SOPAC Refined Velocities Documentation

SOPAC's refined velocities are generated using a model (Nikolaidis, 2002) that takes into account offsets (co-seismic or otherwise), linear velocity, and annual and semi-annual fluctuations (for stations that have at least two years and one year of data, respectively). For stations that experience post-seismic relaxation, an exponentially decaying term is also estimated. In some cases where a station experiences co-seismic and post-seismic deformation, changes in the linear velocity are also estimated.

Input data provided to the model are the daily processed site positions calculated at SOPAC using the GAMIT and GLOBK software packages.

All parameters are estimated with full white noise + flicker noise covariances based on a noise analysis of the GPS position time series. Their uncertainties can thus be considered realistic estimates, based on our data analysis.

Velocities are valid for the reference epoch shown at each site, usually the last day of site data available for the weekly model run. Note that these velocities are calculated using the method described above. Therefore, this "current" velocity changes over time depending on any annual, semi-annual or decay terms present. Further, if a site has multiple velocities, due to co-seismic or other offsets, the old velocities are not shown. Use the refined model terms utility to display all of the model-estimated parameters for a site.

Velocities for IGS core reference frame sites are also derived from the model, and may differ from the official values in the ITRF2000 IGS core site file.

Please contact the webmaster if you have any questions.

Reference:
Nikolaidis, R., Observation of Geodetic and Seismic Deformation with the Global Positioning System, Ph.D. Thesis, University of California, San Diego, 2002.