Abstract
Most organisations who maintain regional terrestrial reference frames base their realisations on the global International Terrestrial Reference Frame (ITRS) that uses space based positioning techniques (e.g. GNSS, ITRF, SLR, DORIS) to realize the International Terrestrial Reference Frame (e.g. ITRF2005, ITRF2008). Where ITRF sites are located within stable plates, the coordinate motion is largely of a linear nature. However, sites that are located within plate boundary zones, can exhibit non linear motion that is the result of other geophysical processes such as earthquake events, volcanic activity, subduction, subsidence and up lift. In such zones, a precise coordinate frame transformation that provides the linkages between global and regional ITRF realisations are not valid. Many of these processes exhibit motion that is time dependent and hence more complex models are required in order to accurately model the deformation. This in turn has implications for national geodetic infrastructure and land surveying applications e.g. Network RTK (NRTK) and GIS.
Although periodic earthquake events that cause co seismic displacement are a consequence of t he tectonic situation in New Zealand, a greater challenge is the time dependent transient deformation caused by post seismic relaxation and slow slip events (SSE). NRTK systems require accurate, current epoch (or instantaneous) site coordinates, which can be di fficult to determine when a site periodically (and unpredictably) undergoes slow slip deformation. In addition, most NRTK systems use simple predictive models (e.g. position and velocity only) and do not have any ability to account for non linear deformation.
This paper describes some of the methods and results that are being developed to model these complex deformation events. In particular, examples from NRTK sites will be used to illustrate the transient nature of the deformation both in time and space.