SNARF Working Group - Report of the First SNARF Workshop

Definition of stable North America as discussed at the first SNARF meeting

To establish an accurate GPS based reference frame for stable North America the largest geographic distribution of sites on the North American plate should be included.  Sites should be continuous GPS (CGPS) sites and not episodic GPS (EGPS) sites with a time-series of at least 2.5 years.  To define the stable part of the plate a priori geologic criteria are needed for site selection:

Sites should be > 100 km away from any seismicity to avoid seismic cycle effects.

Sites should be away from any active faults even if seismically inactive. Use geomorphic criteria to recognize them.

These criteria limit potential sites to those approximately east of the Rocky Mountains and away from near Memphis-TN, Charleston-SC and the St-Lawrence Seaway. Sites located within the Northern Gulf Coast west of Florida should also be excluded due to potential subsidence effects reflecting fault slippage, compaction and crustal loading.

Sites most affected by glacial isostatic adjustment (GIA) should also be excluded however the exact geographical area was not agreed upon. The current geographic distribution of CGPS sites with more than 2.5 years of data does not adequately describe the GIA effects. EGPS sites that provide a denser sampling suggest that sites up to 1800 km form the center of Hudson Bay are most affected by GIA and should be omitted.

c_n² test for rigid plate rotation (two dimensional) for 83 sites has a value of 1.08. This suggests that both the error model used (white + coloured + random walk 1.7√t) and the sites selected correctly describe the motion of the stable North American plate. If we include CGPS sites within 1800 km of Hudson Bay (adding 46 sites)  c_n² increases to 1.33 for 129 sites. The position of the pole of rotation changes by ~0.5° and the rate by < 0.001°/Myrs suggesting that omitting what may be the most GIA sites affected sites does not significantly change the definition of the motion of the stable plate.