Reference Frame NA12 for Studies of Continental Deformation in North America

We present a 300-station GPS reference frame NA12 designed for geodetic studies of deformation in North America (NA). Coordinate time series in NA12 have an RMS scatter of ~1 mm in the horizontal and ~3 mm vertical, about 50% of the scatter in IGS08, as a result of filtering common-mode error. NA12 is a secular frame based on IGS08, the GPS-based realization of ITRF2008. Thus vertical velocities in NA12 are consistent with those in ITRF2008, centered at the Earth system center of mass. The rotation rate of NA12 is defined with the intention that points within a geographical domain representing the "stable NA plate interior" do not appear to move horizontally. Plate boundary deformation to the west limits this stable domain somewhere to the east of the Basin and Range Province and the Rio Grande Rift. Horizontal deformation driven by post-glacial rebound centered around Hudson Bay theoretically peaks around the nodal contour of vertical motion, thus further limiting the stable domain to the south of the US-Canadian border. We chose a domain bounded to the west by longitude 104.6°W and to the north by latitude 40.2°N, which is sufficiently large (~2000 km), while bounding variations in horizontal velocities to <0.5 mm/yr. A total of 300 stations broadly distributed around NA were selected as NA12 frame sites, with a subset of 30 "core stations" used to impose the no-net rotation condition. Velocities were estimated using available data from 1996.0 - 2012.1, that were processed using the GIPSY OASIS II software with WLPB ambiguity resolution, with solutions in IGS08. The broad distribution and large number of stations enhances frame stability and mitigates the impact of stations dropping out of the frame. These frame stations were selected from a set of 378 candidates with time series that are fit well by a constant velocity model over periods >5.5 years with no automatically detected step discontinuities. Further manual screening weeded out 78 stations with visibly suspect behavior. The 30 core stations were required to have vertical velocity magnitudes <0.8 mm/yr to guard against non-tectonic motions. No such constraints were placed on horizontal velocities so that accuracy could be assessed. The resulting RMS velocity of the 30 core stations is 0.2 mm/yr in the north, and 0.3 mm/yr east, some of which must result from far-field post-glacial rebound. This suggests that the NA plate interior is at stable at ~0.2 mm/yr, and that relative horizontal velocities over ~2,000 km have been determined by GPS with an accuracy of ~0.2 mm/yr. Seven parameters are computed every day since January 1996 to transform coordinates from JPL's orbit frame, into NA12. The RMS coordinate residuals to the estimated transformation are at the level of 1.0 mm in the north, 0.9 mm east, and 3.4 mm vertical. These parameters can be applied by other users of the GIPSY OASIS II software (e.g., Choe et al., 2012 Fall AGU abstract), and are publicly available at ftp://gneiss.nbmg.unr.edu/xfiles. Parameters are not given for some days (mostly in 1996), because of poor network geometry in earlier years. However, by 2000 there were 100 contributing frame stations. As of 2012 there are up to ~200 contributing frame stations per day.