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Subduction zones are places of intense seismic activity where the largest ruptures occur. Studies on the causal mechanisms of subduction earthquakes generally focus on the accumulation of tectonic stress and strain at the shallow plates interface, which can be documented from surface displacements and seismic activity. The control exerted by deeper sudbuction processes is however not well understood. It can be addressed from time-varying satellite gravity data, that provide a new and unique means of studying mass redistributions at intermediate spatial and temporal scales throughout the volume around plate boundaries, and in particular at depht. Here we use gravity gradients from GRACE geoid to probe slow deep mass variations and their possible interactions with intraplate seismicity along the Chilean margin. We work with three different GRACE geoid models (GRGS, CSR,ITSG) from 2003 to 2014, over a large region surrounding the rupture zone of the Mw 8.8 2010 Maule earthquake. From these data we reconstruct the Earth’s gravity gradients at different spatial scales in order to better separate signals associated with mass sources of different sizes, shapes or orientations in the GRACE geoids. Our analysis reveals an anomalous gravity gradient signal north-east of the epicentral zone, which amplitude progressively increases during the months preceding the earthquake. This signal is consistently detected in all 3 GRACE solutions and we show that it cannot be explained by a water mass redistribution nor artefacts. Instead, it could be explained by an extension of the plunging Nazca plate near 150 km depth along the subduction direction. The migration of the gravity signal laterally and from the depths to the surface from a weakly coupled zone in the North to a strongly coupled zone in the South suggests that the Mw 8.8 earthquake may have originated the propagation of this deep slab deformation towards the surface. Our results highlight the importance of time series of satellite observations of the Earth’s gravity field, to detect and characterize mass redistributions at depth of major plate boundaries at timescales of month to years.

Retrieving short-term preseismic signals before the occurrence of great subduction earthquakes is a major goal for seismic hazard mitigation. It requires a continuous monitoring of seismic and aseimic deformations within the entire subduction system, which cannot be obtained from space geodetic measurements of ground deformations and seismology only. With its unique sensitivity to mass redistributions at all depths along and around the descending slab, satellite gravity can bring key information to overcome this observational gap. Taking the example of the 2011 Tohoku earthquake, we develop here a global and systematic analysis of time series of GRACE-reconstructed gravity gradients, to test whether the deep gravity signals preceeding the earthquake (Panet et al., 2018 ; Bouih et al. 2022) can be detected before the occurrence of the event, as a specific feature originating from solid Earth. Our approach is based on a refined detection of short-term gravity variations aligned with the subduction direction, and consistency tests between signals shared by different GRACE geoid models, allowing us to evaluate the signals robustness with respect to errors and oceanic corrections in the GRACE gravity solutions. Considering the GRGS and the CSR solutions, the results of these tests underline the unique character of the deep pre-Tohoku gravity variations compared to the usual variability of gravity, confirming its interpretation as pre-seismic signal. We compare the preseismic gravity signals of the 2011 Tohoku and 2010 Maule earthquakes, and conclude that our approach can be applied to the systematic monitoring of the Pacific subduction belt.

International audience; We conduct a global analysis of GRACE-reconstructed gravity gradients from July 2004 to February 2011, to test whether the deep signals preceding the March 2011 Tohoku earthquake can be detected before the event as a specific feature originating from solid Earth. First, we improve the angular resolution of the gravity gradients using two overlapping ranges of azimuthal sensitivity to investigate short-term signals of large amplitude aligned with the orientation of the Northwestern Pacific subduction. Then, we set-up a method to identify consistent solid Earth signals shared by different GRACE gravity models. Robust signals in a model are selected based on their spatial overlap and relative intensity with the signals of another model, so that their sensitivity to the GRACE data processing and ocean dealiasing product can be tested. We show that the dipolar gravity gradient anomaly before the Tohoku earthquake is nearly unique in space and time in the GRACE GRGS03 solutions. A well-resolved dipolar spatial pattern, typical of dislocations within the solid Earth and poorly sensitive to the ocean dealiasing model, is detected. In addition, the preseismic gravity gradient increase is highly consistent between the GRGS03 and CSR06 solutions, independently from their respective oceanic corrections, and can be clearly distinguished from rare anomalies of similar amplitudes all associated with the water cycle over continental areas. Our approach offers solutions for the continuous monitoring of the Pacific subduction belt to document transient slabs motions in real time from global satellite gravity fields, and their relation with shallower deformations and seismic events.

The AlpArray Gravity Research Group (AAGRG), as part of the European AlpArray program, focuses on the compilation of a homogeneous surface-based gravity data set across the Alpine area. In 2017 10 European countries in the Alpine realm agreed to contribute with gravity data for a new compilation of the Alpine gravity field in an area spanning from 2 to 23∘ E and from 41 to 51∘ N. This compilation relies on existing national gravity databases and, for the Ligurian and the Adriatic seas, on shipborne data of the Service Hydrographique et Océanographique de la Marine and of the Bureau Gravimétrique International. Furthermore, for the Ivrea zone in the Western Alps, recently acquired data were added to the database. This first pan-Alpine gravity data map is homogeneous regarding input data sets, applied methods and all corrections, as well as reference frames. Here, the AAGRG presents the data set of the recalculated gravity fields on a 4 km × 4 km grid for public release and a 2 km × 2 km grid for special request. The final products also include calculated values for mass and bathymetry corrections of the measured gravity at each grid point, as well as height. This allows users to use later customized densities for their own calculations of mass corrections. Correction densities used are 2670 kg m−3 for landmasses, 1030 kg m−3 for water masses above the ellipsoid and −1640 kg m−3 for those below the ellipsoid and 1000 kg m−3 for lake water masses. The correction radius was set to the Hayford zone O2 (167 km). The new Bouguer anomaly is station completed (CBA) and compiled according to the most modern criteria and reference frames (both positioning and gravity), including atmospheric corrections. Special emphasis was put on the gravity effect of the numerous lakes in the study area, which can have an effect of up to 5 mGal for gravity stations located at shorelines with steep slopes, e.g., for the rather deep reservoirs in the Alps. The results of an error statistic based on cross validations and/or “interpolation residuals” are provided for the entire database. As an example, the interpolation residuals of the Austrian data set range between about −8 and +8 mGal and the cross-validation residuals between −14 and +10 mGal; standard deviations are well below 1 mGal. The accuracy of the newly compiled gravity database is close to ±5 mGal for most areas. A first interpretation of the new map shows that the resolution of the gravity anomalies is suited for applications ranging from intra-crustal- to crustal-scale modeling to interdisciplinary studies on the regional and continental scales, as well as applications as joint inversion with other data sets. The data are published with the DOI https://doi.org/10.5880/fidgeo.2020.045 (Zahorec et al., 2021) via GFZ Data Services., Earth System Science Data, 13 (5), ISSN:1866-3516, ISSN:1866-3508

International audience; In the realisation of the International Height Reference System, the determination of the geopotential value and its variations in time plays an important role. In this study, the geodetic boundary value problem approach is applied for direct determination of the gravity potential value using a GOCE global gravity field model enhanced with terrestrial gravity data. This determination is carried out on the Global Navigation Satellite System-Continuously Operating Reference Stations (GNSS-CORS) stations towards the realisation of the International Height Reference System in Vietnam. First, the effects of the GOCE global gravity field model omission error, the indirect bias term on the disturbing potential and the systematic cumulative errors in levelling data are estimated. These errors affect the estimated geopotential value. The results calculated on the GNSS/levelling points show that the effect of the GOCE DIR-R5 (up to degree/order 260) omission error on the offset potential value is quite significant. This effect was eliminated using high-resolution terrestrial gravity data using the remove-compute-restore technique. The indirect bias term on the disturbing potential can be safely neglected by using a GOCE global gravity field model for degrees higher than 60 for this study region. The systematic cumulative errors in levelling data can be modelled and removed using a third-order polynomial model. Then, the mean zero-height gravity potential of the Vietnam local vertical datum is estimated equal to ${\rm{W}}_0^{{\rm{LVD}}}$ = 62 636 846.69 m2 s-2 with standard deviation of 0.70 m2 s-2 based on the proposed approach. Finally, the geodetic boundary value problem approach was used to determine the geopotential on the surface of three GNSS-CORS stations in Vietnam. Based on time-series of the vertical component derived from the GNSS observations as well as InSAR data, temporal variations in geopotential are also estimated on these permanent GNSS stations. The purpose is to monitor deformation of the vertical datum. The results indicate that the geopotential value needs to be monitored and determined with the time-dependent component on the three Vietnamese permanent GNSS stations for a vertical datum. These stations may contribute to increase the density of reference points in the International Terrestrial Reference Frame, which is being researched and implemented by the International Association of Geodesy.

Documenting the preparation phase of giant earthquakes and retrieving pre-seismic signals of upcoming events is a crucial challenge. Over the short term, various deformation transients have been detected before large subduction events, emphasizing in particular the role of the slab pull in driving plate motions (e.g., Bouchon et al., 2016 ; Bedford et al., 2020). Among them, we previously evidenced an anomalous gravity gradient signal during the months before the March 2011 Tohoku earthquake, likely originating from the solid Earth (Panet et al., 2018). We showed that it could reflect a broad deformation of the subducted slab prior to the event, generating the giant earthquake as the deformation propagated from depth to surface.Taking the example of the 2011 Tohoku earthquake, we conduct here a systematic and global retrospective analysis of time series of GRACE-reconstructed gravity gradients truncated in February 2011. Our aim is to test whether the gravity gradient variations preceeding the earthquake can be identified as singular and potentially originating from the solid Earth in an automated way and without knowledge on the upcoming event. First, we enhance the angular resolution of the gravity gradients in order to extract signals closely aligned with a chosen plate boundary orientation. Along this preferred orientation, we extract fast temporal variations at the sub-annual timescales of geodetic, gravitational and seismic signals reported before the event in previous studies. To evaluate the significance of the obtained gravity gradient anomalies, we design a method to extract coherent signals between different GRACE gravity field models and assess their sensitivity to the dealiasing ocean model. We present and discuss the results of these analyses applied to different sets of GRACE gravity models: the GRGS03, GRGS04 and CSR06 solutions, as well as their respective ocean dealiasing models. Beyond the case of the Tohoku earthquake, our approach can be applied to the systematic monitoring of the Pacific subduction belt, to detect gravity variations potentially linked with sudden changes in slab motions in-depth these plate boundaries. ReferencesBedford, J. R., et al. (2020). Months-long thousand-kilometre-scale wobbling before great subduction earthquakes, Nature 580, 628-635.Bouchon, M., et al. (2016). Potential slab deformation and plunge prior to the Tohoku, Iquique and Maule earthquakes, Nature Geoscience 9, 380-383.Panet, I., et al. (2018). Migrating pattern of deformation prior to the Tohoku-Oki earthquake revealed by GRACE data, Nature Geoscience 11, 367-373.

The control on megathrust earthquake generation exerted by deeper subduction processes remains poorly understood and still insufficiently documented. Here, we use the 2003-2014 space-time variations of the Earth’s gravity gradients derived from the GRACE geoids in order to probe aseismic mass variations at depth and their possible interactions with intraplate seismicity along the Chilean margin. We work with three different datasets of GRACE geoid models over a large region surrounding the rupture zone of the Mw 8.8 2010 Maule earthquake. In order to separate signals associated with mass sources of differents sizes, shapes or orientations, we reconstruct each month the Earth’s gravity gradients at different spatial scales from these geoid models. Our analysis emphasizes a highly anomalous, large-amplitude gravity gradients signal that appears three months prior to the earthquake North of the epicentral zone, and progressively increases until the megathrustal rupture, in all three datasets. We show that this large signal cannot be caused by a shallow hydrological source nor by GRACE striping artefacts and dealiasing models. Instead, we conclude that its most likely origin is in mass redistributions within the solid Earth on the continental side of the subduction zone. These anomalous gravity gradient variations could be explained by a deep extensional deformation of the slab around 150-km depth along the Nazca Plate subduction direction, driving large-scale fluid motion in the subduction zone and into the overriding lithosphere. Our results highlight the importance of observations of the Earth’s time-varying gravity field from satellites to probe aseismic mass redistributions in-depth major plate boundaries . The detection of such mass redistributions at depth by GRACE and their interactions with interplate seismicity opens a new field of research to better characterize and understand the dynamics of the seismic cycle at megathrusts.

International audience; A high-resolution gravimetric quasigeoid model for Vietnam and its surrounding areas was determined based on new gravity data. A set of 29,121 land gravity measurements was used in combination with fill-in data where no gravity data existed. Global Gravity field Models plus Residual Terrain Model effects and gravity field derived from altimetry satellites were used to provide the fill-in information over land and marine areas. A mixed model up to degree/order 719 was used for the removal of the long and medium wavelengths and the calculation of the quasigeoid restore effects. The residual height anomalies have been determined employing the Stokes integral using the Fast Fourier Transform approach and deterministic kernel modification proposed by Wong-Gore, as well as by means of Least-Squares Collocation. The accuracy of the resulting quasigeoid models was evaluated by comparing with height anomalies derived from 812 co-located GNSS/levelling points. Results are very similar; both local quasigeoid models have a standard deviation of 9.7 cm and 50 cm in mean bias when compared to the GNSS/levelling points. This new local quasigeoid model for Vietnam represents a significant improvement over the global models EIGEN-6C4 and EGM2008, which have standard deviations of 19.2 and 29.1 cm, respectively, for this region.

GOCE-derived gravity anomalies are analyzed over the western Mediterranean in order to evaluate the information brings by these satellite observations on the complex lithospheric structure of this region. Free-air and Bouguer anomalies deduced from the latest GOCE model are analyzed and compared with the theoretical response computed from the CRUST1.0 and EPcrust models. Semi-quantitative and qualitative approaches are performed to highlight major geological domains and discontinuities. We show that GOCE gravity anomalies in Western Mediterranean are mainly caused by the thickness of the crust. We also observe two GOCE gravity anomalies located in the centre of the Mediterranean basin that could be related to ocean spreading process, but their nature is unclear and an inhomogeneity of the GOCE gravity response of continental shelves that are not explain by regional 3D models. Our analyses also suggest the presence of two major regional crustal-scale North-East-trending lineament in the western Mediterranean: the well-known North Balearic transform zone and the Cassidaigne-Asinara lineament that separates the Sardinia and Corsica blocks. This later lineament is associated with an apparent sinistral offset of gravity anomalies in the western Mediterranean suggesting strike-slip displacement during opening of the Mediteranean basin. A normal-sense displacement along the Cassidaigne-Asinara lineament between Corsica and Sardinia is alternatively proposed and could be more consistent with geodynamic reconstructions from previous tectonic and paleomagnetic studies. Although additional local geological and geophysical investigations are required to establish the possible separation of Corsica and Sardinia, the homogeneous observations provided by GOCE on both terrestrial and marine areas brings new views on lithospheric scale structure and evolution of this region.

Since a few years, several laboratories, institutes or organizations through the world have acquired marketed quantum absolute gravimeters AQG developed by Muquans. Among their potentialities, these new generations of instruments are expected to complement the existing capabilities of long term monitoring of the Earth gravity field. A metrological evaluation of their performances for long-term measurements is thus a first step.The LNE-SYRTE gravimetry laboratory in the suburb of Paris, has been designed to accommodate other gravimeters for metrological comparisons, tests and calibrations. Instruments of different classes operate in this well characterized laboratory: a laboratory-based absolute cold atom gravimeter (CAG) and a relative superconducting gravimeter iGrav. Both instruments allow for continuous measurements, Accuracy is guaranteed by the CAG and long-term stability by the iGrav.We there have performed a more than one-year long measurement session with the initial version of the marketed quantum gravimeter AQG (AQG-A01).An improved version of this AQG (AQG-B01) designed for outdoor measurement and recently acquired by RESIF (the French Seismologic and Geodetic Network) has been also implemented to close this session with a last month of simultaneous data recording involving all the instruments. Finally, we also performed supplementary accuracy tests, in particular to evaluate the Coriolis bias of the two AQG commercial sensors.The talk will briefly present the different instruments to rapidly focus on the performances of the AQGs and results of the comparisons.

The AlpArray Gravity Research Group (AAGRG), as part of the European AlpArray program, focuses on the compilation of a homogeneous surface-based gravity dataset across the Alpine area. From this data set, Bouguer- and Free Air anomalies are calculated and presented here. In 2016/17 ten European countries in the Alpine realm have agreed to contribute with gravity data for a new compilation of the Alpine gravity field in an area from 2° to 23° East and from 41° to 51° North. This compilation relies on existing national gravity databases and, for the Ligurian and the Adriatic seas, on ship-borne data of the Bureau Gravimétrique International. Furthermore, for the Ivrea zone in the Western Alps, recently acquired data were added to the database. This first pan-Alpine gravity data map is homogeneous regarding input data sets, applied methods and all corrections as well as reference frames. Here, the AAGRG presents the data set of the recalculated gravity fields on a 4 km × 4 km grid for public release, 2 km × 2 km for special request. The final products also include calculated values for mass/bathymetry corrections of the measured gravity at each grid point, as well as height. This allows users to use later customized densities for their own calculations of mass corrections. Correction densities used are 2670 kg m−3 for landmasses, 1030 kg m−3 for water masses above and −1640 kg m−3 below the ellipsoid. The correction radius was set to the Hayford zone O2 (167 km). The new Bouguer anomaly is station completed (CBA) and compiled according to the most modern criteria and reference frames (both positioning and gravity), including atmospheric corrections. Special emphasis was put on the gravity effect of the numerous lakes in the study area, which can have an effect of up to 5 mGal for gravity stations located at shorelines with steep slopes, e.g., for the rather deep reservoirs in the Alps. The results of an error statistic based on cross validations and/or interpolations residuals is provided for the entire database. As an example, the interpolation residuals of the Austrian data set range between about −8 and +8 mGal, the cross-validation residuals between −14 mGal and +10 mGal; standard deviations are well below 1 mGal. The accuracy of the newly compiled gravity database is close to ±5 mGal for most areas. A first interpretation of the new map shows that the resolution of the gravity anomalies is suited for applications ranging from intra-crustal to crustal scale modelling to interdisciplinary studies on the regional and continental scales as well as applications as joint inversion with other datasets. The data will be published with the DOI https://doi.org/10.5880/fidgeo.2020.045 (Zahorec et al., 2020) when the final paper is accepted. In the meantime, the data is accessible via this temporary review link: https://dataservices.gfz-potsdam.de/panmetaworks/review/fdc35a9f6551b01b6152ee1af7b91a5a0c3de5341d067644522c192ad7f25e7f.

International audience; In this paper GravPSO2D, a Matlab tool for two-dimensional gravity inversion in sedimentary basins using the Particle Swarm Optimization (PSO) algorithm, is presented. The package consists of a collection of functions and scripts that cover the main three parts of the process: (1) the model definition based on the observations, (2) the inversion itself, where the PSO is employed, and (3) the results processing, including best model estimation, uncertainty analysis and plots generation. GravPSO2D is freely available, and represents an effort for providing the scientific community with the first tool based on the PSO algorithm in order perform the inversion and the uncertainty assessment of the sedimentary basin gravity inversion problem, taking into account the gravity regional trend estimation, and vertically and horizontally density contrast variations. Synthetic and real examples are provided in order to show the software capabilities.

A vertical offset model for Vietnam and its surrounding areas was determined based on the differences between height anomalies derived from 779 Global Navigation Satellite System (GNSS)/levelling points and those derived from a dedicated high-resolution gravimetric-only quasigeoid model called GEOID_LSC. First, the deterministic transformation model to effectively fit the differences between the quasigeoid and GNSS/levelling heights was based on a third-order polynomial model. Second, the residual height anomalies have been interpolated to a grid employing Least-Squares Collocation. Finally, the distortions were restored to the residual grid. This model can be used for combination with a gravimetric quasigeoid model in GNSS levelling. The quality of GNSS/levelling data in Vietnam was analyzed and evaluated in this study. The annual subsidence rate from ALOS-1 was also used to analyze the effects of subsidence on the quality of GNSS/levelling data in the Mekong Delta. From this we made corrections to improve the accuracy of GNSS/levelling data in this region. The offset model was evaluated using cross-validation technique by comparing with GNSS/levelling data. Results indicate that the offset model has a standard deviation of 5.9 cm in the absolute sense. Based on this offset model, GNSS levelling can be carried out in most of Vietnam’s territory complying third-order levelling requirements, while the accuracy requirements for fourth-order levelling networks is met for the entire country. This model in combination with the developed gravimetric quasigeoid model should also contribute to the modernization of Vietnam’s height system. We also used high-quality GNSS/levelling data and the determined quasigeoid model to determine the geopotential value W0 for the Vietnam Local Vertical Datum. The gravity potential of the Vietnam Local Vertical Datum is estimated equal to W 0 LVD = 62,636,846.81 ± 0.70 m2s−2 with the global equipotential surface realized by the conventional value W0 = 62,636,853.4 m2s−2.

Recent studies have proposed that nucleation zones, seismic barriers and high-slip patches of megathrust earthquakes correlate with physical heterogeneities, both in the oceanic/subducting plate and in the continental wedge. We present a density-depth model along the Nazca-South America subduction margin, from 18 degrees S to 23 degrees 30'S, where a partial segment of this zone was ruptured by the M-w 8.2 Iquique earthquake on 2014 April 1. The density modelling and interpretation were constrained with seismic reflection profiles, published Vp-depth tomographic models and relocated seismicity. These results were used to estimate the variability of normal stress on the seismogenic contact. Our results show a heterogeneous structure for the Northern Chile marine forearc. In particular, we observed a latitudinal and longitudinal segmentation of continental wedge properties, where changes in density can be explained by changes in fracturing degree, which could have an important control on the Iquique earthquake rupture process. This study provides new insights into the analysis of large earthquakes and seismic/tsunami hazard in this active segment of the Chilean margin.

Gravimetry is a well-established technique for the determination of sub-surface mass distribution needed in several fields of geoscience, and various types of gravimeters have been developed over the last 50 years. Among them, quantum gravimeters based on atom interferometry have shown top-level performance in terms of sensitivity, long-term stability and accuracy. Nevertheless, they have remained confined to laboratories due to their complex operation and high sensitivity to the external environment. Here we report on a novel, transportable, quantum gravimeter that can be operated under real world conditions by non-specialists, and measure the absolute gravitational acceleration continuously with a long-term stability below 10 nm.s−2 (1 μGal). It features several technological innovations that allow for high-precision gravity measurements, while keeping the instrument light and small enough for field measurements. The instrument was characterized in detail and its stability was evaluated during a month-long measurement campaign.

Understanding how and when far-field continuous motions lead to giant subduction earthquakes remains a challenge. An important limitation comes from an incomplete description of aseismic mass fluxes at depth along plate boundaries. Here we analyse Earth’s gravity field variations derived from GRACE satellite data in a wide space-time domain surrounding the Mw 9.0 2011 Tohoku-Oki earthquake. We show that this earthquake is the extreme expression of initially silent deformation migrating from depth to the surface across the entire subduction system. Our analysis indeed reveals large-scale gravity and mass changes throughout three tectonic plates and connected slabs, starting a few months before March 2011. Before the Tohoku-Oki earthquake rupture, the gravity variations can be explained by aseismic extension of the Pacific plate slab at mid-upper mantle depth, concomitant with increasing seismicity in the shallower slab. For more than two years after the rupture, the deformation propagated far into the Pacific and Philippine Sea plate interiors, suggesting that subduction accelerated along 2,000 km of the plate boundaries in March 2011. This gravitational image of the earthquake’s long-term dynamics provides unique information on deep and crustal processes over intermediate timescales, which could be used in seismic hazard assessment. Deformation migrated from depth towards the surface in the months leading up to the 2011 Tohoku-Oki earthquake, according to analyses of satellite gravity data.

Since the last century, several geological and geophysical studies have been developed in the Santiago Basin to understand its morphology and tectonic evolution. However, some uncertainties regarding sedimentary fill properties and possible density anomalies below the sediments/basement boundary remain. Considering that this is an area densely populated with more than 6 million inhabitants in a highly active seismotectonic environment, the physical properties of the Santiago Basin are important to study the geological and structural evolution of the Andean forearc and to characterize its seismic response and related seismic hazard. Two and three-dimensional gravimetric models were developed, based on a database of 797 compiled and 883 newly acquired gravity stations. To produce a well-constrained basement elevation model, a review of 499 wells and 30 transient electromagnetic soundings were used, which contribute with basement depth or minimum sedimentary thickness information. For the 2-D modelling, a total of 49 gravimetric profiles were processed considering a homogeneous density contrast and independent regional trends. A strong positive gravity anomaly was observed in the centre of the basin, which complicated the modelling process but was carefully addressed with the available constrains. The resulting basement elevation models show complex basement geometry with, at least, eight recognizable depocenters with maximum sedimentary infill of 500m. The 3-D density models show alignments in the basement that correlates well with important intrusive units of the Cenozoic and Mesozoic. Along with interpreted fault zones westwards and eastwards of the basin, the observations suggest a structural control of Santiago basin geometry, where recent deformation associated with the Andean contractional deformation front and old structures developed during the Cenozoic extension are superimposed to the variability of river erosion/deposition processes.

We compared the structure of two flat and two conical mud volcanoes (MV) in Azerbaijan in order to understand what governs the morphological contrast. We paid particular attention to the structural evolution of the surface, as observed in the field over 5 years and as monitored by satellite images revealing surface activity over that period; in addition, we acquired microgravity measurements to probe deeper structure. The combination of these data reveals in all studied volcanoes significant mass deficit concentric with the surface edifice. Modeling indicates that the mass deficit is consistent with a chamber underlying the summit caldera of conical MVs and the plateau of flat MVs, with a thickness of several hundred meters for a density contrast of 500 kg/m3 between the chamber mud and the surrounding solid mud. Surface structures and their evolution during and in between eruptions provide insight into the depth of the mud chamber, which lies within a few meters of the surface for flat mud volcanoes and one to several hundred meters deep for conical ones. Surface structures and satellite-derived deformation analysis shows that flat mud volcanoes grow by radial expansion and compression of the flanks distributed all around the edifice. In contrast, conical mud volcanoes grow by flow stacking in the summit caldera, deformation being constrained by the caldera rim; excess mud supply is accommodated by occasional rim breaching and mud overflow into flank gullies.

In this paper we present a methodology to perform geophysical inversion of large scale linear systems via a covariance-free orthogonal transformation: the Discrete Cosine Transform (DCT). The methodology consists in compressing the matrix of the linear system as a digital image and using the interesting properties of orthogonal transformations to define an approximation of the Moore-Penrose pseudo-inverse. This methodology is also highly scalable since the model reduction achieved by these techniques increases with the number of parameters of the linear system involved due to the high correlation needed for these parameters to accomplish very detailed forward predictions, and allows for a very fast computation of the inverse problem solution. We show the application of this methodology to a simple synthetic 2D gravimetric problem for different dimensionalities and different levels of white Gaussian noise, and to a synthetic linear system whose system matrix has been generated via geostatistical simulation to produce a random field with a given spatial correlation. The numerical results show that the DCT pseudoinverse outperforms the classical least-squares techniques, mainly in presence of noise, since the solutions that are obtained are more stable and fit the observed data with a lowest RMS error. Besides, we show that the model reduction is a very effective way of parameter regularization when the conditioning of the reduced DCT matrix is taken into account. We finally show its application to the inversion of a real gravity profile in the Atacama Desert (north Chile) obtaining very successful results in this nonlinear inverse problem. The methodology presented here has a general character and can be applied to solve any linear and nonlinear inverse problems (through linearization) arising in technology and particularly in geophysics, independently of the geophysical model discretization and dimensionality. Nevertheless, the results shown in this paper are better in the case of ill-conditioned inverse problems for which the matrix compression is more efficient. In that sense, a natural extension of this methodology would be its application to the set of normal equations. This article is protected by copyright. All rights reserved

International audience; ilicic systems generate the most explosive eruptions on Earth. In contrast to basaltic systems, they can accumulate large volumes of magma without systematically erupting, confronting the classical interpretation that a volcano inflates when a magmatic intrusion occurs. Understanding the mechanisms of volcanic inflation and unrest is thus one of the most important challenges in volcanic risk assessment. Laguna del Maule (LdM) in the Southern Volcanic Zone (SVZ) of Chile, is one of the most active Holocene silicic complexes in the world and it has been inflating since 2007, accumulating 2 m of uplift without erupting. Several geophysical and geochemical studies conclude that a large crystal rich reservoir would be residing beneath LdM, in consistency with other multi-disciplinary studies showing that such crystal-rich reservoirs (“mush zones”) can be maintained beneath silicic volcanoes, fed by mafic magma recharge from below. Nevertheless, the mechanical state of such reservoirs remains unclear. Here, we characterize for the first time the mechanical properties of such a mush reservoir, able to promote large surface displacements such as those measured at LdM. Using a 3D finite element method we simulate a recharge of magma at the base of a crystal rich reservoir, by assuming an overpressurized source surrounded by a large viscoelastic shell. Inversion results show that this model fits the observed temporal and spatial evolution of ground displacements measured with InSAR data and GNSS data between 2007 and 2017. We interpret the temporal behavior of ground displacement at LdM as resulting from two contributions. A magma recharge occurred within the first 4 yr of the active inflation, followed by the viscous response of the large viscoelastic shell, set to a viscosity of 1017 Pas. Compared to a purely elastic solution, our model suggests that up to 50% of the accumulated surface displacement during the ten-year period can be explained by this viscous response, and predicts ongoing displacements 50 yr after the onset of inflation. This model agrees with geophysical and geochemical observations and offers a simple explanation of the temporal evolution of surface displacements. It further allows to reconsider the mechanical behavior of large partially crystallized domains in the upper crust; such significant transient stress transfer over large viscoelastic areas should thus be accounted for in other studies of silicic volcanic complexes.

Summary Reservoir monitoring, reservoir engineering, 4D gravity, time-lapse gravimetry, gravity survey, absolute gravimetry, subsidence monitoring, geothermal energy, carbon sequestration, enhanced oil recovery.

International audience; We present a new model of the lithosphere and asthenosphere structure down to 300 km depth beneath the Pyrenees from the joint inversion of recent gravity and teleseismic data. Unlike previous studies, crustal correction was not applied on teleseismic data in order (i) to preserve the consistency between gravity data, which are mainly sensitive to the density structure of the crust lithosphere, and traveltime data, and (ii) to avoid the introduction of biases resulting from crustal reductions. The density model down to 100 km depth is preferentially used here to discuss the lithospheric structure of the Pyrenees, whereas the asthenospheric structure from 100 to 300 km depth is discussed from our velocity model. The absence of a high density anomaly in our model between 30 and 100 km depth (except the Labourd density anomaly) in the northern part of the Pyrenees seems to preclude eclogitization of the subducted Iberian crust at the scale of the entire Pyrenean range. Local eclogitization of the deep Pyrenean crust beneath the western part of the Axial Zone (west of Andorra) associated with the positive central density anomaly is proposed. The Pyrenean lithosphere in density and velocity models appears segmented from east to west. No clear relation between the along-strike segmentation and mapped major faults is visible in our models. The Pyrenees’ lithosphere segments are associated with different seismicity pattern in the Pyrenees suggesting a possible relation between the deep structure of the Pyrenees and its seismicity in the upper crust. The concentration of earthquakes localized just straight up the central density anomaly can reflect the subsidence and/or delamination of an eclogitized Pyrenean deep root. The velocity model in the asthenosphere is similar to previous studies. The absence of a high-velocity anomaly in the upper mantle and transition zone (i.e. 125 to 225 km depth) seems to preclude the presence of a detached oceanic lithosphere beneath the European lithosphere.

International audience; We estimate using gravity data the thickness of post-glacial unconsolidated sediment filling two major glacial valleys in northern Pyrenees: the Gave de Pau valley between Pierrefitte-Nestalas and Lourdes, and the Garonne valley between Saint-Béat and Barbazan. One hundred and eighty-four new gravity data complete 74 measurements obtained from the International Gravimetric Bureau database. Negative residual anomalies resulting from the presence of small-density unconsolidated sediment approach 4 mgal in both the Gave de Pau and the Garonne valleys. Estimating the sediment thickness requires knowing the density contrast between Quaternary sediments and the underlying bedrock. Supposing this density contrast is 600 kg/m3, the maximum estimated thickness of post-glacial sediment is ∼ 230 and 300 meters, and the volume of sediment is 2.1 and 3.2 km3 in the Gave de Pau and Garonne valleys, respectively. In both valleys, the depth of Quaternary sediment suddenly increases at the confluence between two major glacial valleys (Gave de Pau – Gave de Cauterets, and Garonne – Pique confluences). Overdeepened basins are less deep downstream when approaching terminal moraines (Lourdes and Barbazan area), illustrating that the efficiency of glacial erosion depends on the ice flux flowing through valleys.

International audience

Co-auteur étranger; International audience; Monitoring ground surface displacement of volcanoes over a long period of time helps improve understanding of the volcano structure, dynamics and mechanisms. In this paper, we investigate the spatio-temporal behavior of the long-term displacement field at Piton de la Fournaise volcano (La Réunion Island) by means of X-band InSAR and GNSS time series analysis from 2009 to 2014. We propose a new correction approach based on principal component analysis to mitigate the long-wavelength artifact in the interferograms over the study area. Our results show that most of the volcanic edifice including the Central Cone, the Eastern Flank and recent lava flow fields was affected by varying degrees of ground motion. More importantly, a widespread sector (~ 20 km2) affected by time-dependent downward and eastward motion was observed on the Eastern Flank of Piton de la Fournaise. The combined analysis of InSAR and GNSS time series and previous studies allows us to confirm that this large mobile sector on the Eastern Flank underwent deformation for at least 7 years following the end of the large March–April 2007 eruption, including a stage of transient strain from the end of eruption to February 2011, followed by a stage of steady-state strain until late 2014. Several possible origins, including a pre-existing structural discontinuity, summit stress associated with magmatism, and activated fault movement are explored to explain the observed widespread long-term seaward motion that is considered to be direct evidence of potential flank instability.

Comparative analysis of coseismic and postseismic variations of the Earth’s gravity field is carried for the regions of three giant earthquakes (Andaman-Sumatra, December 26, 2004, magnitude M w = 9.1; Maule-Chile, February 27, 2010, M w = 8.8, and Tohoku-Oki, March 11, 2011, M w = 9.0) with the use of GRACE satellite data. Within the resolution of GRACE models, the coseismic changes of gravity caused by these seismic events manifest themselves by large negative anomalies located in the rear of the subduction zone. The real data are compared with the synthetic anomalies calculated from the rupture surface models based on different kinds of ground measurements. It is shown that the difference between the gravity anomalies corresponding to different rupture surface models exceeds the uncertainties of the GRACE data. There-fore, the coseismic gravity anomalies are at least suitable for rejecting part of the models that are equivalent in the ground data. Within the first few months after the Andaman-Sumatra earthquake, a positive gravity anomaly started to grow above the deep trench. This anomaly rapidly captured the area of the back-arc basin and largely compensated the negative coseismic anomaly. The processes of viscoelastic stress relaxation do not fully allow for these rapid changes of gravity. According to the calculations, even with a sufficiently low viscosity of the upper mantle, relaxation only covers about a half of the observed change of the field. In order to explain the remaining temporal variations, we suggested the process of downdip propagation of the coseismic rupture surface. The feasibility of such a process was supported by numerical simulations. The sum of the gravity anomalies caused by this process and the anomaly generated by the processes of viscoelastic relaxation accounts well for the observed changes of the gravity field in the region of the earthquake. The similar postseismic changes of gravity were also detected for the region of the Tohoku-Oki earthquake. Just as in the case discussed above, this earthquake was also followed by a rapid growth of a positive postseismic anomaly, which partially counterbalanced the negative coseismic anomaly. The time variations of the gravity field in the region of the Maule-Chile earthquake differ from the pattern of changes observed in the island arcs described above. The postseismic gravity variations are in this case concentrated in a narrower band above the deep trench and shelf, and they do not spread over the continental territory, where the negative coseismic anomaly is located. These discrepancies reflect the difference in the geodynamical settings of the studied earthquakes.

[1] Reprocessed Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) gravity gradient data were combined with data from Laser Geodynamics Satellite (LAGEOS) 1/2 and Gravity Recovery and Climate Experiment (GRACE) to generate a satellite-only gravity field model to degree 260 using the direct approach, named DIR-R4. When compared to Earth Gravitational Model 2008 (EGM2008), it is more accurate at low to medium resolution thanks to GOCE and GRACE data. When compared to earlier releases of ESA GOCE models, it is more accurate at high degrees owing to the larger amount of data ingested. It is also slightly more accurate than ESA's fourth release of the time-wise model (TIM-R4), as demonstrated by GPS/leveling, orbit determination tests, and an oceanographic evaluation. According to the formal, probably too optimistic by a factor of 2–2.5, cumulated geoid (1.3 cm) and gravity anomaly (0.4 mGal) errors at 100 km resolution, the GOCE mission objectives have been reached.

Over the oceans, data from altimetry are currently the only input data for the latest global geoid models such as EGM08 or EIGEN6C. Over the Mediterranean sea, satellite altimetry does not give good results for gravity models. In particular because of the high ocean variability and in the vicinity of the coast in some areas. A marine gravity data compilation and screening was conducted as part of an international project GEOMED2 for calculating the geoid of the Mediterranean sea. In this paper, marine gravity data and their processing are described. The shipborne gravity data are validated and an estimation of the error is done. Then the cleaned data are used to validate the Global Geoid models (GGm).

The availability of high-resolution global digital elevation data sets has raised a growing interest in the feasibility of obtaining their spherical harmonic representation at matching resolution, and from there in the modelling of induced gravity perturbations. We have therefore estimated spherical Bouguer and Airy isostatic anomalies whose spherical harmonic models are derived from the Earth's topography harmonic expansion. These spherical anomalies differ from the classical planar ones and may be used in the context of new applications. We succeeded in meeting a number of challenges to build spherical harmonic models with no theoretical limitation on the resolution. A specific algorithm was developed to enable the computation of associated Legendre functions to any degree and order. It was successfully tested up to degree 32,400. All analyses and syntheses were performed, in 64 bits arithmetic and with semi-empirical control of the significant terms to prevent from calculus underflows and overflows, according to IEEE limitations, also in preserving the speed of a specific regular grid processing scheme. Finally, the continuation from the reference ellipsoid's surface to the Earth's surface was performed by high-order Taylor expansion with all grids of required partial derivatives being computed in parallel. The main application was the production of a 1' x 1' equiangular global Bouguer anomaly grid which was computed by spherical harmonic analysis of the Earth's topography-bathymetry ETOPO1 data set up to degree and order 10,800, taking into account the precise boundaries and densities of major lakes and inner seas, with their own altitude, polar caps with bedrock information, and land areas below sea level. The harmonic coefficients for each entity were derived by analyzing the corresponding ETOPO1 part, and free surface data when required, at one arc minute resolution. The following approximations were made: the land, ocean and ice cap gravity spherical harmonic coefficients were computed up to the third degree of the altitude, and the harmonics of the other, smaller parts up to the second degree. Their sum constitutes what we call ETOPG1, the Earth's TOPography derived Gravity model at 1' resolution (half-wavelength). The EGM2008 gravity field model and ETOPG1 were then used to rigorously compute 1' x 1' point values of surface gravity anomalies and disturbances, respectively, worldwide, at the real Earth's surface, i.e. at the lower limit of the atmosphere. The disturbance grid is the most interesting product of this study and can be used in various contexts. The surface gravity anomaly grid is an accurate product associated with EGM2008 and ETOPO1, but its gravity information contents are those of EGM2008. Our method was validated by comparison with a direct numerical integration approach applied to a test area in Morocco-South of Spain (Kuhn, private communication 2011) and the agreement was satisfactory. Finally isostatic corrections according to the Airy model, but in spherical geometry, with harmonic coefficients derived from the sets of the ETOPO1 different parts, were computed with a uniform depth of compensation of 30 km. The new world Bouguer and isostatic gravity maps and grids here produced will be made available through the Commission for the Geological Map of the World. Since gravity values are those of the EGM2008 model, geophysical interpretation from these products should not be done for spatial scales below 5 arc minutes (half-wavelength).

International audience; The use of Synthetic Aperture Radar interferometry (InSAR) in northern Chile, one of the most seismically active regions in the world, is of great importance. InSAR enables geodesists not only to accurately measure Earth's motions but also to improve fault slip map resolution and our knowledge of the time evolution of the earthquake cycle processes. Fault slip mapping is critical to better understand the mechanical behavior of seismogenic zones and has fundamental implications for assessing hazards associated with megathrust earthquakes. However, numerous sources of errors can significantly affect the accuracy of the geophysical parameters deduced by InSAR. Among them, atmospheric phase delays caused by changes in the distribution of water vapor can lead to biased model parameter estimates and/or to difficulties in interpreting deformation events captured with InSAR. The hyper-arid climate of northern Chile might suggest that differential delays are of a minor importance for the application of InSAR techniques. Based on GPS, Moderate Resolution Imaging Spectroradiometer (MODIS) data our analysis shows that differential phase delays have typical amplitudes of about 20 mm and may exceptionally exceed 100 mm and then may impact the inferences of fault slip for even a Mw 8 earthquakes at 10% level. In this work, procedures for mitigating atmospheric effects in InSAR data using simultaneous MODIS time series are evaluated. We show that atmospheric filtering combined with stacking methods are particularly well suited to minimize atmospheric contamination in InSAR imaging and significantly reduce the impact of atmospheric delay on the determination of fundamental earthquake parameters.

The Mw 7.7 2007 November 14 earthquake had an epicentre located close to the city of Tocopilla, at the southern end of a known seismic gap in North Chile. Through modelling of Global Positioning System (GPS) and radar interferometry (InSAR) data, we show that this event ruptured the deeper part of the seismogenic interface (30–50 km) and did not reach the surface. The earthquake initiated at the hypocentre and was arrested ∼150 km south, beneath the Mejillones Peninsula, an area already identified as an important structural barrier between two segments of the Peru–Chile subduction zone. Our preferred models for the Tocopilla main shock show slip concentrated in two main asperities, consistent with previous inversions of seismological data. Slip appears to have propagated towards relatively shallow depths at its southern extremity, under the Mejillones Peninsula. Our analysis of post-seismic deformation suggests that small but still significant post-seismic slip occurred within the first 10 d after the main shock, and that it was mostly concentrated at the southern end of the rupture. The post-seismic deformation occurring in this period represents ∼12–19 per cent of the coseismic deformation, of which ∼30–55 per cent has been released aseismically. Postseismic slip appears to concentrate within regions that exhibit low coseismic slip, suggesting that the afterslip distribution during the first month of the post-seismic interval complements the coseismic slip. The 2007 Tocopilla earthquake released only ∼2.5 per cent of the moment deficit accumulated on the interface during the past 130 yr and may be regarded as a possible precursor of a larger subduction earthquake rupturing partially or completely the 500-km-long North Chile seismic gap.

Most inverse problems in the industry (and particularly in geophysical exploration) are highly underdetermined because the number of model parameters too high to achieve accurate data predictions and because the sampling of the data space is scarce and incomplete; it is always affected by different kinds of noise. Additionally, the physics of the forward problem is a simplification of the reality. All these facts result in that the inverse problem solution is not unique; that is, there are different inverse solutions (called equivalent), compatible with the prior information that fits the observed data within similar error bounds. In the case of nonlinear inverse problems, these equivalent models are located in disconnected flat curvilinear valleys of the cost-function topography. The uncertainty analysis consists of obtaining a representation of this complex topography via different sampling methodologies. In this paper, we focus on the use of a particle swarm optimization (PSO) algorithm to sample the region of equivalence in nonlinear inverse problems. Although this methodology has a general purpose, we show its application for the uncertainty assessment of the solution of a geophysical problem concerning gravity inversion in sedimentary basins, showing that it is possible to efficiently perform this task in a sampling-while-optimizing mode. Particularly, we explain how to use and analyze the geophysical models sampled by exploratory PSO family members to infer different descriptors of nonlinear uncertainty.

Nine ERS-1 and ERS-2 descending orbit data ac- quired over Aira Caldera between June 1995 and November 1998 were used to create 36 differential interferograms. Al- though the interferograms exhibit a relatively low level of coherence, even for couples sampling short time intervals (6 months), Synthetique Aperture Radar (SAR) observations re- veal a distinct range change pattern over Kokubu urban area whose amplitude increases with the time separation between SAR images. The analysis of the ground deformation time series relative to the earliest ERS images showed a maxi- mum uplift of about 20 mm between the north and the south of the urban area during the period covered by our satellite observations. Taking the reduced surface of the coherent area into account, we performed a simple modeling of the defor- mation field assuming an inflating spherical source within an elastic half-space medium located beneath the centre of the Aira Caldera. This simple model predicts a maximum volume increase of 20-30◊10 6 m 3 between 1995 and 1998, which would produce an inflation of about 70 mm at the cen- tre of the Aira Caldera and 40 mm in the Kokubu south urban area. These results are in good agreement with other geo- physical observations carried out on the Aira caldera during this period. Despite the limited spatial extent of the coherent areas around the Aira Caldera, this study shows that Din- SAR method using data collected in C band can be success- fully used to detect subtle ground displacement changes of the volcanic complex and thus provides complementary in- formation to ground-based geodetic monitoring of dynamic processes of the Aira Caldera and Sakurajima volcano.

International audience; We determine here for the first time the geometry and location of the hydrothermal and magmatic reservoirs in the Lazufre volcanic area. This furthers the understanding of the origin of one of the largest worldwide volcanic uplift regions, both in space and amplitude. The exact locations and shapes of the sources generating a double-wide uplift region in the Lazufre found by past deformation data (InSAR and GPS) and generating hydrothermal and magmatic fluids found by geochemical gas analysis have not been well-delimited. In this study, we use seismological data to perform a 3-D high-resolution S-wave velocity model, which allows defining better the locations and shapes of the sources of the deformations and the hydrothermal and magmatic reservoirs. We find three anomalies. Two of them (with S-wave velocity of about 1.2–1.8 km/s) are located below the Lastarria volcano. The shallow one (

International audience; Gravity inversion is a classical tool in applied geophysics that corresponds, both, to a linear (density unknown) or nonlinear (geometry unknown) inverse problem depending on the model parameters. Inversion of basement relief of sedimentary basins is an important application among the nonlinear techniques. A common way to approach this problem consists in discretizing the basin using polygons (or other geometries), and iteratively solving the nonlinear inverse problem by local optimization. Nevertheless, this kind of approach is highly dependent of the prior information that is used and lacks from a correct solution appraisal (nonlinear uncertainty analysis). In this paper, we present the application of a full family Particle Swarm Optimizers (PSO) to the 20 gravity inversion and model appraisal (uncertainty assessment) of basement relief in sedimentary basins. The application of these algorithms to synthetic and real cases (a gravimetric profile from Atacama Desert in north Chile) shows that it is possible to perform a fast inversion and uncertainty assessment of the gravimetric model using a sampling while optimizing procedure. Besides, the parameters of these exploratory PSO optimizers are automatically tuned and selected based on stability criteria. We also show that the result is robust to the presence of noise in data. The fact that these algorithms do not require large computational resources makes them very attractive to solve this kind of gravity inversion problems. (C) 2015 Elsevier B.V. All rights reserved.

International audience; We analyzed C band and L band interferometric synthetic aperture radar (InSAR) data acquired from 2003 to 2011 to search for volcanic deformations at Llaima volcano, Southern Andes (38.69°S, 71.73°W). There, specific environmental conditions (steep slopes, snow- or ice-capped summit, dense vegetation cover, and strong tropospheric artifacts) and limited amount of radar data available make it challenging to accurately measure ground surface displacement with InSAR. To overcome these difficulties, we first performed a careful analysis of the water vapor variations using Medium-Resolution Imaging Spectrometer and Moderate Resolution Imaging Spectroradiometer near-infrared water vapor products and then we inverted wrapped interferograms for both topographic correlated phase delays and a simple model source strength. In the light of our results, we conclude that there is no detectable ground displacement related to a deep magmatic source for the 2003–2011 period and that most of the fringes observed in the interferograms were produced by tropospheric delays.

The Yellowstone-East Snake River Plain hotspot track has been intensely studied since several decades and is widely considered to result from the interaction of a mantle plume with the North American plate. An integrated conclusive geodynamic interpretation of this extensive data set is however presently still lacking, and our knowledge of the dynamical processes beneath Yellowstone is patchy. It has been argued that the Yellowstone plume has delaminated the lower part of the thick Wyoming cratonic lithosphere. We derive an original dynamic model to quantify delamination processes related to mantle plume-lithosphere interactions. We show that fast (similar to 300 ka) lithospheric delamination is consistent with the observed timing of formation of successive volcanic centres along the Yellowstone hotspot track and requires (i) a tensile stress regime within the whole lithosphere exceeding its failure threshold, (ii) a purely plastic rheology in the lithosphere when stresses reach this yield limit, (iii) a dense lower part of the 200 km thick Wyoming lithosphere and (iv) a decoupling melt horizon inside the median part of the lithosphere. We demonstrate that all these conditions are verified and that similar to 150 km large and similar to 100 km thick lithospheric blocks delaminate within 300 ka when the Yellowstone plume ponded below the 200 km thick Wyoming cratonic lithosphere. Furthermore, we take advantage of the available extensive regional geophysical and geological observation data sets to design a numerical 3-D upper-mantle convective model. We propose a map of the ascending convective sheets contouring the Yellowstone plume. The model further evidences the development of a counter-flow within the lower part of the lithosphere centred just above the Yellowstone mantle plume axis. This counter-flow controls the local lithospheric stress field, and as a result the trajectories of feeder dykes linking the partial melting source within the core of the mantle plume with the crust by crosscutting the lithospheric mantle. This counter-flow further explains the 50 km NE shift observed between the mantle plume axis and the present-day Yellowstone Caldera as well as the peculiar shaped crustal magma chambers.

The accuracy of surface deformation maps derived from DinSAR (differential interferomery synthetic aperture radar) in volcanic areas is limited by temporal variations of the vertical refractivity profile in the troposphere between the radar data acquisitions. Due to their dependence on altitude, such variations may produce topography-related fringes in the computed interferograms. The variations should be carefully estimated in a given area so that they can be discriminated with real ground deformation signals. In this paper, we present a clear example of tropospheric induced fringes (reaching up to 2.9 fringes over 1200 m), evidenced on Sakurajima volcano (Japan) from ERS-1 and ERS-2 data. The analysis of phase–altitude correlations based on network adjustments of the interferometric dataset leads to a simple linear model of the tropospheric excess path delay (with an rms accuracy of 0.4 cm) that might be used for further corrections. A more detailed investigation of these phase variations providing accurate measurements of linear and non-linear contributions of tropospheric effects is also presented and shows that tropospheric phase delays can be better modeled on Sakurajima with a non-linear model.

A newly developed interactive computer program, CG3TOOL, has been dedicated to the processing of the gravity data acquired by the Scintrex CG-3/3M automated gravity meter. The aim of CG3TOOL is two fold: to allow for an objective evaluation of Scintrex data and to provide a higher resolution in data reductions than those computed in real time by the microprocessor-controlled instrument. The program reads the gravity data acquired in either field or cycle mode (field surveys and continuous recordings, respectively) and then downloaded from the meter to a PC computer. The processing tasks are divided into two successive levels. Level 1 is dedicated to the reduction of the daily data files by applying standard or accurate corrections (earth tide, instrumental drift, atmospheric pressure). The precise corrections are performed up to the microGal (µGal) level, in accordance with the specifications of high-resolution surveys. Level 2 contains a series of processing tools (including network adjustment, anomaly computation, and gravity meter calibration) that will precisely compute and adjust the gravity values with error estimates. The interactive procedures and the program output (plot and text files) have been designed to ease data handling and archiving as well as to provide useful information for future purposes of data interpretation or modeling. CG3TOOL was developed in a standard C language for Unix Sun workstations and uses the standard graphical and mathematical Generic Mapping Tools (GMT) free library, available from the web. The objectives and principles of the computer program are presented below along with corresponding examples of the main processing tasks applied to observed data.

International audience; The Sixth International Comparison of Absolute Gravimeters was held from 5 June to 28 August 2001 at the Bureau International des Poids et Mesures (BIPM), Sèvres. Seventeen absolute gravimeters were used to make measurements at five sites of the BIPM gravity network. The vertical gravity gradients at the sites and the ties between them were also measured using seventeen relative gravimeters. For the first time the ties were also measured using absolute gravimeters. Various methods of processing the absolute and relative data were tested to calculate the results. The final results of ICAG-2001 are presented. The acceleration due to gravity at a height of 0.90 m is given as (980 925 701.2 ± 5.5) µGal and (980 928 018.8 ± 5.5) µGal for sites A and B, respectively, calculated using a combined adjustment of the absolute and relative data.