Forrest, N.D. (2010) The determination of subtle deformation signals using a permanent CGPS network in the Aegean. Doctoral thesis, UCL (University College London).
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Geophysical motions can occur over a broad temporal spectrum, from high frequency seismic movements to very long period tectonic deformation. The Aegean region is tectonically one of the most active areas on Earth. There have, over the past 15 years, been a range of campaign style GPS studies which have looked to increase our knowledge of the area and better define the geodynamic processes involved. In 2002 the Center for the Observation and Modelling of Earthquakes and Tectonics (COMET) established a network of continuously operating GPS receivers (CGPS) throughout the region in order to add to the knowledge gained from previous studies. This thesis focuses on which tectonic motions can be observed using the COMET continuous GPS network. Approaches for the precise analytical estimation of subtle tectonic motion are presented. Daily coordinate estimates of COMET sites and a number of ITRF (International Terrestrial Reference Frame) sites around Europe were calculated using a precise point positioning strategy and ambiguity resolution using NASA’s GIPSY – OASIS II processing software and IGS (International GPS Service) precise products. Time series produced showed post fit standard deviations of 2-3 mm in the horizontal and 6-8 mm in the vertical. Significant annual periodic variation is observed in the time series. The coordinate time series studies were further refined using a selection of filters. Firstly, gross and sigma filters were applied to remove outliers, the data then had a range of regional filters applied looking to best define and remove the common mode error in the area. These filters produced mixed results with time series improvement occurring on a site by site basis. In some cases noise was reduced by a factor of 2 whilst in other cases there was little or no improvement. This combined with a lack of knowledge of the individual site movements led to the use of a filtered baseline method, whereby common mode error was removed purely on a site by site basis. This method revealed expansion across the Hellenic arc of the order of a few millimetres per year and sub millimetre north-south compaction behind the arc. It also revealed first evidence of transient motion at a number of sites parallel to the Hellenic arc. The transient signals occurred every 12 months ±1.5 and lasting for 40 – 100 days. These signals were not so much a reversal of tectonic motion akin to the silent earthquakes observed in Cascadia, Japan and Mexico, instead they appeared more as a pause in the otherwise consistent movement of the Aegean microplate overriding the subducting African lithosphere. In addition to the observed tectonic signals, the effects and implications of the two post processing strategies are analysed and discussed. Higher temporal frequency positioning is carried out on seismic events (Mw 6.7 earthquake Kithera, Mw 8.1 and Mw 6.7 earthquakes, Macquarie island) using instantaneous positioning followed by “sidereal filtering” whereby integer-cycle phase ambiguities are resolved using only single epochs of dual frequency phase and pseudorange data. These positions are then siderealy stacked to reduce the effects of geometry related error. The technique reduces geometry related noise by a factor ≈2 using epoch by epoch 30 second data. The feasibility of the technique for observing pre, co and post seismic signals is demonstrated. A visualisation tool was developed to allow the simultaneous observation of the tectonic motion of a CGPS network data over any spatial and temporal regimes.
|Title:||The determination of subtle deformation signals using a permanent CGPS network in the Aegean|
|Open access status:||An open access version is available from UCL Discovery|
|UCL classification:||UCL > School of BEAMS > Faculty of Engineering Science > Civil, Environmental and Geomatic Engineering|
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