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Pearson C.F.,National Geodetic Survey | Pearson C.F.,University of Otago | Snay R.S.,National Geodetic Survey Retired | McCaffrey R.,Portland State University
International Association of Geodesy Symposia | Year: 2014

This paper describes a block model of tectonic deformation for the western part of the contiguous United States (i.e. west of longitude 100°W and between latitudes 31°N and 49°N). NOAA’s National Geodetic Survey (NGS) uses velocities predicted by this model as the basis of the horizontal velocity grids incorporated in Horizontal Time-Dependent Positioning (HTDP) software. We model the deformation of this area using 46 rotating blocks. Along with the poles of rotation, we also solve for 38 independent horizontal strain rate tensors and the elastic coupling coefficients on faults that bound adjacent blocks. For the release of HTDP 3.1, we updated estimates of model parameters by using 6,287 GPSderived velocity vectors, that include vectors from the 2009 Plate Boundary Observatory (PBO) solution and the NGS Multiyear CORS (MYCORS) solution, and 330 geological measurements of fault slip rates and/or fault orientation. In general, the fault slip rates and the interseismic coupling coefficients are consistent with the results of previous studies; however, because of the comprehensive nature of this model, we are able to quantitatively map deformation rates over the entire plate boundary zone within the contiguous United States. Slip rates on the faults range from over 30 mm/year for the Cascadia subduction zone and parts of the San Andreas system to near zero for faults adjacent to stable North America. Block rotations play a significant role in accommodating deformation in the Pacific Northwest but make amuch smaller contribution south of CapeMendocino. Because of the variable gradient of the velocity field, HTDP3.1 incorporates a hierarchy of 5 grids with a spacing ranging from 4 nodes per degree to 100 nodes per degree. © Springer-Verlag Berlin Heidelberg 2014.

Pearson C.,University of Otago | Freymueller J.,University of Alaska Fairbanks | Snay R.,National Geodetic Survey Retired
ISCORD 2013: Planning for Sustainable Cold Regions - Proceedings of the 10th International Symposium on Cold Regions Development | Year: 2013

Crustal motion in the western United States and Alaska, due primarily to tectonic forces, causes positional coordinates of points on the Earth's surface to change over time. As a result, accurate surveying in these areas requires an equally accurate description of this deformation to allow survey measurements conducted at different times to be corrected for such movement. NOAA's National Geodetic Survey (NGS) has developed the horizontal time-dependent positioning (HTDP) software that enables its users to make these corrections. HTDP may also be used to transform coordinates measured at one time to corresponding coordinates that would be measured at some other time. To accomplish these tasks, HTDP contains numerical models describing interseismic horizontal crustal velocities. HTDP also contains models describing the displacements associated with 30 major earthquakes, including two in Alaska. NGS updates the HTDP software periodically to incorporate newer models. Over the last few years, NGS has made substantial advances in developing models to allow coordinates and survey measurements in Alaska to be corrected for crustal motion. This includes models describing Alaska's horizontal velocity field, and models for both the coseismic and postseismic displacements associated with the 2002 Denali Earthquake. The Denali postseismic model is the first such model included in a version of HTDP. © 2013 American Society of Civil Engineers.

Griffiths J.,Washington Technology | Ray J.,National Geodetic Survey retired
Geophysical Journal International | Year: 2016

While it has been known for some time that offsets in the time-series of Global Navigation Satellite System (GNSS) position estimates degrade station velocity determinations, the magnitude of the effect has not been clear. Using products of the International GNSS Service (IGS), we assess the impact empirically by injecting progressively larger numbers of artificial offsets and solving for a series of long-term secular GNSS frames. Our results show that the stability of the IGS global frame datum is fairly robust, with significant effects at the formal error level only for the Rx (and Y-pole) and Rz rotational orientations. On the other hand, station velocity estimates are more seriously affected, especially the vertical component. For the typical IGS station, the mean vertical rate uncertainty is already limited to 0.34 mm yr-1 for the current set of position discontinuities. If the number of breaks doubles, which might occur using newer detection schemes, then that uncertainty will worsen by ~40 per cent to 0.48 mm yr-1. This error source is generally a more important component of realistic velocity uncertainties than any other, including accounting for temporal correlations in the GNSS data. The only way to improve future GNSS velocity estimates is to severely limit manmade displacements at the tracking stations. © The Authors 2015.

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