Base de données FMHex20 pour la France métropolitaine et les régions voisines

This illustration is from the FMHex20 database for metropolitan France and neighboring regions. (a) Stereographic projections (lower hemisphere) of individual focal mechanisms. Symbol sizes are proportional to magnitudes. White dashed boxes indicate regional figures in the supplementary material. (b) Average grid fault style. Blue (1): normal, yellow (0): lateral slip, red (1): reverse. Symbol size is inversely proportional to standard deviations s (large: s 0.25, medium: 0.25 0.5). (c) and (d) Average grid orientations of the quasi-horizontal axes P (red) and T (blue). Light-colored fan shapes indicate standard deviations s. The FMHex20 database is a product of the Hazards axis of the Resif-Epos Seismicity Transverse Action. Résif-Epos is a national research infrastructure dedicated to the observation and understanding of the structure and dynamics of the Internal Earth. Résif is based on high-tech observation networks, composed of seismological, geodetic and gravimetric instruments deployed in a dense manner throughout France. The data collected make it possible to study with high spatial and temporal resolution the deformation of the ground, surface and deep structures, seismicity on a local and global scale and natural hazards, and more particularly seismic events, on French territory. Résif is part of the European (Epos - European Plate Observing System) and global systems of instruments used to image the Earth's interior as a whole and to study many natural phenomena.Cette illustration est issue de la base de données FMHex20 pour la France métropolitaine et les régions voisines. (a) Projections stéréographiques (hémisphère inférieur) des mécanismes focaux individuels. Les tailles des symboles sont proportionnelles aux magnitudes. Les cases en pointillés blancs indiquent les figures régionales dans le matériel supplémentaire. (b) Style de faille moyen de la grille. Bleu (1) : normal, jaune (0) : glissement latéral, rouge (1) : inverse. La taille des symboles est inversement proportionnelle aux écarts types s (grand : s 0,25, moyen : 0,25 0,5). (c) et (d) Orientations moyennes sur la grille des axes quasi-horizontaux P (rouge) et T (bleu). Les formes en éventail de couleur claire indiquent les écarts types s. La base de données FMHex20 est un produit de l'axe Aléas de l'Action transverse sismicité de Résif-Epos. Résif-Epos est une infrastructure une infrastructure de recherche nationale dédiée à l’observation et la compréhension de la structure et de la dynamique Terre interne. Résif-Epos se base sur des réseaux d’observation de haut niveau technologique, composés d’instruments sismologiques, géodésiques et gravimétriques déployés de manière dense sur tout le territoire français. Les données recueillies permettent d’étudier avec une haute résolution spatio-temporelle la déformation du sol, les structures superficielles et profondes, la sismicité à l’échelle locale et globale et les aléas naturels, et plus particulièrement sismiques, sur le territoire français. Résif s’intègre aux dispositifs européens (Epos - European Plate Observing System) et mondiaux d’instruments permettant d’imager l’intérieur de la Terre dans sa globalité et d’étudier de nombreux phénomènes naturels

Sea-level rise assessment for Brotish Columbia coastal communities, In : Climate Change Geoscience program : 2006-2011 program final report, A.N. Rencz (ed.), Geological Survey of Canada Open File, 6879

ReportThis activity combines tide gauge and geodetic data to determine the 20th century trends and patterns of relative and absolute sea level changes along the coastline of British Columbia. This information is then used to derive scenarios of relative sea-level rise in coastalcommunities for the 21st century. Particular emphasis is put on the Fraser River Delta and Greater Vancouver region. The projections of 21 st century sea-level rise are provided to provincial and municipal partners to inform local and regional land-use and planning policies

Control of tectonic inheritance on continental intraplate strain rate and seismicity

International audiencePresent-day deformation and seismicity of continental lithosphere are characterized by a first-order dichotomy between Plate Boundary Zones (PBZ) and Stable Continental Regions (SCR). Whereas the former are associated with high strain and seismicity rates, the latter tend to remain un-deformed, except in localized regions of higher strain and seismicity, commonly related to fossilized paleo-PBZ acting as locally weaker domains. Because of their low amplitudes, these intraplate strain and seismicity rates are particularly difficult to measure and characterize. In this study, we propose a simple model to explain and quantify first-order continental strain rate variations, focusing on intraplate regions. Assuming near-failure equilibrium on 1D lithosphere profiles, we derive steady-state strain rates driven by tectonic forces as a function of rheological models that include new strain-weakening rheologies in order to simulate tectonic inheritance. Within this framework, inherited strain-weakening plays a fundamental role in allowing for and explaining strain and seismicity concentration in intraplate weak zones: our model predicts strain rates in intraplate weak zones up to two to three orders of magnitude higher compared to stable cratons due to the effect of tectonic inheritance on rheology weakening. These model predictions are in agreement with empirical estimations in intraplate regions and can provide a conceptual framework for characterization of SCR strain and seismicity rates

Comparative analysis between three GNSS datasets

International audienceIn Western Europe, the Western Alps is one of the most seismically active regions with a large density of earthquakes and some historical damaging earthquakes. There is no consensus on the driver of seismicity in the region, although the African-Eurasian tectonic convergence hypothesis is not consistent with the seismic extension in the inner part of the Alps. The last decade's improvement of GNSS coverage allows a sufficient resolution on strain rate map to point out deformation of 10-9 yr-1, necessary to study active processes in low deformation region. With the final goal to study surface processes as a driver of seismicity, the objective of this study is to give a robust strain rate map of the Western Alps. We used three datasets processed independently (two velocity fields communicated; one velocity field computed from available position time series in this study) to identify common signals. The precise comparison between datasets confirms the radial extension rate in the inner part of the Alps. The strain rate map computed in this study shows significant deformation, that could be linked with surface processes and micro-plate rotation

Quantitative impact of structural inheritance on present-day deformation and seismicity concentration in intraplate deformation zones

International audienceStructural inheritance (i.e. paleo-tectonic) areas, acting as weakened domains, appear to be a key element localizing the seismicity in intraplate deformation zones. However, the impact of structural inheritance on the observed present-day seismicity and strain rate concentration remains to be quantified. In this study, we quantify through 2D numerical modeling the localization and amplification factor of upper crustal strain rates induced by structural inheritance. Our 2D models are constrained by intraplate velocity boundary conditions and include rheology laws that accounts for inherited strain weakening in both the brittle and ductile layers of the lithosphere. The role of structural inheritance is investigated for different localization of the weakened domain in the lithosphere. For an average intraplate geotherm (Moho temperature ca. 500 °C), brittle weakening (i.e. inherited faults) alone induces a limited amplification factor of upper crustal strain rates of ca. 4. Ductile weakening can increase the amplification factor to ca. 7 when localized in the lower crust, but has no effect when localized in the lithospheric mantle. Overall, the amplification factors of upper crustal strain rates vary between 1 and 27 depending on the location of the weakened area in the lithosphere and on the different possible net driving forces, crustal strengths, amounts of weakening, and geotherms. These model amplification factors are in reasonable agreement with those derived from GPS and seismicity data over large spatial scale (several hundreds of kilometers) in North America

GravProcess: An easy-to-use MATLAB software to process campaign gravity data and evaluate the associated uncertainties

International audienceWe present GravProcess, a set of MATLAB routines to process gravity data from complex campaign surveys and calculate the associated gravity field. Data reduction, analysis, and representation are done using the MATLAB Graphical User Interface Tool, which can be installed on most systems and platforms. Data processing is divided into several steps: (1) Integration of gravity data, station location, and gravity line connection input files; (2) Gravity data reduction applying solid-Earth tide and instrumental drift corrections and, depending on the required processing level, air pressure and oceanic tidal corrections; (3) Automatic network adjustment and alignment to absolute base stations; (4) Free air and terrain corrections to calculate gravity values and anomalies, and to estimate the associated errors. The final step is dedicated to post-processing and includes graphical representations of data and an output text file, which can be used by Geographic Information System software. An example of this processing chain applied to a recent survey in northern Morocco is given and compared with previous available results

Analysis of GPS Measurements in Eastern Canada Using Principal Component Analysis

International audienceContinuous Global Positioning System (CGPS) position time series from eastern North America constrain the pattern and magnitude of regional crustal deformation. Initial analysis delineates consistent uplift patterns, as expected from glacial isostatic adjustment (GIA) predictions, but the associated horizontal deformation is not definitive, in part due to the short time periods for a significant number of the available stations. We employ an eigenpattern decomposition in order to define a unique, finite set of deformation patterns for this continuous GPS data. Similar in nature to the empirical orthogonal functions historically employed in the analysis of atmospheric and oceanographic phenomena, the method derives the eigenvalues and eigenstates from the diagonalization of the correlation matrix using a Karhunen-Loeve expansion (KLE). The KLE technique is used to identify the important modes in both time and space for the CGPS data, modes that potentially include signals such as horizontal and vertical GIA, tectonic strain, and seasonal effects. Here we filter both the vertical and horizontal velocity patterns on different spatiotemporal scales in order to study the potential geophysical sources, after the removal of correlated and random noise. The method is successful in disaggregating the linear vertical signal from more variable and less spatially correlated signals. The vertical and horizontal results are compared to the predictions of the ICE-3G GIA loading model with a number of plausible mantle viscosity profiles. The horizontal velocity analysis allows for qualitative differentiation between several potential GIA models and suggests that, with longer time series, this technique can be employed to remove correlated noise and improve estimates of crustal strain patterns and their sources

New analytical solution and associated software for computing full-tensor gravitational field due to irregularly shaped bodies

International audienceWe present a new analytical solution to compute the full-tensor gravity gradient due to a body mass of uniform density with arbitrary geometry. The solution is an extension of an existing analytical computation of gravitational anomalies of a polyhedron source, based on a transition of the general expressions from surface to line integrals. These developments enable the computation of the gravity gradient tensor using the same simple procedures as the gravitational field. The method is validated by comparing with a closed analytical solution, including on/in the near field of the body surface. The algorithm is implemented in the freely available MATLAB-based software called Gal Eötvös Earth Calculator. It is tested successfully for various measurement distances and body mass sizes, enabling applications from local geophysical prospecting to global topographic effect for satellite data. Due to its flexibility, the new solution, and the associated software, is particularly well suited for joint analyses of all types of gravity measurements regardless of the extent, altitude and irregularity of their spatial distribution

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