1. GLOBK Velocity Solutions
Thomas Herring

2. Overview Basic velocity solution
Generating realistic velocity sigmas (eariler lecture)
Accounting for earthquakes and site offsets
Pre-stacking and speeding up results
09/25/2008
Unavco GLOBK Velocity

3. Basic velocity solution
Basic process is:
Make a file with list of binary h-files (usually .glx files). These hfiles span multiple years normally
glist can be used to see list of stations and durations and check for large differences in coordinates.
globk command file add:
apr_neu all
Assigns 1 m/yr uncertainty to velocities at all sites
glorg command file add:
rate_org xtran ytran ztran <xrot> <yrot> <zrot> <scale>
Entries depend on if rotations and scale are estimated in globk
09/25/2008
Unavco GLOBK Velocity

4. Basic solution: Orbits
Considerations in globk:
If ‘Choice of Experiment = RELAX’ in sestbl. then orbits needed to accounted for.
Merge with global orbit files (available from MIT as combined binary h-files)
Constrain or fix the orbits. Post-2000 IGS orbits can generally be fixed,
There are advatages to generated updated h-files with orbits contsrained or fixed. (Speeds up velocity solution because apr_svs command not needed in velocity solution).
If ‘Choice of Experiment = BASELINE’ then orbits are fixed so do not need to be treated specially.
With older data (pre-2000) still useful to estimate orbits. Also depends on next size (<1000km).
09/25/2008
Unavco GLOBK Velocity

5. Basic solution: EOP
When RELAX mode, EOP parameters (apr_wob and apr_ut1) are estimated; when BASELINE mode used, they are not estimated in gamit and therefore orientation of system is fixed.
If apr_wob/apr_ut1 used, then mar_wob and mar_ut1 should also be used to allow these to vary with time.
For long baselines and BASELINE mode, these should be included in command file.
For short baselines (<1000km) and RELAX mode, they can be not included (not further constraint), or constrained (even F is OK).
When tightly constrained (<0.2 mas), xrot yrot zrot should be included in pos/rate_org commands
09/25/2008
Unavco GLOBK Velocity

6. Basic Solution: Scale
Treatment of scale is unclear as to best approach
For long baseline and long time series, apr_scale and mar_scale probably should be used until re-processed IGS products are available.
Shorter baseline, probably don’t need scale estimated.
Suggestion is to try both ways and see impact on results (good use of OPTION string in globk and glorg command files).
09/25/2008
Unavco GLOBK Velocity

7. Accounting for earthquakes and site offsets
Earthquakes and offsets are accounted for with the eq_file.
Generally, after and earthquake there is post-seismic motion which can be accounted for with the eq_post process noise or eq_log log estimates.
Parameters for some earthquakes are given in distributed tables directory. Ideally for other earthquakes, model for coseismic motion can be used to determine parameters, Postseismic is often assumed to 10-20% of coseismic.
The glorg commends equate and eq_dist can be used to make velocities the same before and after offsets. Apriori velocity be the same; FIXA org_opt can bs used but should be checked.
09/25/2008
Unavco GLOBK Velocity

8. Offsets
The RNRP org_opt can be used to get an estimate of the offset magnitude to see if it is significant.
The script sh_exeqs can be used to extract equate lines from glorg output. These can be used then in glorg to apply equates.
09/25/2008
Unavco GLOBK Velocity

9. Pre-stacking and speeding up results
With large numbers of days of data and stations, velocity solutions can take a long time. Some methods of speeding up runs:
The globk command use_num can specify the minimum number of times a site is used. Also glist output can be used for this (with grep and awk)
The command decimate to process every n’th file. With process noise, decimation make little difference to results. (decimate 7 or even 30 works well with continuous data).
09/25/2008
Unavco GLOBK Velocity

10. Pre-stacking Pre-stacking hfiles also works well
Several approaches here:
Combination of local and global hfiles to get orbits:
Orbits loose in combination and then not used afterwards (I.e., no further constraints)
Combined hfiles or individual hfiles can be combined in time to form n-day averages (7 to 30 days work fine).
With process noise used: 7-30 day combination should estimate only position and not velocity. MIDP org_opt will refer result to center of data span.
Same process noise model should be used when making 7-30 day combination and when combining the combined h-files. (Statistically, the results are same as direct solution)
Care needed in duration of combinations during times of large post-seismic deformation (use shorter combinations)
With time-stacks, the chi^2 increments will be greater than 1 because random walk is too step a spectrum.
09/25/2008
Unavco GLOBK Velocity

11. Extracting and displaying results
There are shell scripts to extract results (e.g., sh_exglk) and shell scripts to plot velocity field (e.g., sh_plotvel)
velview and tsview can be used to display velocities and time series.
tsview outputs rename files that can be used in globk. It can also read an eq_file from globk to apply breaks and earthquakes.
09/25/2008
Unavco GLOBK Velocity

12. Iteration of results Common approach is to:
Run velocity solution and extract coordinates for new apriori coordinate file.
Use this new apr_file and “good” stations to generate time series with glred.
Use sh_gen_stats to get process noise on the sites and get lists fo stabile sites. Command sig_neu can used to account for occasional error such as campaign with bad height,
Use these estimates to re-estimate velocity and iterate the system
Time series can be used to detect outliers (use rename to <name>_XCL or <name>_XPS in eq_file) and breaks (can rename in eq_file to <name>_?PS where ? Is letter or number)
09/25/2008
Unavco GLOBK Velocity

13. Summary
Basic velocity solution: Useful when small number of days and stations.
Accounting for earthquakes and site offsets:
Can be complex with large data sets
Use source command to allow reading in of standard values. Runstring option can be used to have same command file for velocity and repeatability runs
Pre-stacking and speeding up results: Useful with large continuous data solutions. Multiple techniques can be used.
09/25/2008
Unavco GLOBK Velocity