MAGNET GPS Station - Photo by Geoff Blewitt

Explanation of Time Series Figures on NGL’s Station Pages

Plots

Time series plots are provided for 24 hour and 5 minute sample rate solutions. Each blue dot is an individual estimate of site position, the plot axes are scaled automatically to accommodate the data time span and range of positions (which have been demeaned).

Times of nearby earthquakes and known equipment change events are marked with gray and cyan vertical dashed lines, respectively. Information about the earthquake and equipment events are provided in a table below the time series on the station page, with links to the USGS earthquake pages for that event. A “nearby” earthquake is one that is within 10^(M/2 - 0.8) km of the station, where M is the magnitude of the event. This is an approximation of the radius of maximum influence of the event and does not guarantee that a significant offset will appear in the time series at that time, or will not appear for stations at greater distance.

“Cleaned” time series are provided. These have had outlying position solutions removed to clarify the plots in cases where the presence of outliers expand the vertical scale of the plots, making it difficult to see details of most of the data.

The red curve on the plots is an estimate of a model that fits the data. The slope of the curve is estimated using the MIDAS algorithm (Blewitt et al., 2016), while the other parameters (intercept, annual and semiannual oscillations, step magnitudes) are estimated using least squares with the value of the trend assumed. MIDAS rates are estimated for all stations with at least one year of data and sufficiently dense time series. Tables of these rates are available on our data products page. If no MIDAS rate is available, then we solve for a rate using damped least squares. In cases where a large earthquake (M>6.9) has occurred near enough to the station we solve for an exponential decay function with a form of A(1-exp(-(t-t0)/tau))H(t-t0) where t0 is the time of the earthquake, tau is a relaxation time, A is the amplitude of the decay, and H is the Heaviside step function. In these cases we re-solve for the background trend after the exponential terms have been removed to obtain a self-consistent model for the time series.

Graphics are generated using the GMT software (Wessel et al., 2013), and provided in .png format.

Reference Frames

Plots are currently provided in several reference frames. For all stations time series are provided in IGS14. This frame is a global Earth-centered Earth-fixed, GPS-only realization of the International Terrestrial Reference Frame 2014 (Altamimi et al., 2016). ITRF14 has a coordinate origin that is aligned to the Earth center of mass.

Time series and MIDAS velocities are also provided in 25 plate fixed reference frames. For each station the frame used depends on the location of the station, i.e., on which plate the station lies. These products are still based on the IGS14 solutions, however, the horizontal component time series have the predicted plate motion trends removed so that portrayed motion is with respect to that plate. Each station is associated with at least one tectonic plate. Although in some cases where a station is near one or more other tectonic plate boundaries (within 100 km) the time series are detrended according to the additional plate(s) motion as well. Vertical component time series are unaffected.

Detrended time series are also provided. For these plots the all three component time series are detrended with the MIDAS rate, if it is available. If MIDAS rates are not available for the station, least squares rates are determined and used to detrended the time series.

References

University of Nevada, Reno
Last edited 27 November 2019.