plate is considered fixed on average. None of our derived velocities exceeds 6 mm/yr in magnitude. Moreover, the derived velocity at each GPS site is statistically zero at the 95% confidence level except for the site BOLTON in central Ohio and the site BEARTOWN in southeastern Pennsylvania. However, as statistical theory would allow approximately 5% of the 64 GPS sites to fall our zero-velocity hypothesis, we are uncertain whether or not these estimated velocities for BOLTON and BEARTOWN reflect actual motion relative to the North American plate. We also computed horizontal strain rates for the cells formed by a 1{degrees} by 1{degrees} grid spanning the central and eastern United States. Corresponding shearing rates are everywhere less than 60 nanoradians/yr in magnitude, and no shearing rate differs statistically from zero at the 95% confidence level except for a grid cell near BEARTOWN whose rate is 57 {+-} 26 nanoradians/yr. Also corresponding areal dilatation rates are everywhere less than 40 nanostrain/yr in magnitude, and no dilatation rate differs statistically from zero at the 95% confidence level