Quantification Of Geodetic Strain Rate Uncertainties And Implications For Seismic Hazard Estimates

Geodetic velocity data provide first-order constraints on crustal surface strain rates, which in turn are linked to seismic hazard. Estimating the 2-D surface strain tensor everywhere requires knowledge of the surface velocity field everywhere, while geodetic data such as Global Navigation Satellite System (GNSS) only have spatially scattered measurements on the surface of the Earth. To use these data to estimate strain rates, some type of interpolation is required. In this study, we review methodologies for strain rate estimation and compare a suite of methods, including a new implementation based on the geostatistical method of kriging, to compare variation between methods with uncertainty based on one method. We estimate the velocity field and calculate strain rates in southern California using a GNSS velocity field and five different interpolation methods to understand the sources of variability in inferred strain rates. Uncertainty related to data noise and station spacing (aleatoric uncertainty) is minimal where station spacing is dense and maximum far from observations. Differences between methods, related to epistemic uncertainty, are usually highest in areas of high strain rate due to differences in how gradients in the velocity field are handled by different interpolation methods. Parameter choices, unsurprisingly, have a strong influence on strain rate field, and we propose the traditional L-curve approach as one method for quantifying the inherent trade-off between fit to the data and models that are reflective of tectonic strain rates. Doing so, we find total variability between five representative strain rate models to be roughly 40 per cent, a much lower value than roughly 100 per cent that was found in previous studies (Hearn et al.). Using multiple methods to tune parameters and calculate strain rates provides a better understanding of the range of acceptable models for a given velocity field. Finally, we present an open-source Python package (Materna et al.) for calculating strain rates, Strain 2D, which allows for the same data and model grid to be used in multiple strain rate methods, can be extended with other methods from the community, and provides an interface for comparing strain rate models, calculating statistics and estimating strain rate uncertainty for a given GNSS data set

NASA Geodynamics Program

Activities and achievements for the period of May 1983 to May 1984 for the NASA geodynamics program are summarized. Abstracts of papers presented at the Conference are inlcuded. Current publications associated with the NASA Geodynamics Program are listed

Strategies for long-term monitoring of tide gauges using GPS

Changes in mean sea (MSL) level recorded relative to tide gauge benchmarks (TGBM) are corrupted by vertical land movements. Accurate estimates of changes in absolute sea level, require these MSL records to be corrected for ground level changes at tide gauge sites. For more than a decade, the Global Positioning System (GPS) has been used to determine positions of TGBMs and to monitor their position changes, i.e. station velocities, over time in the International Terrestrial Reference System (ITRS). This was initially carried out by episodic GPS campaigns and later on by continuous GPS (CGPS) or a combination of both. Highly accurate realizations of the ITRS, satellite orbits and models for the mitigation of systematic effects currently enable the determination of station positions using GPS at the centimetre or even millimetre level. It is however argued that accurate long--term estimates of changes in the vertical component at the 1mm/yr level cannot be achieved, making intercomparisons between GPS estimates and other techniques necessary. Daily processing and analysis of continuous GPS networks requires automated procedures. The modifications and improvements to the existing procedures at the IESSG are described. The newly developed tools include the monitoring and quality control of daily archived GPS observations and of processing results. A special focus is on the coordinate time series analysis and methodologies used to obtain the best possible estimates of vertical station velocities and associated uncertainties. The coordinate time series of 21 CGPS stations in the UK and France are analysed. Eight of these stations are co-located with tide gauges. The effects of two processing strategies and two realizations of the ITRS on the coordinate time series are investigated. Filtered coordinate time series are obtained by application of a regional filtering technique. Station velocity estimates are obtained by fitting a model including a linear and annual term, and offsets to the unfiltered and filtered coordinate time series. Realistic uncertainties for these velocities are obtained from the application of two empirical methods which account for coloured noise in the coordinate time series. Results from these are compared to the Maximum Likelihood Estimation (MLE), which allows for more rigorous and accurate, simultaneous estimation of the model parameters and their uncertainties. Strategies for coordinate time series analysis on a daily or monthly, and annual or bi-annual basis are defined. At two CGPS stations the dual-CGPS station concept is tested and compared to the single baseline analysis and the application of an adaptive filter. An empirical method to obtain coordinate time series specific filter parameters is described. This investigation shows that reliable relative vertical station velocity estimates can be obtained after much shorter observation spans than absolute vertical station velocity estimates. The availability of dual-CGPS station pairs allows a simplified processing strategy and a multitude of coordinate time series analysis methods, all contributing to a better understanding of the variations in the positions of CGPS stations. Vertical station velocity estimates for the unfiltered and filtered coordinate time series and different analysis strategies are compared for 17 of the CGPS stations and show disagreements of up to 2mm/yr. At the eight CGPS stations co-located with or close to tide gauges alternative estimates of vertical land/crustal movements from absolute gravimetry, geological information and glacial isostatic adjustment models are compared to the GPS estimates, and it is suggested that the latter are systematically offset. An alignment procedure is demonstrated, correcting the vertical station velocity estimates of all 17 CGPS stations for this offset. The correlation of the geology-aligned vertical station velocity estimates and the MSL records from eight tide gauges suggests changes in absolute sea level of approximately +1mm/yr around the UK

Strategies for long-term monitoring of tide gauges using GPS

Changes in mean sea (MSL) level recorded relative to tide gauge benchmarks (TGBM) are corrupted by vertical land movements. Accurate estimates of changes in absolute sea level, require these MSL records to be corrected for ground level changes at tide gauge sites. For more than a decade, the Global Positioning System (GPS) has been used to determine positions of TGBMs and to monitor their position changes, i.e. station velocities, over time in the International Terrestrial Reference System (ITRS). This was initially carried out by episodic GPS campaigns and later on by continuous GPS (CGPS) or a combination of both. Highly accurate realizations of the ITRS, satellite orbits and models for the mitigation of systematic effects currently enable the determination of station positions using GPS at the centimetre or even millimetre level. It is however argued that accurate long--term estimates of changes in the vertical component at the 1mm/yr level cannot be achieved, making intercomparisons between GPS estimates and other techniques necessary. Daily processing and analysis of continuous GPS networks requires automated procedures. The modifications and improvements to the existing procedures at the IESSG are described. The newly developed tools include the monitoring and quality control of daily archived GPS observations and of processing results. A special focus is on the coordinate time series analysis and methodologies used to obtain the best possible estimates of vertical station velocities and associated uncertainties. The coordinate time series of 21 CGPS stations in the UK and France are analysed. Eight of these stations are co-located with tide gauges. The effects of two processing strategies and two realizations of the ITRS on the coordinate time series are investigated. Filtered coordinate time series are obtained by application of a regional filtering technique. Station velocity estimates are obtained by fitting a model including a linear and annual term, and offsets to the unfiltered and filtered coordinate time series. Realistic uncertainties for these velocities are obtained from the application of two empirical methods which account for coloured noise in the coordinate time series. Results from these are compared to the Maximum Likelihood Estimation (MLE), which allows for more rigorous and accurate, simultaneous estimation of the model parameters and their uncertainties. Strategies for coordinate time series analysis on a daily or monthly, and annual or bi-annual basis are defined. At two CGPS stations the dual-CGPS station concept is tested and compared to the single baseline analysis and the application of an adaptive filter. An empirical method to obtain coordinate time series specific filter parameters is described. This investigation shows that reliable relative vertical station velocity estimates can be obtained after much shorter observation spans than absolute vertical station velocity estimates. The availability of dual-CGPS station pairs allows a simplified processing strategy and a multitude of coordinate time series analysis methods, all contributing to a better understanding of the variations in the positions of CGPS stations. Vertical station velocity estimates for the unfiltered and filtered coordinate time series and different analysis strategies are compared for 17 of the CGPS stations and show disagreements of up to 2mm/yr. At the eight CGPS stations co-located with or close to tide gauges alternative estimates of vertical land/crustal movements from absolute gravimetry, geological information and glacial isostatic adjustment models are compared to the GPS estimates, and it is suggested that the latter are systematically offset. An alignment procedure is demonstrated, correcting the vertical station velocity estimates of all 17 CGPS stations for this offset. The correlation of the geology-aligned vertical station velocity estimates and the MSL records from eight tide gauges suggests changes in absolute sea level of approximately +1mm/yr around the UK

Geodetic Sciences

Space geodetic techniques, e.g., global navigation satellite systems (GNSS), Very Long Baseline Interferometry (VLBI), satellite gravimetry and altimetry, and GNSS Reflectometry & Radio Occultation, are capable of measuring small changes of the Earth�s shape, rotation, and gravity field, as well as mass changes in the Earth system with an unprecedented accuracy. This book is devoted to presenting recent results and development in space geodetic techniques and sciences, including GNSS, VLBI, gravimetry, geoid, geodetic atmosphere, geodetic geophysics and geodetic mass transport associated with the ocean, hydrology, cryosphere and solid-Earth. This book provides a good reference for geodetic techniques, engineers, scientists as well as user community

Eighth International Workshop on Laser Ranging Instrumentation

The Eighth International Workshop for Laser Ranging Instrumentation was held in Annapolis, Maryland in May 1992, and was sponsored by the NASA Goddard Space Flight Center in Greenbelt, Maryland. The workshop is held once every 2 to 3 years under differing institutional sponsorship and provides a forum for participants to exchange information on the latest developments in satellite and lunar laser ranging hardware, software, science applications, and data analysis techniques. The satellite laser ranging (SLR) technique provides sub-centimeter precision range measurements to artificial satellites and the Moon. The data has application to a wide range of Earth and lunar science issues including precise orbit determination, terrestrial reference frames, geodesy, geodynamics, oceanography, time transfer, lunar dynamics, gravity and relativity