Laurent Métivier

ITRF2008 plate motion model

[1]xa0The ITRF2008 velocity field is demonstrated to be of higher quality and more precise than past ITRF solutions. We estimated an absolute tectonic plate motion model made up of 14 major plates, using velocities of 206 sites of high geodetic quality (far from plate boundaries, deformation zones and Glacial Isostatic Adjustment (GIA) regions), derived from and consistent with ITRF2008. The precision of the estimated model is evaluated to be at the level of 0.3xa0mm/a WRMS. No GIA corrections were applied to site velocities prior to estimating plate rotation poles, as our selected sites are outside the Fennoscandia regions where the GIA models we tested are performing reasonably well, and far from GIA areas where the models would degrade the fit (Antarctica and North America). Our selected velocity field has small origin rate bias components following the three axis (X, Y, Z), respectively 0.41xa0±xa00.54, 0.22xa0±xa00.64 and 0.41xa0±xa00.60 (95 per cent confidence limits). Comparing our model to NNR-NUVEL-1A and the newly available NNR-MORVEL56, we found better agreement with NNR-MORVEL56 than with NNR-NUVEL-1A for all plates, except for Australia where we observe an average residual rotation rate of 4xa0mm/a. Using our selection of sites, we found large global X-rotation rates between the two models (0.016°/Ma) and between our model and NNR-MORVEL56 of 0.023°/Ma, equivalent to 2.5xa0mm/a at the Earth surface.

Strategies to mitigate aliasing of loading signals while estimating GPS frame parameters

Although GNSS techniques are theoretically sensitive to the Earth center of mass, it is often preferable to remove intrinsic origin and scale information from the estimated station positions since they are known to be affected by systematic errors. This is usually done by estimating the parameters of a linearized similarity transformation which relates the quasi-instantaneous frames to a long-term frame such as the International Terrestrial Reference Frame (ITRF). It is well known that non-linear station motions can partially alias into these parameters. We discuss in this paper some procedures that may allow reducing these aliasing effects in the case of the GPS techniques. The options include the use of well-distributed sub-networks for the frame transformation estimation, the use of site loading corrections, a modification of the stochastic model by downweighting heights, or the joint estimation of the low degrees of the deformation field. We confirm that the standard approach consisting of estimating the transformation over the whole network is particularly harmful for the loading signals if the network is not well distributed. Downweighting the height component, using a uniform sub-network, or estimating the deformation field perform similarly in drastically reducing the amplitude of the aliasing effect. The application of these methods to reprocessed GPS terrestrial frames permits an assessment of the level of agreement between GPS and our loading model, which is found to be about 1.5xa0mm WRMS in height and 0.8xa0mm WRMS in the horizontal at the annual frequency. Aliased loading signals are not the main source of discrepancies between loading displacement models and GPS position time series.

Hydrological deformation induced by the West African Monsoon: Comparison of GPS, GRACE and loading models

Three-dimensional ground deformation measured with permanent GPS stations in nWest Africa was used for investigating the hydrological loading deformation associated nwith Monsoon precipitation. The GPS data were processed within a global network for the n2003–2008 period. Weekly station positions were retrieved with a repeatability (including nunmodeled loading effects) of 1–2 mm in the horizontal components and between 2.5 nand 6 mm in the vertical component. The annual signal in the vertical component for nsites located between 9.6N and 16.7N is in the range 10–15 mm. It is consistent at the n3 mm-level with the annual regional-scale loading deformations estimated from GRACE nsatellite products and modeled with a combination of hydrological, atmospheric, and nnontidal oceanic models. An additional 6 month transient signal was detected in the vertical ncomponent of GPS estimates at most of the West African sites. It takes the form of an noscillation occurring between September and March, and reaching a maximum amplitude of n12–16 mm at Ouagadougou (12.5N). The analysis of in situ hydro-geological data revealed na strong coincidence between this transient signal and peak river discharge at three sites nlocated along the Niger River (Timbuktu, Gao, and Niamey). At Ouagadougou, a similar ncoincidence was found with the seasonal variations of the water table depth. We propose a nmechanism to account for this signal that involves a sequence of swelling/shrinking of clays ncombined with local loading effects associated with flooding of the Niger River.

Characterizing long-time scale hydrological effects on gravity for improved distinction of tectonic signals

The influence of the hydrological noise on repeated gravity measurements has been investigated on the basis of the time series of 18 superconducting gravimeters (SGs) and on predictions inferred from the Land Dynamics (LaD) world-Gascoyne land water-energy balances model. Presently, the global hydrologic models are not precise enough to fulfill the geodetic requirements and are not efficient enough to separate the hydrology from tectonic motion in the land-based gravity time series. However, although the LaD model predictions and the gravity observations present significant differences in the time domain, it is shown that they have similar amplitudes in the frequency domain in most of the cases. The time series of the Global Geodynamics Project make it possible to investigate phenomena of a few years in the best case. Given the similarity between the power spectral densities (PSDs) of the LaD model predictions and the SG measurements when taken at the same epoch, it makes sense to use the LaD model to study the spectral behavior of the hydrological effects down to the decadal time scale, which is not yet possible with land-based measurements. It is shown that the PSDs of the hydrological effects flattens at low frequency and is characterized by a generalized Gauss-Markov structure. With such a noise level, the time necessary to measure a gravity rate of change of 1 nm/s(2)/a, at the 1 sigma level should not extend any longer than 17 years at the locations where the hydrological effects play a major role

ITRF2008 contribution to glacial isostatic adjustment and recent ice melting assessment

[1]xa0We investigate what information station vertical velocities of the ITRF2008 provide on global geodetic parameters and by extension on glacial isostatic adjustment (GIA) and recent ice melting (RIM) processes. We infer degree-2 spherical harmonic coefficients (SHC) of the Earth figure change and theJ2 gravity rate (J˙2), which we compare with five GIA models. We find J˙2 to be (0.0 ± 2.4) × 10−11 yr−1, which is consistent with recent studies that propose a J˙2 change in the 1990s, due to RIM whose contribution to the J˙2 would be today around (3.5–4.0 ± 2.4) × 10−11 yr−1. Such results favor Peltier (2004) VM2 or Paulson et al. (2007) GIA models. The ITRF2008 SHC that are directly impacted by the GIA rotational feedback, confirm with a good precision recent results from GRACE mission that initiated a debate on GIA rotational feedback and about Peltier GIA model quality. We find a coefficient consistent with Paulsons (and other) model and more than 7 times smaller than coefficients in Peltiers models. Two explanations are possible: (1) if the model of Peltier (2004) VM2 were to be correct, then the strong rotational feedback in the model must be counteracted by a strong rotational feedback in the opposite direction generated by current ice loss, (2) if the model of Paulson et al. (2007) were to be correct, therefore GIA and RIM separately induce negligible rotational feedbacks. Both answers are quite extreme and call for more investigation on GIA modeling and rotational feedback.