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