
The result shown here are derived from the network of continuous GPS sites
covering the New Madrid area (GAMA) installed by CERI and from additional
continuous GPS sites of the CORS and IGS networks.
The picture on the left (courtesy G. Mattioli, University of Arkansas) shows two GPS antennas at site HCES in the New Madrid seismc zone. Note the different type of monumentation. 
GAMITGLOBK:
The GAMITGLOBK analysis uses doubledifferenced GPS phase and solves for
station coordinates, satellite state vectors, 7 tropospheric delay parameters
per site and day, horizontal tropospheric gradients, and phase ambiguities
using IGS final orbits and earth orientation parameters. We apply elevation
dependant antenna phase center models following the tables recommended by
the IGS, solid Earth, polar tide, and ocean loading corrections following
the IERS standards (IERS, 1996). We process 15 subnetworks of about 25
stations each, totalling about 400 sites covering the stable interior
of the North American plate. 10 IGS sites are common to all subnetworks
to tie them together and with a global reference frame, if needed. We then
calculate position time series for each site, from which we estimate process
noise parameters. The daily solutions for each subnetwork are then combined
into a single looselyconstrained positionvelocity solution covering 19962005.
We finally implement a North Americafixed reference frame by minimizing
the velocities of 23 stations in the North American interior while estimating
an orientation and translation transformation. These stations are chosen to
(1) have a randomwalk noise component lower than 0.6 mm/sqrt(year),
(2) have at least 5 years of continuous operation, and (3) be located
south of 40N in order to avoid glacial isostatic adjustment effects
and east of 110W in order to avoid tectonic deformation in the Western U.S.
GIPSY/OASIS:
The GIPSYOASISII solution uses final precise orbits, clocks, and earth
orientation parameters (all from JPL's global analysis) with a freenetwork,
absolute pointpositioning strategy. Oceanloading coefficients for each
site were obtained using the Scherneck model of the Onsala Space Observatory.
Site coordinates determined from the analysis of the GPS phase pseudorange
observables are initially defined in a fiducialfree reference frame and
subsequently transformed into the ITRF2000 using 7parameter Helmert
transformations that align the daily coordinates of a selected subset
of ITRF2000 fiducial sites with the fiducialfree coordinates of the same
subset of sites. The secular motion of the North American plate was removed
based on current realization of the DeMets et al. (2000) NOAMITRF00 euler
pole. Common mode errors at each site were removed (up to Sep. 2004) using
the NLIB cGPS site as a regional reference with a method modified after
Wdowinski et al., 1997. Velocities are calculated by a linear fit to position
time series using a linear leastsquares inversion with formal estimates of
white and flicker noise and an assumed random walk monument noise of
1 mm/sqrt(yr) as discussed by Mao et al. (1997).