Past, Present and Future GNSS Applications for Geodynamics
Nocquet, J-M
CNRS - University of Nice Sophia-Antipolis

The constantly increasing accuracy of GNSS positioning, the denser spatial resolution provided by GNSS networks and the better temporal resolution achieved by GNSS positioning has revolutionized our knowledge of the Earth’s surface deformation. My presentation will show a few examples of GPS contribution to Geodynamics in the past years, introduce the present challenges in tectonics and possible applications of GNSS for the next years.

About two decades separates the onset of the plate tectonics theory to the first direct measurement of plate motion using space geodetic techniques. During those two decades, kinematics of plates could only be derived using indirect and partial measurements and required to assume that the motion of plate has remained constant over the past million years. Space geodetic techniques achieved the accuracy to detect the present-day motion of tectonics plate in the mid 1980’s. In the early 1990’s, the first space-based geodetic plate kinematic models were proposed and are now regularly refined. While a general good agreement is usually found between plate motion averaged over a few million years and present-day motion monitored by geodesy, new global space geodesy solutions revealed significant discrepancies, indicating possible recent changes in plate motion in the Mediterranean (Eurasia/Nubia/Arabia plates) and the Andes (South America/Nazca). Documenting recent plate motion changes provides new insight on the forces driving plate motion and their time evolution. However, the application of high-accuracy positioning has not been restricted to the determination of plate motion. Recent space geodesy studies provide new quantitative information on the rigidity of continental plate interior. Residual velocities after removing are usually around 0.5 mm/yr and do not decrease as longer time series become available. Several studies on the noise properties of GNSS time series (Williams et al., 2004) shows that the power spectrum of GNSS of residual position time series are dominated by long term time correlated noise. As a consequence, velocity estimates are improving very slowly with longer data span. The presence of time correlated noise in GNSS position time series therefore still limits the accuracy of GNSS ability to measure slow deformation. On the other hand, other studies indicate that for Europe and North America, the signature of Glacial Isostatic Adjustment can be detected in the northern part of those continents. Understanding the origin of time correlated noise and improving the velocity determination in order to monitor intraplate deformation remains a challenge for the next years. Indeed, any improvement of velocity estimates, and in particular on the vertical component, would bring new quantification of GIA effects and a better knowledge of the viscosity of the Earth mantle and contribute to the seismic hazard assessment of intraplate domain where large earthquakes have occur in the past (like the Rhine Graben in western Europe and the New Madrid Zone in central US) with little convincing explanation on the stress accumulation in those areas.

Another major frontier in geophysics is to understand the processes leading to the occurrence of large earthquakes. Until recently, it was assumed that steady constant motion of fault was increasing the stress along faults. However, permanent GPS network have revealed the presence of transient motions in several subduction zones. These transients show that the loading of fault before earthquake was more complex and not constant in time. Better monitoring these transient movements has become a major issue for the next years. Their characteristics, the amount of stress released in such events, their role in the whole earthquake cycle has still to be understood any properly modelled. Because some of these transient might constitute precursory signal of large future earthquakes, it is essential to develop new observation and strategies to detect them.