Continuous GPS monitoring of crustal deformation with the Western Canada Deformation Array: 1992-1995
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The objective of this thesis is to conduct research on high-precision. Continuous GPS monitoring of crustal deformation in the northern Cascadia subduction zone, which is located in southwestern British Columbia and known to be one of the most seismically active regions in North America.
Although conventional geodetic measurements were made in the past showing a consistent deformation pattern in the region, these measurements have relatively large uncertainties. In contrast, GPS, as a modern geodetic techniques, provides the best means for deformation monitoring: higher accuracy, lower cost, more efficient, and near-real time. The Western Canada Deformation Array (WCDA) is a GPS network designed to monitor the crustal deformation with high precision. It has been in operation since its establishment in the summer of 1992 and is still under development.
The daily data collected from the WCDA stations have been reduced using the CGPS22 software package and the precise orbits generated by Natural Resources Canada (NRCan) or the International GPS Service for Geodynamics (IGS). A specific estimation strategy has been designed and different measurement models have been tested in order to achieve high accuracies. Based on the estimation strategy, 811 days of daily solutions spanning from September 1992 to April 1995 have been obtained and analyzed in particular.
Problems associated with hardware and errors due to scattering effects have been detected and identified from the results. The scattering effects at stations DRAO, ALBH and WILL caused offsets in the vertical component and the east component of the baselines (Latter for the ALBH-DRAO only) in the solutions. Meanwhile, the east component of the baseline ALBH-DRAO also suffered an apparent annual variation due possible to the same scattering effects. All these offsets and variation display a systematic dependence on the choice of elevation or cutoff angle.
Small annual variations were found in the north component of most, if not all, of the WCDA baselines and these variations were analyzed with great emphasis. Five tests were carried out in order to identify the source of these annual variations. From the results of these tests, some factors, such as tidal effects and tropospheric mismodelling, can probably be discounted. A test on the GPS satellite antenna offsets reveals a similarity in features between the annual variations and an error caused by an uncalibrated GPS antenna offset along the satellite local x-axis. This leads to a suspicion that a systematic error at a few cm level in the precise orbit/EOP data is the cause of the annual variations, though further efforts are required to verify this claim.
The precision assessment for the WCDA solutions shows long-term repeatabilities of2 to 5 mm in horizontal components and 6 to 9 mm in vertical components for baseline lengths ranging from 302 to 627 km. Linear rates in each of the baseline components have been estimated by the Least Squares Spectral Analysis algorithm, along with offsets and periodic constituents. In general, these linear rates show a deformation pattern consistent with that obtained from previous conventional measurements. However, a high resolution on crustal deformation signals and a better separation of GPS measurement errors from the deformation signals would require a longer set of solutions and more effort on error handling.