GLOBK Velocity Solutions Thomas Herring 09252008 Unavco GLOBK

GLOBK Velocity Solutions Thomas Herring 09/25/2008 Unavco GLOBK Velocity

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 2

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 10 10 10 1 1 1 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 3

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 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 (<1000 km). 09/25/2008 Unavco GLOBK Velocity 4

Basic solution: EOP • When RELAX mode, EOP parameters (apr_wob and apr_ut 1) are estimated; when BASELINE mode used, they are not estimated in gamit and therefore orientation of system is fixed. • If apr_wob/apr_ut 1 used, then mar_wob and mar_ut 1 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 (<1000 km) 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 5

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 reprocessed 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 6

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 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 7

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 8

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 9

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 10

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 11

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 12

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 13