Lateral uniformity of India Plate strength over central and eastern Nepal

The current understanding of the Himalayan lithosphere stems mostly from cross-sections through the range at the longitude of the Kathmandu Basin. In this paper we laterally extend the analyses of structures and rheology along the Nepal Himalayas between the Pokhara valley and the Arun river. We take advantage of available information and a new data set including gravity measurements and a receiver function profile. It appears that the geometry of the Moho inferred from seismological profiles and long-wavelength gravity anomalies does not exhibit major East-West variations within the 350-km-wide study area. Using thermomechanical modelling, we show that the northward deepening of the Moho observed along profiles perpendicular to the main thrust faults can be interpreted simply as the bending of a strong India Plate. This result suggests a gradual mechanical decoupling between the crust and the mantle, leading to a northward decrease of the effective elastic thickness of the Indian lithosphere from∼75 km to∼25 km beneath the Ganga Basin and the Tibetan Plateau, respectively. Our results also confirm (partially) eclogitized lower Indian crust beneath southern Tibet. At shorter wavelengths, the observed gravity profiles exhibit some small lateral variations that can be interpreted in terms of east-west variations of the thickness of subsurface geological structures such as the Ganga Basin and the Tethyan Sedimentary Sequenc

A GPS network for tropospheric tomography in the framework of the Mediterranean hydrometeorological observatory Cévennes-Vivarais (south-eastern France)

International audienceThe Mediterranean hydrometeorological observatory Cévennes-Vivarais (OHM-CV) coordinates hydrometeorological observations (radars, rain gauges, water level stations) on a regional scale in southeastern France. In the framework of OHM-CV, temporary GPS measurements have been carried out for 2 months in autumn 2002, when the heaviest rainfall are expected. These measurements increase the spatial density of the existing permanent GPS network, by adding three more receivers between the Mediterranean coast and the Cévennes-Vivarais range to monitor maritime source of water vapour flow feeding the precipitating systems over the Cévennes-Vivarais region. In addition, a local network of 18 receivers covered an area of 30 by 30 km within the field of view of the meteorological radar. These regional and local networks of permanent and temporary stations are used to monitor the precipitable water vapour (PWV) with high temporal resolution (15 min). Also, the dense local network provided data which have been inverted using tomographic techniques to obtain the 3-D field of tropospheric water vapour content. This study presents methodological tests for retrieving GPS tropospheric observations from dense networks, with the aim of assessing the uncertainties of GPS retrievals. Using optimal tropospheric GPS retrieval methods, high resolution measurements of PWV on a local scale (a few kilometres) are discussed for rain events. Finally, the results of 3-D fields of water vapour densities from GPS tomography are analysed with respect to precipitation fields derived from a meteorological radar, showing a good correlation between precipitation and water vapour depletion areas

Crowdsourcing hypothesis tests: Making transparent how design choices shape research results

To what extent are research results influenced by subjective decisions that scientists make as they design studies? Fifteen research teams independently designed studies to answer fiveoriginal research questions related to moral judgments, negotiations, and implicit cognition. Participants from two separate large samples (total N > 15,000) were then randomly assigned to complete one version of each study. Effect sizes varied dramatically across different sets of materials designed to test the same hypothesis: materials from different teams renderedstatistically significant effects in opposite directions for four out of five hypotheses, with the narrowest range in estimates being d = -0.37 to +0.26. Meta-analysis and a Bayesian perspective on the results revealed overall support for two hypotheses, and a lack of support for three hypotheses. Overall, practically none of the variability in effect sizes was attributable to the skill of the research team in designing materials, while considerable variability was attributable to the hypothesis being tested. In a forecasting survey, predictions of other scientists were significantly correlated with study results, both across and within hypotheses. Crowdsourced testing of research hypotheses helps reveal the true consistency of empirical support for a scientific claim.</div

Extracting small deformation beyond individual station precision from dense Global Navigation Satellite System (GNSS) networks in France and western Europe

International audienceWe use 2 decades of data from a dense geodetic network to extract regionally coherent velocities and deformation rates in France and neighboring western European countries. This analysis is combined with statistical tests on synthetic data to quantify the deformation detection thresholds and significance levels. By combining two distinct methods – Gaussian smoothing and k-means clustering – we extract horizontal deformations with a 95 % confidence level of ca. 0.1–0.2 mm yr−1 (ca. 0.5–1×10−9 yr−1) on spatial scales of 100–200 km or more. From these analyses, we show that the regionally average velocity and strain rate fields are statistically significant in most of our study area. The first-order deformation signal in France and neighboring western European countries is a belt of N–S to NE–SW shortening of ca. 0.2–0.4 mm yr−1 (1–2×10−9 yr−1) in central and eastern France. In addition to this large-scale signal, patterns of orogen-normal extension are observed in the Alps and the Pyrenees, but methodological biases, mainly related to GPS (Global Positioning System) solution combinations, limit the spatial resolution and preclude associations with specific geological structures. The patterns of deformation in western France show either tantalizing correlation (Brittany) or anticorrelation (Aquitaine Basin) with the seismicity. Overall, more detailed analyses are required to address the possible origin of these signals and the potential role of aseismic deformation

Assessment of GPS data for meteorological applications over Africa: Study of error sources and analysis of positioning accuracy

International audienceThe aim of this study is to assess the availability and quality of data from the International GNSS Service (IGS) Global Positioning System (GPS) network in Africa, especially for retrieving zenith tropospheric delay (ZTD), from which precipitable water vapour (PWV) can be derived, in view of application to the African Monsoon Multidisciplinary Analysis (AMMA) project. Three major error sources for the GPS data analysis evaluating PWV in Africa are the accuracy of the satellite orbits, the correction for the radio delay induced by the ionosphere and the vertical site displacements due to ocean loading. The first part of this study examines these error sources and the validity of GPS data for meteorological applications in Africa in dedicated analyses spanning the year 2001. These analyses were performed using the IGS precise orbits. Weak degradation of baseline precision with increasing baseline lengths suggests that the average orbital error is not limiting the GPS analysis in Africa. The impact of the ionosphere has been evaluated during a maximum of solar activity in 2001. The loss of L2 data has actually been observed. It amounts to 2% on average for 2001, with maxima of 8% during magnetic storm events. A slight decrease in formal accuracy of ZTD seems to be related to the loss of L2 data at the end of the day. This indicates that scintillation effects are present in the GPS observations but however are not a major limitation. The impact of ocean loading is found to be significant on ZTD estimates (up to ±2 mm in equivalent PWV). The use of a proper ocean loading model eliminates this effect. The second aspect of this study concerns the IGS analysis quality for the African stations. The accuracy has been assessed through position dispersion between individual solutions and the most recent version of the IGS combined solution IGb00, and residuals from the transformation of the IGS combined solution into the International Terrestrial Reference Frame 2005. The positioning performance of the IGS analysis is consistent with an accuracy in ZTD of ±6 mm (±1 mm in PWV), as requested for meteorological applications such as planned in AMMA

Contribution of artificial tracing and geophysical methods to understanding flows in the non-saturated area of karst (St Ferron site, France)

peer reviewedGeophysical methods allow imaging the structure of the epikarst. However, studying groundwater transfer in a non-saturated zone is still a challenging task. Tracing is then used to remedy this problem and to relate water flow with geological structures. These studies are all the more important than the epikarst is a vulnerable area with important water reserve. In order to validate the hypotheses issued on the basis of the geophysical methods, a multi tracer experiment was realized on the St Ferron site (Larzac, France). The study showed the importance of using tracing in addition to geophysical methods to validate the hypotheses on the flows and thus better know the epikarst and its functioning.Les méthodes géophysiques permettent d’imager la structure de l’épikarst, mais l’étude des transferts dans la zone non saturée reste une tâche difficile. Les traçages sont dès lors utilisés pour remédier à ce problème et relier les écoulements aux structures géologiques. Ces études sont d’autant plus importantes que l’épikarst est une zone vulnérable, présentant d’importants stocks d’eau. Afin de valider les hypothèses émises sur la base des méthodes géophysiques, un multi-traçage a été réalisé sur le site de St Ferron (Larzac, France). L’étude montre l’importance d’utiliser des traçages en complément des méthodes géophysiques pour valider les hypothèses sur les écoulements et ainsi mieux connaître l’épikarst et son fonctionnement

IWV observations from a network of ground-based GNSS receivers during EUREC4A

International audienceIWV data were retrieved from a network of nearly fifty Global Navigation Satellite System (GNSS) stations distributed over the Caribbean arc for the period 1 January-29 February 2020 encompassing the EUREC4A field campaign. Two of the stations had been installed at the Barbados Cloud Observatory (BCO) during fall 2019 in the framework of the project and are still running. All other stations are permanent stations operated routinely from various geodetic and geophysical organisations in the region. High spatial and temporal Integrated Water Vapour (IWV) observations will be used to investigate the atmospheric environment during the life cycle of convection and its feedback on the large-scale circulation and energy budget.&lt;/p&gt;&lt;p&gt;This paper describes the ground-based GNSS data processing details and assesses the quality of the GNSS IWV retrievals as well as the IWV estimates from radiosoundings, microwave radiometer measurements and ERA5 reanalysis.&lt;/p&gt;&lt;p&gt;The GNSS results from five different processing streams run by IGN and ENSTA-B/IPGP are first intercompared. Four of the streams were run operationally, among one was in near-real time, and one was run after the campaign in a reprocessing mode. The uncertainties associated with each of the data sets, including the zenith tropospheric delay to IWV conversion methods and auxiliary data, are quantified and discussed. The IWV estimates from the reprocessed data set are compared to the Vaisala RS41 radiosonde measurements operated from the BCO and to the measurements from the operational radiosonde station at Grantley Adams international airport (GAIA). A significant dry bias is found in the GAIA humidity observations with respect to the BCO sondes (-2.9 kg/m2) and the GNSS results (-1.2 kg/m2). A systematic bias between the BCO sondes and GNSS is also observed (1.7 kg/m2) where the Vaisala RS41 measurements are moister than the GNSS retrievals. The HATPRO IWV estimates agree with the BCO soundings after an instrumental update on 27 January, while they exhibit a dry bias compared to GNSS and BCO sondes before that date. ERA5 IWV estimates are overall close to the GAIA observations, probably due to the assimilation of these observations in the reanalysis. However, during several events where strong peaks in IWV occurred, ERA5 is shown to significantly underestimate the IWV peaks. Two successive peaks are observed on 22 January and 23/24 January which were associated with heavy rain and deep moist layers extending from the surface up to altitudes of 3.5 and 5 km, respectively. ERA5 significantly underestimates the moisture content in the upper part of these layers. The origins of the various moisture biases are currently being investigated.&lt;/p&gt

On the small‐scale fractal geometrical structure of a living coral reef barrier

International audienceTopographical complexity of coral reefs is of primary importance for a number of hydrodynamical and ecological processes. The present study is based on a series of high‐resolution seabottom elevation measurements along the Maupiti barrier reef, French Polynesia. Several statistical metrics and spectral analysis are used to characterize the spatial evolution of the coral geometrical structure from the reef crest to the backreef. A consistent fractal‐like power law exists in the spectral density of bottom elevation for length‐scales between 0.1 and 7m while, at larger scale, the reef structure shows a different pattern. Such a fine characterization of the reef geometrical structure provides key elements to reconstruct the reef history, to improve the representation of reef roughness in hydrodynamical models and to monitor the evolution of coral reef systems in the global change context

Integrated water vapour observations in the Caribbean arc from a network of ground-based GNSS receivers during EUREC4A

International audienceGround-based Global Navigation Satellite System (GNSS) measurements from nearly fifty stations distributed over the Caribbean Arc have been analysed for the period 1 January-29 February 2020 in the framework of the EUREC 4 A (Elucidate the Couplings Between Clouds, Convection and Circulation) field campaign. The aim of this effort is to deliver high-quality Integrated Water Vapour (IWV) estimates to investigate the moisture environment of mesoscale cloud patterns in the Tradewinds and their feedback on the large-scale circulation and energy budget. This paper describes the GNSS data processing procedures and assesses the quality of the GNSS IWV retrievals from four operational streams and one reprocessed research stream which is the main data set used for offline scientific applications

Co-Seismic Surface Displacement Induced by the Bam Earthquake, Iran (26/12/2003, M=6.6): Insights from InSAR, GPS, SPOT5 Analyses and Levelling

Co-seismic surface deformation measurements in the vicinity of a ruptured fault provide constraints on detailed fault geometry and slip distribution at depth. Together with seismological data, they give unique insights on the mechanical behaviour of a seismic fault. Three different satellite and ground geodetic measurements of Bam earthquake (Mw 6.6, December 26, 2003) induced surface deformation are presented. Envisat ASAR interferometry provides precise and dense information. However, due to this strike-slip fault orientation, sub-pixel correlation technique applied to Spot-5 images makes more explicit the horizontal component of surface deformation. We complete these oblique and horizontal estimations of deformation with a levelling profile along the main road crossing the rupture from west to east. This geodetic data allows us to propose a dislocation model at depth. The slip vector, on a quasi-vertical fault, slightly dipping towards east, has a strike-slip component as high as 2m, while the dip-slip component appears to be small. We suggest that rupture may have been initiated at depth on the Bam fault and propagated towards surface along this new fault branch. In addition to co-seismic deformation, InSAR analysis and levelling data reveal the presence of a high-rate subsiding zone southeast of Bam city. The phenomenon is likely due to heavy water withdrawal for cultivation purpose or water supply to the Bam and Baravat inhabitants.Ultimately, we present a work in progress involving GPS and InSAR which aims to map post-seismic deformation in the vicinity of Bam. However, technical problems in GPS campaigns and atmospheric artifacts in InSAR acquisitions did not enable us to show any evidence of such a deformation so far