SCIGN MIT Report

June 14, 2003

Thomas Herring, MIT contribution

Last Updated Wednesday, June 18, 2003

Combined SIO/JPL Velocity solution

This report updates the March 2003 report, which updated the Dec 2002 report on the comparison of the JPL and SIO analyses of the SCIGN GPS data collected between Jan 1996 and 21 March 2003. As in previous reports we include is this distribution, time series from the processing of SIO and JPL SINEX files, and from the analyses carried out at each center. These time series directories replace the ones available in previous reports. All of the time series files have been brought to a common format for both the velocity fields and the time series, and these files can be viewed with the GAMIT/GLOBK Matlab toolkit distributed as GGMatlab.

Basic framework of combined analysis

The combined SCIGN motion model is realized in a multi-step analysis. The details of the analysis methods are discussed in the Analysis methods section. The details of the data sets used and access to the data sets are discussed in the Basic Data Sets section.

Changes from March 2003 report.

In this report, we have made several major changes from previous analyses.

(1) In order to realize the SCIGN site motions in a North America fixed frame, we have included a selection of the North American and Pacific plate sites in the SIO combined analysis. The JPL stacov files only contain sites from Southern California and the JPL analysis was aligned to the North American frame by aligning it with the SIO solution. In previous reports we had aligned both SIO and JPL with the SCEC Crustal Motion Model 3.0.

(2) Based on the time series behaviors after the Hector Mine earthquake, we have added estimation of logarithmic time series behavior after earthquakes. The model used in documented under tsview in GGMatlab. The use of the logarithm fits for post seismic behavior reduces the need for difference pre- and post-earthquake velocity estimates. There is still many avenues of exploration in this area including the dependence of the time constant of the log function on distance from the rupture and for the different components of the position. For this analysis was use a constant 10-day value for all sites.

The following files and directories are included in the distribution SCIGN_COM.tar.Z (Compressed this is a 89 Mbyte file, uncompressed it is 380Mbtes).

File/Directory	Description
COM_96_0303.vel	Combined JPL/SIO velocity field for the sites that show small non-secular motions. Generated by GLOBK with site-specific process noise for all sites and logarithmic time variations estimated after the Hector Mine earthquake (10-day time constant used).
COM_96_0303.ln	Estimates of the logarithm terms in North, East and Up. This file is formatted the same as the velocity file above and can be viewed in velview. Units are mm but velview will write labels with mm/yr.
JPL_96_0303.vel	Same style of file as COM_96_0303.vel except only JPL STACOV files used in the analysis
JPL_96_0303_TS.vel	Time series analysis for the velocity estimates. After the Hector Mine earthquake, logarithm terms are estimated with a +- 3mm apriori sigma on the estimates
JPL_96_0303.ln	Logarithm terms estimated from the JPL only analysis
JPL_96_0303_TS.vel	Time series analysis for JPL only with logarithm terms after the Hector Mine earthquake
JPL_96_0303_NL.vel	Non-logarithmic estimates of velocities. These are unreliable near the Hector Mine rupture but are less noisy than the velocities in JPL_96_0303_TS.vel for sites away from the rupture.
JPL_0303_rep.ens	Estimates of the respeatabilities of the sites in the JPL times series (may be used as the ENSUM file in tsview to sort sites)
jpl_0306.noam.vel	JPL Velocity estimates from table2.txt rotated from ITRF00 to North America Fixed using the ITRF 2000 NOAM rotation pole.
SIO_96_0303.vel	Same style of file as COM_96_0303.vel except only SIO H-files used in the analysis
SIO_96_0303_TS.vel	Time series analysis for the velocity estimates. After the Hector Mine earthquake, logarithm terms are estimated with a +- 3mm apriori sigma on the estimates
SIO_96_0303.ln	Logarithm terms estimated from the SIO only analysis
SIO_96_0303_TS.vel	Time series analysis for SIO only with logarithm terms after the Hector Mine earthquake
SIO_96_0303_NL.vel	Non-logarithmic estimates of velocities. These are unreliable near the Hector Mine rupture but are less noisy than the velocities in SIO_96_0303_TS.vel for sites away from the rupture.
SIO_0303_rep.ens	Estimates of the respeatabilities of the sites in the SIO times series (may be used as the ENSUM file in tsview to sort sites)
sio_0306.noam.vel	SIO Velocity estimates from refinedModelVelocities.cgi rotated into a NOAM frame using ITRF 2000 pole. Latitude and longitudes added the file so that results may be plotted. These velocities are the current velocities of the GPS sites (see documentation on the SOPAC web site. Some standards deviations have been replaced with non-zero values at sites where zero sigma is reported in the SOPAC file.
mpb_jpl	Time series from the JPL analysis. File names are of the form mb_NNNN_XXX.datY where NNNN is the four character code for the site, XXX is the extent that indicates post-earthquake and/or jumps in the time series, and Y is 1 for North, 2 for East and 3 for Height components. Time is in years. Time series are in a NOAM frame
jpl_fil	Time series from the JPL web site series.html with results ftp’d from sideshow.jpl.nasa.gov. The mb_xxxx files are in the tsview format. The original files from the JPL are also included in the directory. NOTE: These time series are in the ITRF 2000 frame. The itrf00_noami00.dvel file linked below gives the offsets in velocities between ITRF 2000 and NOAM for sites in the California region.
mpb_sio	Similar to mpb_jpl but from processing SIO H-files
sio_fil	Times series from SIO web site obtained with ftp from ftp://garner.ucsd.edu/pub/timeseries These files have been converted to the mb_xxxx files and the time series limited to start in 1996 to match the JPL time series availability. The original SIO files are also included in the directory. These time series are also in the ITRF 2000 frame. In this version of the time series files, no jumps have been removed. The original SIO files contain the times of any discontinuities in the header records.
itrf00_noami00.dvel	File containing the difference in North and East velocities between ITRF 2000 and NOAM for sites in California.
CCVelV.xy	Simple file of longitudes and latitudes of the California coast appropriate for use with Features option in velview. NOTE: The features option does not currently work in the executable version of velview; velview must by run from within Matlab to use this feature.
CF3VelV.xy	Simple file of longitudes and latitudes of the California Faults appropriate for use with Features option in velview
CEQ2VelV.xy	Simple file of longitudes and latitudes of the California earthquakes since 1996 with magnitude >2 appropriate for use with Features option in velview (use the brown dots load, Dbrown). Data obtained from the USGS NEIC
CEQ3VelV.xy	Earthquakes >3 magnitude since 1996
CEQ4VelV.xy	Earthquakes >4 magnitude since 1996

Earthquake names:

LA – Landers, LB – close in region to Landers with extended postseismic relaxation time, NR – Northridge, NA – After shocks of Northridge; HT – Hector mine. The file labn.eq contains the GLOBK earthquake definitions and site renames used in this analysis. (Sites renamed to end in XCL are excluded from the analysis).

The Random walk process noise model values sorted alphabetically and by magnitude are given in files: RW_Stats_alpha.dat and RW_Stats_mag.dat.

The combined motion field was generated by combining the JPL and SIO motion models generated from the data sets described below using GLOBK.

Motion Maps:

SCIGN Velocity field in North America fixed frame. Red vectors are from the GLOBK analysis of sites showing small non-secular motions; only sites with velocity sigmas less than 2 mm/yr are shown. 50 % Error ellipses are shown; site names are not shown.

SCIGN combined velocity field with the average motions on the Sierra Nevada block set to zero. NOTE: Much of the motion seen is due to strain accumulation on the San Andreas Fault,

Comparisons between the analysis of JPL and SIO loosely constrained solution files

Overlay of velocities from the analysis of the JPL and SIO loosely constrained solution files. The red vectors are from the JPL stacov files and the blue vectors are from the SIO H-files. The RMS differences between the two fields are (from 192 sites) 0.6 mm, North; 0.5 mm, East; and 1.4 mm, Height. The square-root of chi-squared per degree of freedom for the components are 0.6 North; 0.5 East and 1.2 Height.

Detail view of the JPL and SIO results. The frame is relative to the three island sites, CAT1, CAT2 and SNI1. The RMS differences in this region are similar to the over all statistics given above. Red vectors are JPL and blue vectors are SIO.

Basic Data Sets

The “data sets” used in this analysis were:

1. From JPL: loosely constrained “stacov” files that contain station position estimates, covariance matrix, and limited information about antenna heights. For each day, the stacov files contain all the sites used on that day obtained by merging results from up to six network. The JPL merged stacov files were used in this analysis. The files made available to the SCIGN analysis group ran from Jan 1, 1996 to March 21, 2003. The sites include were in the box between longitudes 239-250 and latitudes 28-39 degrees. All the SCIGN sites and some, but not all, of the BARGEN sites this box were included. The stacov files are available at ftp://sideshow.jpl.nasa.gov/pub/mbh/stacov/loosefinal.

2. From SOPAC: loosely constrained GAMIT “h-files” that contain station positions, satellite orbital parameter, and Earth Orientation parameter estimates; covariance matrix and antenna type and height information. For each day, the h-files are divided into networks with up to 15 networks per day containing about 450 sites distributed around the world. The SCIGN group of stations is represented in up to 6 networks again with a common group of stations tying the networks together. In this case, the common sites are distributed across North America. These files are available for data collected since 1990 and updated each week with about a 3-week delay from real-time (ftp://garner,ucsd.edu/pub/hfiles). For this analysis, we used sites in the latitude/longitude box and time interval given above, and approximately 35 other stations on the North American plate and 6 sites on the Pacific plate..

Analysis methods

The approach in the analysis method was to treat the data from both groups in the same way using the GLOBK software, which is designed to combined position estimates and covariance matrices using a Kalman filter approach.

For the JPL stacov files we changed the antennas at 46 sites to make them the same as the heights in SOPAC site database. For the Dorne Margolin antennas this change can be made exactly. For other antenna types we had to assume that phase center model in the JPL analysis was the same as the SOPAC analysis and that JPL had not applied horizontal position changes for these antennas (such as the TRM14532.00 antenna at MATH) because only an LC height is given in the stacov files with no information about the models used for either the antenna type or its characteristics as the L1 and L2 frequencies.

Both sets of solution files were combined into daily files that contained all the stations processed by each group. For the SOPAC h-files, this combination used all the sites in all the networks and the orbital parameters were included in the combination. From these combined files, the sites in the SCIGN latitude/longitude box and the reference sites on the North American and Pacific plates were used in the analysis. The orbits were loosely constrained in these combinations.

The combinations resulted in 2622 daily files for SOPAC and 2604 files for JPL.

For this preliminary comparison the velocity field and times series from these daily files were constructed in two steps: “Good site combination” and “Remaining Sites Analysis.” The production of the combined velocity is being carried out in stages. In the initial analysis, http://bowie.mit.edu/~tah/SCIGN/SCIGN_AC09_2002.html, we used 86 “good” sites that had more than 500 days of data and horizontal RMS scatters less than 1 mm. With this selection of sites there were problems straddling the discontinuities in the time series due to the Hector Mine earthquake. In the current analysis, we base the good site selection on the estimated random walk process noise for the sites. Sites with horizontal RW walk noise of less than 1 mm^2/yr were included in the good site combination. The process noise for all site are given in RW_Stats_alpha.dat and RW_Stats_mag.dat

“Good Site Combination”

In separate analyses, the JPL and SOPAC files were combined with GLOBK to estimate the position and velocities of the sites. After the Hector mine earthquake (1999/10/16), we renamed all sites within 250km of the epicenter (34.2752N –118.5962E) and estimated logarithmic time dependence to the position after the earthquake. We also renamed sites that had unexplained jumps. This information is contained in the globk earthquake file labn.eq. We automatically added jumps when antennas are changed from the injected Ashtech Choke ring antennas (ASH700936E_C) to the milled choke ring antennas (ASH701945B_M). This change in antenna type causes several millimeters of horizontal position change and does not appear to be the same at all locations.

For the full GLOBK solution, we decimated the experiments and used values only once every seven days. Because of missing days in the JPL stacov files, there was little overlap in the selection of days. (Every seventh file was used). Also, because the velocity sigmas are dominated by the process noise, there is little effect on the final sigmas of the velocity estimates.

The SIO frame was realized by rotating and translating the velocity field onto zero motion for the North American sites and the Pacific plate motion velocities of the Pacific sites given by the ITRF2000 North America/Pacific Euler pole. The RMS fit to this frame was 0.7 mm/yr in both North and East components. The JPL frame was realized by rotating and translating on the SIO site velocities of site in California. (The JPL stacov files contain only stations in this region).

The statistics of the differences are:

For the 374 site names in this analysis:

Horizontal components only (2-D): WRMS difference 0.44 mm/yr, NRMS 0.50

3-D components: WRMS 0.75 mm/yr, NRMS 0.8

If we estimate a rotation and translation between the two fields, these differences are changed by only a small amount

2-D WRMS 0.42 mm/yr; NRMS 0.48 and

3-D WRMS 0.74 mm/yr, NRMS 0.8.

(WRMS is Weighted Root-Mean-Square, and NRMS is Normalized Root-Mean-Square and is equal to (square root of Chi squared per degree of freedom).

“Remaining Sites Analysis”

The second analysis adopted a reference frame based on the 374-sites from the above analysis. Time series were generated in this analysis by rotating and translating each daily combined file on the reference frame. Linear trends were fit to North, East and Height components from the time series, and for sites affected by the Hector Mine earthquake, logarithmic time variations after the earthquake

The overall statistics of these time series are for JPL for 448 site names:

NORTH : 50% < 1.1 (mm) 70% < 1.4 (mm) 95% < 2.6 (mm)

NORTH : Mean (mm) : 1.3 Sigma (mm) : 0.6 Stations: 473

EAST : 50% < 1.3 (mm) 70% < 1.6 (mm) 95% < 3.1 (mm)

EAST : Mean (mm) : 1.6 Sigma (mm) : 0.9 Stations: 474

UP : 50% < 4.1 (mm) 70% < 4.8 (mm) 95% < 10.0 (mm)

UP : Mean (mm) : 4.9 Sigma (mm) : 2.8 Stations: 472

For SIO:

Statistics are for 508 sites (not individual locations):

NORTH : 50% < 0.8 (mm) 70% < 1.0 (mm) 95% < 2.1 (mm)

NORTH : Mean (mm) : 1.0 Sigma (mm) : 0.6 Stations: 572

EAST : 50% < 0.8 (mm) 70% < 1.0 (mm) 95% < 2.3 (mm)

EAST : Mean (mm) : 1.1 Sigma (mm) : 0.7 Stations: 572

UP : 50% < 2.7 (mm) 70% < 3.3 (mm) 95% < 6.3 (mm)

UP : Mean (mm) : 3.2 Sigma (mm) : 1.6 Stations: 572

Analysis of the Times series determined velocities

The velocity estimates from the time series used a correlated noise model to compute the standard deviations of the estimates. (The algorithm used is described the tsview documentation in GGMatlab under the Realistic Sigma option.

Between the two time series based velocities fields there are 454 common site names (separate velocities before and after earthquakes). The statistics of the comparison of the two fields are:

For 454 sites names:

2-D WRMS 0.50 mm/yr; NRMS 1.31

3-D WRMS 0.60 mm/yr; NRMS 1.33

If a rotation and translation between the two fields is estimated, the statistics are:

2-D WRMS 0.48 mm/yr; NRMS 1.24

3-D WRMS 0.58 mm/yr; NRMS 1.29

The NRMS values are high because of the white noise statistics assumption.

Terms and Definitions

Stacov files

JPL distribution of position results with full covariance matrices. Results are from point positioning with bias fixing applied in networks. After Dec 2002, stacov files are available only in separate networks that are combined into single networks using the GLOBK software. Prior to Dec 2002, the combined files we generated by JPL. Stacov files contain only position estimates, covariance matrices and heights of the dual frequency position of most of the antennas in the network. No antenna type or phase center models are given and we assume that the antennas are Dorne-Margolin antennas with zero phase center models. In some cases, we know this is not the case but there is no information in the files about the actual values used.

H-files

SIO distribution of position. Earth orientation and orbit results will full covariance matrices and antenna information. The files are available as separate networks that cover the globe. We combine the networks for each day into a single network. The files contain full information on antenna types and phase center models used.