Overview of the SNARF Working Group, its activities, and accomplishments
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Transcript of Overview of the SNARF Working Group, its activities, and accomplishments
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