SCIGN MIT Report

December 7, 2002

Thomas Herring, MIT contribution

 

Last Updated Wednesday, December 11, 2002

 

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 October 12, 2002.  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.
  1. 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 only sites in the latitude/longitude box and time interval given above.

 

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, only the sites in the SCIGN latitude/longitude box were used for analysis.  (Since the representation of the solution is in covariance matrix space, the step simply requires creating a covariance matrix that contains the rows and columns of the desired sites.) The orbits were loosely constrained in these combinations.

 

The combinations resulted in 2472 daily files for SOPAC (the expected number) and 2456 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 have been less restrictive in the selection of the core sites and we had added an additional 6 months of data (recently made available by JPL).

 

“Good Site Combination”

 

Because a number of SCIGN sites have systematic temporal variations and in some cases poor data quality during some intervals, we selected 264 “sites” in the SCIGN box that had more than 500-days of data and an RMS scatter in the horizontal position estimates of less than 2 mm.  This selection was based on previous on the preliminary SCIGN analysis of September 2002.  The 264 “sites” represent 195 distinct stations because in our treatment of sites, we rename stations after earthquakes and if there are non-modeled jumps in time series (often due to raydome and/or antenna changes). 

 

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 included random walk process noise in the filter for 160 days after the earthquake.  The minimum process noise was 0.1 mm2/day with it increasing for sites closer to the epicenter.  We also renamed sites that had unexplained jumps.  This information is contained in the globk earthquake file labn.eq.  A notable change to this file from the September analysis is that we now allow 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.

 

 To keep the velocity sigmas realistic we added random walk process noise to all sites of 0.3 mm2/yr for both analyses.  The effect of the process noise is to limit the velocity sigmas to no less than 0.17 mm/yr.  The combination was performed with loose constraints on the positions and velocities.  The final loose solutions were rotated and translated onto a common, nominally North America fixed frame derived from the SCEC Crustal Motion Model Version 3.0.  (Since the JPL stacov files were limited to sites in Southern California, it was not possible to use site located on stable North America).

 

The two sets of velocity estimates are given in http://bowie.mit.edu/~tah/SCIGN/JPL_VEL_1202.vel for the JPL analysis http://bowie.mit.edu/~tah/SCIGN/SIO_VEL_1202.vel for the SOPAC analysis.

In these files the columns labeled dNv, dEv and dHv are the adjustments to the apriori velocity of the site.  The site names are in two parts:

Example SCMS_CHT.  The first 4-characters are the standard code for the site; the character after the underscore (_) is normally G but if there is a break in the time series then a numeric value or letter is used (1 denoting the site name after the first break, the letter C as in the case above denotes the change to the milled choke rings); the last two characters if not PS denote the name after an earthquake.  For these files, LA is Landers, NR is Northridge and HT is Hector mine (see labn.eq).  For antenna changes, the velocities of sites before and after the change have been forced to be the same.  (Breaks due to earthquakes have different before and after earthquake velocities.)

 

The results are shown graphically in the following figures (all error ellipses are 95% confidence intervals):

http://bowie.mit.edu/~tah/SCIGN/Fig1_1202.jpg is the over laid plot of the two sets of results.  Black is SOPAC and Red is JPL.  Multiple vectors at the one site are pre- and post-Hector Mine velocities.

http://bowie.mit.edu/~tah/SCIGN/Fig1_1202.A0.jpg is an A0 size version of this figure.

 

http://bowie.mit.edu/~tah/SCIGN/Fig2_1202.jpg is the direct difference between the two velocity fields.  Note the change in scale; most of the differences are < 1 mm.yr

http://bowie.mit.edu/~tah/SCIGN/Fig2_1202.A0.jpg is an A) size version of the previous field and

http://bowie.mit.edu/~tah/SCIGN/Fig2_1202.LA.jpg is a zoom of the LA Basin area and

http://bowie.mit.edu/~tah/SCIGN/Fig2_1202.LA.A0.jpg is an A0 size zoom of the LA Basin area.

 

The statistics of the differences are:

For the 264 site names in this analysis:

Horizontal components only (2-D): WRMS difference 0.71 mm/yr, NRMS 2.56

3-D components: WRMS 0.86 mm/yr, NRMS 2.66

If we estimate a rotation and translation between the two fields, these difference reduce to:

2-D WRMS 0.60 mm/yr; NRMS 2.18 and

3-D WRMS 0.74 mm/yr, NRMS 2.28.

(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 264-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.

 

For JPL:

http://bowie.mit.edu/~tah/SCIGN/sujpl_rep_1202.ens contains the RMS scatters by component and site; and

http://bowie.mit.edu/~tah/SCIGN/jpl_rep_1202.vel contains a velocity field extracted from the time series.  Warning: Since the linear fits assume white noise, the sigmas on the velocities here are very small.

Plots of the time-series can be found at:

http://bowie.mit.edu/~tah/SCIGN/TimeSeries/JPL_plots/ where the blue points and statistics are for the JPL files.  The red points are an overlay of the SOPAC results.

http://bowie.mit.edu/~tah/SCIGN/TimeSeries/JPL_1202.tar is a tar file containing the ASCII time series values.  Files ending in 1 are for North, 2 for East and 3 for height.  In these files, the part of the component greater than 10 m is removed.  The full value is given on the second line of the file.  The removal of the large part of the values allows the files to be used in low precision plotting programs.  (The tar file is 27Mbytes)

 

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

NORTH   :  50% < 1.2 (mm)    70% < 1.5 (mm)    95% < 3.8 (mm)

NORTH   :  Mean (mm) : 1.6    Sigma (mm) : 1.2      Stations: 448

EAST    :  50% < 1.4 (mm)    70% < 1.8 (mm)    95% < 3.1 (mm)

EAST    :  Mean (mm) : 1.7    Sigma (mm) : 1.1      Stations: 448

UP      :  50% < 4.2 (mm)    70% < 4.8 (mm)    95% < 9.1 (mm)

UP      :  Mean (mm) : 4.9    Sigma (mm) : 3.3      Stations: 448

 

 

For SIO:

A similar set of files exists for the SIO files:

http://bowie.mit.edu/~tah/SCIGN/susio_rep_1202.ens

http://bowie.mit.edu/~tah/SCIGN/sio_rep_1202.vel. Warning: Since the linear fits assume white noise, the sigmas on the velocities here are very small.

 

http://bowie.mit.edu/~tah/SCIGN/TimeSeries/SIO_plots/

For these plots SIO is blue and JPL is red.  Statistics are for SIO solution,

http://bowie.mit.edu/~tah/SCIGN/SIO_1202.tar (The tar file is 33Mbytes)

 

Statistics are for 508 sites (not individual locations):

NORTH   :  50% < 0.8 (mm)    70% < 1.1 (mm)    95% < 2.4 (mm)

NORTH   :  Mean (mm) : 1.0    Sigma (mm) : 0.8      Stations: 508

EAST    :  50% < 0.8 (mm)    70% < 1.0 (mm)    95% < 2.5 (mm)

EAST    :  Mean (mm) : 1.1    Sigma (mm) : 0.8      Stations: 508

UP     :  50% < 2.9 (mm)    70% < 3.6 (mm)    95% < 6.9 (mm)

UP     :  Mean (mm) : 3.6    Sigma (mm) : 2.7      Stations: 508

 

Analysis of the Times series determined velocities

 

The standard deviations of the velocities from the times series are too small because of the white noise assumption which can be clearly seen to be violated by many (and on close examination all) time series.  The comparisons below use the standard deviation for weighting the root-mean-square calculation and sites whose velocities have very small standard deviations will tend to dominate the statistics.

 

Between the two time series based velocities fields there are 440 common site names (separate velocities before and after earthquakes).  The statistics of the comparison of the two fields are:

 

For 440 sites:

2-D WRMS 0.60 mm/yr; NRMS 5.59

3-D WRMS 0.76 mm/yr; NRMS 5.93

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

2-D WRMS 0.56 mm/yr; NRMS 5.27

3-D WRMS 0.71 mm/yr; NRMS 5.52

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

 

Details of the field based in the time series analysis

 

In the figures below we show some details of the fields from time series analysis.  In these figures we have multiplied the velocity standard deviations by 6 as rough means of generating more realistic error ellipses.  We limit the result shown to velocities with sigmas of less than 3 mm/yr after rescaling.  In the figures, the SIO results are the red vectors with green error ellipses and the JPL results are blue vectors with blue error ellipses.  The error ellipses are 95% confidence intervals.
The tools we are using for this comparison are available at
http://www-gpsg.mit.edu/~tah/GGMatlab

 

 

We firstly show all the sites with re-scaled sigmas of less than 3 mm/yr (in either North or East).

Through the velview controls we can examine individual parts of the field in detail.

 

North Eastern Region: Offset removed –3.0 mm/yr East, 0.1 mm/yr North (see Offset box). 

The offset is estimated from the SIO analysis (Field 1).

Offset Results from 16 sites (right-click to remove)

Comp         Wmean (mm)     WRMS  NRMS

North     0.06 ±   0.09    0.35  1.221

East     -3.00 ±   0.03    0.14  0.502

Height   -0.78 ±   0.07    0.28  1.808

Site        NRes +-       ERes +-        URes +-

BEAT_GLA  -0.20   0.24  -0.02   0.24  -0.72   0.14

BULL_GLA   0.51   0.24  -0.03   0.24  -0.00   0.13

BUST_GHT   0.27   0.30  -0.01   0.30   0.17   0.16

CHLO_GHT   0.59   0.30  -0.31   0.30  -0.19   0.17

CRAT_GHT   0.29   0.36  -0.07   0.36   0.11   0.20

JOHN_GHT  -0.09   0.42   0.21   0.42  -0.10   0.25

LITT_GHT  -0.08   0.36   0.11   0.30  -0.18   0.20

MERC_GHT  -0.35   0.36   0.12   0.30   0.42   0.19

PERL_GLA   0.06   0.24  -0.13   0.24   0.15   0.12

POIN_GHT  -0.18   0.36   0.21   0.30   0.12   0.19

RELA_GHT   0.25   0.30  -0.11   0.30  -0.02   0.16

REPO_GLA  -0.34   0.24  -0.13   0.24  -0.13   0.13

SKUL_GHT  -0.01   0.30  -0.04   0.30  -0.12   0.18

STRI_GHT   0.31   0.30   0.22   0.30   0.04   0.15

TATE_GLA  -0.16   0.24   0.14   0.24   0.42   0.13

TIVA_GLA  -0.60   0.24   0.01   0.24   0.04   0.12

 

 

Northern Area around Yellowstone.

 

In this figure we also show the vertical rates, which are large at some sites, and the estimates are consistent between the JPL and SIO analysis.  The CASA site is known to have large non-secular variations, which explain the rate differences at this site (due to sampling over different intervals).  The MINS site also disagrees between the JPL and SIO analysis and we show the time series for this sites generated using tsview (tsview can be invoked by clicking on the velocity vectors in the velview plots).  The difference in rate is due to additional data in the SIO analysis at a time when the site deviated by a substantial amount from its nominal secular rate.

 

 

Time series plot for SIO analysis:

 

Time series plot for JPL analysis:

 

Hector Mine region

 

Velocities around the Hector Mine earthquake region showing points in a profile across the region.

 

 

Values shown on the profile plot.  A simple “arc-tangent “ fit to the profile values.  (Results shown are only for the SIO analysis.)

There are 10 points in profile

Profile: From 244.230 34.306 to 242.823 34.810

Length   140.52 km, Azimuth -65.96 deg; Width   9.87 km.

Velocities projected along azimith -48.33 deg

 Dst km          Along          Cross              Height      OffProf (km)   long     lat  Name

   15.29   -0.77 ±    0.33   -1.84 ±    0.33   -1.33 ±    0.19    -0.96 244.0819  34.3699 OPBL_GHT

   29.04    0.20 ±    0.45   -5.81 ±    0.45   -1.46 ±    0.29     5.47 243.9166  34.3671 OPCP_GHT

   37.42   -0.46 ±    0.31   -5.31 ±    0.31   -0.11 ±    0.16     1.50 243.8505  34.4301 OPCX_GHT

   40.82   -1.18 ±    0.48   -1.38 ±    0.48   -0.79 ±    0.29    -6.55 243.8518  34.5086 NBPS_GHT

   50.40    7.91 ±    0.36   -7.78 ±    0.36    2.04 ±    0.20     7.52 243.6945  34.4277 OPCL_GHT

   54.00    8.19 ±    0.51   -3.44 ±    0.51    2.14 ±    0.32    -3.73 243.7077  34.5330 OPRD_GHT

   68.23   13.45 ±    0.30   -3.11 ±    0.30   -1.69 ±    0.18    -4.96 243.5706  34.5943 AGMT_GHT

   86.82   15.87 ±    0.30    0.71 ±    0.30   -1.15 ±    0.19    -2.89 243.3753  34.6439 RDMT_GHT

  104.17   16.40 ±    0.81   -0.59 ±    0.81   -1.13 ±    0.49     0.86 243.1849  34.6749 ORMT_GHT

  113.70   18.72 ±    0.45   -0.21 ±    0.45   -1.79 ±    0.28     5.70 243.0683  34.6690 WOMT_1HT

 

 

Los Angeles Basin

 

In this figure we show a detailed view of the LA basin area.  To avoid to much clutter we have restricted the velocities to sites with re-scaled sigmas of less than 1.0 mm/yr and we have shown 50% confidence ellipses.