1. G21C-01: First Report of the Stable North America Reference Frame (SNARF) Working Group G21C-01: First Report of the Stable North America Reference Frame (SNARF) Working Group an EarthScope workshop held at UNAVCO Inc., 27 Jan 2004 Full report available at: http://www.unavco.org/research_science/workinggroups_projects/snarf/snarf.html Geoffrey Blewitt (Chair)University of Nevada, Reno (gblewitt@unr.edu) Richard A. BennettHarvard-Smithsonian Center for Astrophysics Eric CalaisPurdue University Thomas A. HerringMassachusetts Institute of Technology Kristine M. LarsonUniversity of Colorado Meghan M. MillerCentral Washington University Giovanni SellaNorthwestern University Richard A. SnayNational Geodetic Survey Mark E. TamisieaUniversity of Colorado EarthScope’s Plate Boundary Observatory (PBO) requires:EarthScope’s Plate Boundary Observatory (PBO) requires: –“…that plate boundary deformation be adequately characterized over the maximum ranges of spatial and temporal scales common to active continental tectonic processes.” [ES Facility Proposal] –Questions: How broad is the North America-Pacific plate boundary? Is there a “stable plate interior” and how should it be defined? –keeping in mind potential GPS accuracy ~ 0.1 mm/yr Where is this stable plate interior? Can non-tectonic deformation such as glacial isostatic adjustment (GIA) be modeled with sufficient accuracy? –Hence the need for a Stable North America Reference Frame (SNARF) EarthScope PBO will address this requirement by:EarthScope PBO will address this requirement by: GPS network design at various levels of spatial resolution, with broad coverage across North America Research and development of the SNARF through the SNARF Working Group appointed by UNAVCO Discussion and decision making through a series of SNARF Workshops funded by NSF EarthScope 1.January 27, 2004 at UNAVCO Inc., Boulder CO 2.May 19, 2004, concurrent with AGU/CGU Joint Assembly, Montreal 3.January 2005 Reno NV? 4.October 2005 Cambridge MA? Tasks UnderwayTasks Underway Design ideal reference system and realize initial SNARF by Fall 2004 Procedures for users Transformations and software Final SNARF and report by 2005 Charge from UNAVCO Board of Directors “The Stable North America Reference Frame (SNARF) Working Group (WG) is charged with producing a standard reference frame (for studies in North America) and specifying standard procedures to realize such a frame to meet the needs of the UNAVCO community.” Charge from International Association of Geodesy (IAG) SC1.3c-WG3: Stable North American Reference Frame (SNARF) “The objectives of this working group are to establish a high- accuracy, standard reference frame, including velocity models, procedures and transformations, tied to a "stable North America" which would serve the scientific and geomatics communities by providing a consistent and stable reference with which scientific and geomatics results (e.g., positioning in tectonically active areas) can be produced and compared.”

2. Highlights from the Report of the First Workshop Highlights from the Report of the First Workshop http://www.unavco.org/research_science/workinggroups_projects/snarf/snarf.html Reference Frame Theory and Practice, and Implications Two types of approaches to reference frame definition: 1. Dynamic - equations of motions of satellites could be integrated after the system is rotated into an inertial system - the relationship must be known between this system and the system in which the gravity field coefficients are given - this would require globally distributed sites to ensure the origin of the frame is consistent with degree-one gravity field 2. Kinematic - defined to have zero velocity over the portion of North America that is thought to represent the stable plate - other non-zero velocity sites could be included if the site motions are known (or reliably estimated) relative to the stable plate Ideally a SNARF reference frame would satisfy both types of systems. An initial version of SNARF can be defined by estimating the positions and velocities of a group of sites on the North American plates and then realizing the frame using those sites that do not appear to move relative to each other. - The internal consistency of such a frame is between 0.5 and 0.7 mm/yr, horizontally (using data between 1996 and 2004) - How to solve the day-to-day realization of the SNARF frame? Analysis of the time series of some of the sites in the SNARF definition shows that there are systematic deviations from linear motions. These deviations can be partly explained by atmospheric pressure and water loading deformations of the Earth. The atmospheric pressure loading can be determined with high temporal frequency (4 cycles per day) but water-loading models are only available for monthly averages. Almost certainly there could be large deviations with sub-monthly periods. These non-secular variations limit the accuracy with which daily frame realizations are possible. Models for the non-secular deviations can be developed and caution then is needed in carefully defining the meaning of station positions. The other complication in daily realizations is data quality at the sites used to realize the frame. Instrument failures and local site effects such as snow on radomes and antennas will affect the frame realization if the sites are retained in the analysis. These classes of problems can be minimized if a large number of stations are included in the reference frame. In this case, stations that are deemed problematic on a specific day or over an interval of time can be removed from the frame realization, without adverse effects on the realization. Ambiguity resolution can also strengthen daily frame realizations but ambiguity resolution approaches often rely on high-quality pseudo range data being available which is not always the case. Underlying Models: What is Useful, and what is Possible? Intraplate deformation occurs across North America on a variety of timescales. Removing accurate predictions generated from geophysical models of the deformational processes will allow us to obtain a more reliable realization of SNARF. On daily to annual timescales, atmospheric and hydrological variations cause changes in vertical station positions on the order of several millimeters. Vertical motions from atmospheric variations can be removed by using pressure data from models developed by the U.S. National Centers for Environmental Prediction (NCEP) and European Center for Medium-Range Weather Forecasts (ECMWF) combined with software based on the elastic response of an elastic Earth model. Similar, position changes cause by water loading can be removed with using hydrological data sets (e.g., the Land Dynamics model), which will benefit from additional constraints obtained from the Gravity Recovery and Climate Experiment (GRACE) satellite mission. A significant contribution to the secular signal across North America is the continued recovery from loading effects caused by the Late Pleistocene ice sheets (glacial isostatic adjustment or GIA). The crustal motions caused by this process are quite large (horizontal motions on the order of 1 mm/yr across the plate and vertical motions of over 1 cm/yr near the loading centers). Unfortunately, both the Late Pleistocene ice history and the viscosity model of the solid Earth remain uncertain. The range of model predictions caused by reasonable variations in these inputs can be as large as the signals. Thus, the use of predictions from a single model at this initial stage is unreasonable. However, we propose an iterative process to determine the best model for use with SNARF. Initially, we will generate a range of model predictions using both ICE-I (a history based on geological data alone) and a suite of more recent models constrained, in part, from geophysical data sets (e.g., ICE-3G) as well as viscosity models that include a range of lithospheric thicknesses and upper and lower mantle viscosities. These model results will be examined to determine if there are regions were the predictions are either consistent or small. If such regions exist, they could form a basis for site selection criteria. Once these sites are used to determine an initial realization of SNARF, motions from a larger set of North American stations could then be considered to further constrain the GIA models. As with all model data, it is important to remember the reference frame in which the predictions are calculated. Most modeling uses loading theory developed in Farrell (1972), which calculates results in a center of mass of the solid Earth frame. Results from all models should be represented in a reference frame that is consistent with the observational data. A Working Definition of Stable North America and Site Location Criteria A Working Definition of Stable North America and Site Location Criteria 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: 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 > 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. 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. A chi-square 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) cn2 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. Station Selection Criteria Considerations: 1.Appropriateness of location with respect to Stable North America (previously discussed) 2.Length of time series. 3.Quality of monument, assessed in a quantitative manner. 4.Number of jumps in time series, related to equipment changes. 5.Type of antenna being used (phase center corrections required) 6.Quality of station support and ability to influence owners of site to maintain geodetic standards. Initial Recommendations: 1.Detailed information was needed for every site, including photos and descriptions of the monument and its location (e.g. bedrock). 2.Site operators should be invited to become a reference frame site, but that no promise of inclusion would be made. 3.UNAVCO should maintain an online database on sites. 4.Initially, sites would need a minimum of 2.5 years with no equipment changes to be included as a reference site. 5.Sites should be removed from the reference frame if they no longer keep up to PBO standards. Regional Arrays: Current Practice, Limitiation, and Future Needs North America plate reference frame realizations serve several purposes in the analysis of North American regional GPS arrays: 1.North America plate realizations provide a tectonically motivated perspective from which to visualize velocity estimates, compare estimates against tectonic intuition, and otherwise qualitatively interpret estimates. 2.North America frames provide a common ground for comparisons between solutions obtained by different analysis centers. 3.North America reference frames provide an important kinematic boundary condition for studies of plate boundary (or intra-plate) deformation and an important link between plate boundary zone deformation processes and global plate tectonic motions. Although the specifics differ among regional array analysis centers, the North America plate frame is comonly realized as follows: 1.Analysis of raw GPS data from the regional array together with data from stations within the stable interior regions of North America adopting a well-defined external reference frame such as ITRF2000. 2.Selection of a set of stations whose velocity estimates represent motion of the North America plate and estimation of transformation parameters so as to minimize the velocities at the stations deemed representative of the North America plate. 3.Make a decision about how to account for reference frame uncertainty in the variance-covariance matrix associated with the velocity estimates. 4.An alternative to steps (1) and (2) above is to apply a “known” rotation from the regional velocity solution in a global reference frame to the North America frame (e.g., from ITRF2000 to North America using an NNR-NUVEL-1A-North America Euler vector). Experience based on analyses of data from regional arrays exposes several limitations in the utility of present North America reference frame determinations as outlined in the next section. A variety of factors limit the precision and accuracy of reference frame realizations using GPS. These factors lead to differences between frame realizations determined (1) at different times by the same analysis center, and (2) by different analysis centers. Reference frame differences among successive solutions determined by a given analysis center can result from a variety of factors. 1.Reference frames are based on a selection of available data, which evolves with time, both in terms of the amount of data at any given station and the number of available stations producing (useful) data. 2.Plate-fixed reference frames are determined under the assumption that the secular motions of the stations used to define the plate frame represent plate motion only. That is, any apparently secular motions of the sites due, for example, to strain associated with nearby plate boundary zone faults, intraplate faults, hydrological loads, Glacial Isostatic Adjustment, etc, can bias estimates for plate motion at a site. 3.Periodic and episodic motions of sites, including apparent episodic displacements associated with equipment changes, etc, if not modeled or averaged carefully, can also lead to biased estimates for secular motion that depend critically on the time spans of the data. Comparison of successive solutions, to assess the quality of the solutions for example, requires an assessment of the stability of the reference frame realization from solution to solution. Reference frame differences between analysis centers can arise from all of the data-set dependent factors affecting individual analysis centers listed above. But in addition, comparisons between analysis centers must also take into consideration the specifics of the respective procedures used. The transformation from global to North America reference frames (a) by estimation of transformation parameters unique to the data set, or (b) by application of a known set of transformation parameters. Quantitative comparisons between solutions determined by different analysis centers may require re-alignment of one or both frames if precision of the order of 1 mm/yr is required, even if identical subsets of data are analyzed by each center. One of the primary goals of SNARF is to overcome or mitigate the limitations listed in the previous section. Following is a summary the needs of regional network analysts as identified at the January 2004 SNARF Workshop: 1.Decide upon a common set of procedures for determining a GPS-based SNARF, including specification of a set of high-quality GPS stations in the North America interior through which the SNARF frame can be realized, and a prescription for the proper determination of variance-covariance matrices associated with solutions in the SNARF frame. 2.Provide guidelines for authors on how to describe their reference frame determinations in order to facilitate comparisons among published solutions. 3.Provide “directions for use” for solutions provided in the SNARF frame. The directions should be aimed at a non-geodesist audience, but should include a description of how to treat the variance-covariance matrices provided with SNARF solutions, and recommendations for how to use velocities provided in the SNARF frame to constrain geophysical models. Plan for producing and testing SNARF 1.The SNARF data product is intended to address a range of solid earth deformation goals, including optimization for regional deformation studies, in addition to providing a direct correction to NAREF, ITRF 2000 and other commonly used reference frames and plate models. The plan for producing a SNARF baseline or beta version (SNARF 0) is to develop a set of stations of well- known position and velocity, and to test sets of a priori (e.g., geologic) and/or statistical criteria for selection of the subset of zero order stations. Some zero order stations may even be selected from the interior of the Pacific plate, where a robust and standard independent plate motion correction can be applied to the GPS velocity determination. The set of all primary stations shall initially be drawn from existing high quality time series of the IGS, CORS, and existing regional arrays. Ultimately, SNARF will include all PBO backbone stations that meet data continuity and quality criteria. 2.Testing of the application of SNARF will be undertaken by participating data analysis labs at the regional arrays in order to assess its robustness, ease of implementation and for optimization for crustal deformation (including GIA) studies. Strategies for testing will focus on stability of velocities and improvement of uncertainties relative to the defined North America datum. It will also be tested for stability or improvement to short baseline determinations using criteria such as strain, stability or improvement on vertical deformation rates, and optimization for regional stabilization.