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Stable North America Reference Frame Working Group (SNARF) Chair: Geoff Blewitt. Overview of the SNARF Working Group, its activities, and accomplishments. What is SNARF and Why is it Important?. Objective define a reference frame that represents the stable interior of North America Why? - PowerPoint PPT Presentation
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Overview of the SNARF Working Group, its activities, and accomplishments
Stable North America Reference Frame Working Group (SNARF)
Chair: Geoff Blewitt
What is SNARF and Why is it Important?
● Objective– define a reference frame that represents the stable interior of
North America
● Why?– Appropriate frame to describe relative motions of sites
spanning the N.A. - Pacific plate boundary
– Facilitate geophysical interpretation
– Facilitate inter-comparison of solutions
– Standardization and documentation
Why do we Need a Reference Frame?
● GPS alone does not provide unambiguous coordinates– Can arbitrarily rotate your solution
– Fixing the rotation can facilitate interpretation
● Why not simply use, say, ITRF and NUVEL-1A?– Difficult to interpret N.A. deformations in ITRF
– NUVEL-1A has known deficiencies● For example, African Rift not included
– Glacial isostatic adjustment (GIA) is significant
Velocities in ITRF – not appropriate for interpretation
M. Craymer
Velocities in NUVEL-1A
M. Craymer
Vertical Velocities: Not dominated by tectonics! GIA is the issue.
M. Craymer
GIA Predicted Velocities: Very sensitive to model parameters
J. Davis, M. Tamisea, and T. Herring
J. Davis, M. Tamisea, and T. Herring
Example of problem we need to address: Horizontal GIA motions are sensitive to lateral heterogeneity in Earth’s structure.
M. Tamisea
Questions
● Where does the plate boundary begin?– and why?
– what is the extent of the stable plate interior?● and how tectonically stable is the plate interior?
– is the Colorado Plateau still rotating?● and how active is the Rio Grande Rift?
– extends to Bermuda, Greenland, Alaska, Siberia…?
– is Alaska rigidly attached to North America?● empirical evidence is weak
Questions
● What is the vertical velocity field across North America?– what is GIA versus tectonic?
– role of body forces and mantle dynamics?
– Deceptively simple question:● Is the Basin and Range going up or down?● Not straightforward to determine using GPS● Reference frame dependent
Questions
● How can we design geodetic products that are stable over decadal time-scales and beyond?– will we be able to detect a >5-year transient?
– can we detect the “ghosts” of historic earthquakes?
– is tectonic activity “constant” (steady-state) ?● or does it switch on and off?● and migrate from one region to another?● can we confidently compare and relate geodetic rates to geologic rates?
NSF Proposals● To support SNARF workshops
– No salary – volunteer work
– Proposal 1: Feb 2004 – Jan 2006
– Proposal 2: Jan 2006 – Dec 2007
– No new proposals – “operational” work supported by NRCAN, NGS, PBO
● Goals– Tools and products to help users realize a stable North America-fixed
frame
– Provide the reference frame for PBO (Analysis Coordinator: Tom Herring)
– SNARF operations and maintenance jointly by NGS (Richard Snay) and NRCan (Mike Craymer), under the auspices of the IAG “NAREF” projects
SNARF Workshops
1) 2004-01, UNAVCO Inc., Boulder, CO.
2) 2004-05, Joint Assembly, Montreal, Canada.
3) 2005-03, EarthScope National Meeting, Santa Ana, NM..
4) 2005-06, UNAVCO/IRIS Meeting, Stevenson, WA..
5) 2006-03, UNAVCO Science Meeting., Denver, CO.
6) 2006-11, Natural Resources Canada, Ottawa, Canada.
7) 2007-03, EarthScope National Meeting, Monterey, CA
8) 2008-12, AGU, San Franscisco, CA
9) 2009-03, UNAVCO Science Workshop, Boulder, CO.
Working Group Progress
● Have identified and tackled the major issues:– GPS velocity field that is accurate (representative), and
relatively dense to select a base model for GIA
– Site selection criteria to define “frame” sites● geological considerations● monumentation and equipment● data quality and duration
– Subset of “frame” sites used to define “datum” that can represent a non-rotating stable plate interior
– Define products to be distributed for general use
SNARF Products
● First Release: SNARF 1.0 in June 2005– rotation rate vector: (North America – ITRF2000)
– gridded/site velocities from assimilation model
– site epoch coordinates (X, Y, Z) and velocities
– SNARF web page at www.unavco.org
● Has been adopted by PBO Data Analysis Centers– products in Stable North America Reference Frame
– In production-mode: October 2005
● SNARF 2.0 to be release April 2008– ITRF2005, longer time series, improved models
Example: UNR Solution: 3790 stations, 1994-2008 (now ~2600 daily)Ambiguity resolved (Ambizap)
G. Blewitt and C. Kreemer
UNR NA-NNR Frame: 45 sites, 2000-2008Horizontal Velocities
Provides frame for daily transformations
(GIPSY x-files)
G. Blewitt and C. Kreemer
UNR NA-NNR Frame: Horizontal Velocities (zoom)18 Core sites provide the NNR condition
G. Blewitt and C. Kreemer
UNR NA-NNR Frame: Vertical Velocities
G. Blewitt and C. Kreemer
UNR NA-NNR Frame: Vertical Velocities (zoom)
G. Blewitt and C. Kreemer
Application of Daily X-Files (preliminary):Where Does the Plate Boundary Begin? N
G. Blewitt and C. Kreemer
Lessons Learned● Pattern of GIA uplift (Hudson Bay) and peripheral bulge (Canada-US) clearly delineated
● GIA can cause systematic error in pole of rotation that leads to ~1mm/yr velocity bias
– Large variations and model sensitivity in GIA models
– Method (currently) requires a data-model assimilation approach.
● Plate interior (east of Rockies/Rio Grande Rift) is stable << 1 mm/yr
● Vertical motions below peripheral bulge consistently << 1 mm/yr
– Also across the Basin and Range, Sierra Nevada, and NA-Pacific transform (San Andreas,..)
– ITRF2005 works very well
● Bermuda is on stable North America (no apparent passive margin deformation)
● Greenland, Alaska, Siberia motions are significant compared to frame stability
– 1-3 mm/yr motions, perhaps due to mix of GIA, current ice (de-)loading, permafrost, …
● Monument stability and jumps in time series remains an issue for many sites
– But the best sites have 0.1 mm/yr monument stability (inferred by local baselines)
● Daily transformations into SNARF are recommended versus rotation of velocity field
– Example: UNR will provide daily x-files to transform fiducial-free positions into SNARF
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