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De nouveaux âges d'enfouissement par mesure des rapports isotopiques 26Al/10Be au sein de trois cavités proches du canyon de la Vis (sud de la France, limite sud-est des Grands-Causses) remettent en question le modèle de karstogenèse classiquement utilisé pour expliquer la structuration des réseaux. En effet, les niveaux subhorizontaux de galeries souterraines sont généralement utilisés comme marqueurs de la position du niveau de base régional (i.e., rivière). Si la relation âge d’enfouissement et hauteur relative au niveau de base régional attendue en utilisant le modèle classique per descensum pour les grottes des Scorpions (1,2 ± 0,4 Ma, +105 m relative au niveau de base -r.n.b.-) et de Bergougnous (1,2 ± 0,3 Ma, +117 m r.n.b.) est en accord avec la quantification du taux moyen d'incision régional (83 ± 35 m/Ma), elle ne permet pas d’expliquer l'âge de 0,94 ± 0,07 Ma obtenu pour les remplissages des niveaux horizontaux du Rocas situés bien plus haut en altitude (+317 m r.n.b.). Les deux premières cavités s’ouvrent directement dans les gorges, alors que la troisième s’ouvre sur le plateau à moins de 4 km des gorges. C’est à notre connaissance la première fois que des données quantitatives étayent la proposition que le modèle de karstogenèse régional contrôlé par la position du niveau de base n’est pas universel. Ces datations permettent de proposer que la fantômisation joue un rôle primordial en contrôlant la localisation des niveaux sub-horizontaux. Ce processus interne au karst relègue le rôle de la position du niveau de base régional à celui d’un simple déclenchement externe de l'évidement des produits d’altération générés par la fantômisation. New burial ages of cave deposits, based on the 26Al/10Be cosmogenic isotope ratio technique, from three caves in the Vis Valley, located in the south-east perimeter of the “Grands-Causses” in southern France are challenging the validity of classic karstic evolution models to explain the structure and evolution of the karstic network in the region. Generally speaking, the formation of horizontal galleries at different elevations above the canyon floor is commonly used as time-markers of former regional base level positions. The burial ages obtained for the Bergougnous cave (1.2 ± 0.3 Ma, +117 m above base level -a.b.l.-) and the Scorpions cave 1.2 ± 0.4 Ma, +105 m a.b.l.) are in agreement with the regional river-incision rate (83 ± 35 m.Ma-1) and conform to a classical per descensum evolution of the karst network. However, the emplacement of alluvium deposits within the Rocas cave at 0.94 ± 0.07 Ma is not consistent with a regular per descensum model. The Rocas cave is 317 m a.b.l. but its entrance is located on the plateau about 4 kilometres from the Vis canyon crest whereas, in contrast, the two caves listed above open directly onto the wall of the Vis canyon. To our knowledge, this is the first presentation of quantitative data which supports the observation that karstic horizontal levels are not always directly controlled by the regional base level position. We support our findings with absolute ages, which for the first time, demonstrates the primary importance of the ghost-rock process on the location of horizontal levels. Base level location is then a secondary parameter, mainly involved in a triggering event for ghost-rock erosion.

International audience; Surface deformation measured by geodetic data is the sum of single strain sources deforming at depth. A combination of volume changes from several analytical models (e.g. a point source or dislocation along a plane) can be used to model the different sources. However, solving for the best fit of volume variations, dislocations, position and orientation parameters of all sources is a non-linear problem, and its solution is generally non-unique. This problem can be converted into a linear one by assimilating the sum of sources to a simplified model formed by three orthogonal planes of dislocations at fixed position and orientation. This strain source model is equivalent to having all neighbouring deformation sources contained in a small size volume. The determination of the strain tensor components can be performed by inverting geodetic data. Because of their high resolution, tiltmeters are well adapted to survey shallow deformation of volcanoes and geological reservoirs. However, they are known to display unknown long-term drift. We propose an approach to jointly estimate the temporal evolution of the strain source and time-dependent instrumental parameters. We verify the approach using synthetic data, giving confidence intervals for each component of the strain tensor. Finally, we link geological information to the internal deformation by interpreting the strain tensor as principal directions of deformation. This approach seems promising for the identification of fracture onset and fault reactivation in geothermal, hydrocarbon exploitations or volcanic systems.

The Western Alps represent the zone of highest seismicity density in metropolitan France. The seismicity is mainly located along two NE-SW strike-slip fault systems: the right-lateral Belledonne Fault and the left-lateral Durance Fault. Glacial Isostatic Adjustment (GIA) is one of the most common processes given to explain intraplate seismicity (e.g., Scandinavia, North America) and is also proposed as a cause of present-day deformation in the Alps. In order to test the impact of deglaciation from the Last Glacial Maximum on pre-existing vertical strike-slip faults in the Western Alps (Belledonne and Durance Faults), we use a finite-element approach to model fault reactivation throughout the deglaciation period, from ca. 18 kyr up to today. The models are tuned to fit present-day deformation rates observed by geodesy (uplift rate up to 2 mm/yr and horizontal radial extension). Simplified models (homogeneous icecap and Earth rheology) show that, under optimum conditions, GIA stress perturbations can activate a NE-SW right-lateral strike-slip fault such as the Belledonne Fault, requiring the fault to have been pre-stressed up to near-failure equilibrium before the onset of deglaciation. The maximum effect of GIA is 1.7 meters of right-lateral slip over 20 kyr, with a peak of displacement between 20 and 10 ka. These models indicate that GIA can result in a maximum slip rate of 0.08 mm/yr averaged over the Holocene, in association with earthquakes up to Mw = 7 (if all displacement is taken in one event). These results are consistent with local paleoseismicity and geomorphology evidence on the Durance fault. However, the impact of GIA on the left-lateral Belledonne Fault is poorly constrained by these simple models. Additional models based on realistic Alpine icecap reconstructions and regional rheology structures will also be presented, that allow us to test the specific effects of GIA on Holocene deformation along both the Belledone and Durance Fault systems.

Although more and more processes are discussed and discovered on the genesis and evolution of cave systems, the tiered karsts are often explained by a control of the base level evolution. In this classical model, the horizontal galleries are explained by a stability of the base level elevation. To the contrary, the shafts and network segments with steep slopes are related to incision periods with a base level lowering.We use Terrestrial Cosmogenic Nuclide Geochronology to estimate burial ages of alluvium trapped in several caves of the Larzac plateau in Southern France. All the samples are collected in horizontal cave levels, sometimes located between steeper segments. Some caves are opened in river gorge walls, while others are located below the Larzac plateau not farther than 5km away from the river gorges.The burial ages for the caves opening in the gorges are consistent with the incision rates given for the area and could be interpreted using the classical model. However, the cave within the plateau show a horizontal level with alluvium deposited 200m above the caves in the gorge with the same burial ages (~1 Myr). Since then, new shafts have been opened without alluvium and are hydrologically connected to the river by deeper[jfr1] hypogenic galleries. The cave morphologies and the geochronological data suggest that the classical model fails to explain the horizontal levels in cave below the plateau. We postulate that the geometry of the caves in these limestone and dolomite plateaus are related to a previous period of ghost-rock and alteration roots formations. Without the opening of an efficient connection between this primokarst and the valley, no alluvium can flow through the cave. Therefore, we think that our burial ages constrain the emptying of the ghost-rocks leading to the genesis of the cave where water and possibly alluvium can flow through. Furthermore, these new finding explain why the horizontal levels in the caves are not clearly related to horizontal markers in the surface geomorphology and why large shafts (>100m) exist in the area without evidences of long periods of base level stability followed by large drop of the regional base level.

In the 60’s, the formulation of the plate tectonic theory changed our understanding of the Earth dynamics. Aiming at explaining the earth first order kinematics, this primary theory of plate tectonic assumed rigid plates, a necessity to efficiently transfer stress from one boundary to another. If successful to explain, at first order, the plate-boundary evolutions, this theory fails when compared to the unpredicted but identified deformation located inside the plate-domains: the intraplate orogens. Indeed, the intraplate regions are thought to be slowly, if at all, deforming. Therefore, it is expected that intraplate regions do not show important finite deformation, that is to say, no mountains. Some intraplate regions, however, have important relief: the Snowy Mountains (Australia), the Ural Mountains (Russia) or the Massif Central (France) for examples. Traditionally, such regions are interpreted as old structures that are slowly eroded, interpretations that are most of the time weakly constrained. Our study is aiming at providing stronger constraints and then a better understanding of such challenging area that are the intraplate orogen domains. Because direct measurements of deformations (e.g. GNSS: Global Navigation Satellite System or InSAR: Interferometric Synthetic Aperture Radar) are most of the time below the precision level, it is necessary to derive this information from the landscape evolution. To do so, terrestrial cosmogenic nuclide (TCN) technics are a key method, allowing to constraint the temporal landscape evolution. Classically, two TCN-based approaches are used to quantify the landscape evolution rate: burial ages and watershed-wide denudation rates, based on measurement in quartz sediment of 10Be and 26Al concentrations, two radioactive cosmogenic isotopes. Using the Massif Central (France) as study area, we show that this region is currently deforming.From new geochronological constraints and a geomorphometric study, we propose that the region undergoes an active uplift encompassing the last c.a. 4 Ma. It can be explained by the combination of at least two phenomena: the first one is the uplift triggering event, that has yet to be clearly identified, and the second one: the erosional isostatic adjustment enhancing the first one and possibly continuing after the end of the first one.

The evolution of intraplate orogens is still poorly understood. Yet, it is of major importance for understanding the Earth and plate dynamics, as well as the link between surface and deep geodynamic processes. The French Massif Central is an intraplate orogen with a mean elevation of 1000 m, with the highest peak elevations ranging from 1500 to 1885 m. However, active deformation of the region is still debated due to scarce evidence either from geomorphological or geodetic and seismologic data. We focus our study on the southern part of the Massif Central, known as the Cévennes and Grands Causses, which is a key area to study the relationship between the recent geological deformation and landscape evolution. This can be done through the study of numerous karst systems with trapped sediments combined with the analysis of a high-resolution digital elevation model (DEM). Using the ability of karst to durably record morphological evolution, we first quantify the incision rates. We then investigate tilting of geomorphological benchmarks by means of a high-resolution DEM. We finally use the newly quantified incision rates to constrain numerical models and compare the results with the geomorphometric study. We show that absolute burial age (10Be∕26Al on quartz cobbles) and the paleomagnetic analysis of karstic clay deposits for multiple cave system over a large elevation range correlate consistently. This correlation indicates a regional incision rate of 83 +17/-5 m Ma−1 during the last ca. 4 Myr (Pliocene–Quaternary). Moreover, we point out through the analysis of 55 morphological benchmarks that the studied region has undergone a regional southward tilting. This tilting is expected as being due to a differential vertical motion between the northern and southern part of the studied area. Numerical models show that erosion-induced isostatic rebound can explain up to two-thirds of the regional uplift deduced from the geochronological results and are consistent with the southward tilting derived from morphological analysis. We presume that the remaining unexplained uplift is related to dynamic topography or thermal isostasy due to the Massif Central Pliocene–Quaternary magmatism. Integrating both geochronology and morphometrical results into lithospheric-scale numerical models allows a better understanding of this intraplate–orogen evolution and dynamic. We assume that the main conclusions are true to the general case of intraplate deformation. That is to say, once the topography has been generated by a triggering process, rock uplift is then enhanced by erosion and isostatic adjustment leading to a significant accumulation of mainly vertical deformation.

We report here the design, performance, and in situ demonstration, on‐land and offshore, of an innovative high‐resolution low‐cost optical (laser) seismometer. The instrument was developed within the Laser Interferometry for Earth Strain project (French Agence Nationale de la Recherche [ANR] program), and first tested at the low‐noise underground laboratory Laboratoire Souterrain à Bas Bruit (LSBB, France). It is based on Fabry–Pérot optical interferometry between the extremity of a probing optical fiber and a reflecting mirror secured to the mobile mass of a passive 2 Hz geophone. The detection technique is based on the wavelength modulation of the laser diode (1310 nm), which allows the separation of the optical power into two signals in quadrature, thanks to an heterodyne technique. The relative displacement of the mobile mass is retrieved in real time by the phase unwrapping of these two signals. At LSBB, the fiber was 3 km long. It recorded many teleseismic earthquakes and a few regional ones, and resolves the low‐seismic noise of the Earth for periods up to 6 s, presenting an acceleration noise floor lower than 1 ng/Hz in the 0.3–5 Hz range. A three‐component version of this fiber‐based interferometric 2 Hz geophone has been recently constructed, shielded in a hyperbaric container, and installed offshore for test in Brittany (France) in April 2018, with an improved control system. Its record of the marine ambient noise matches those of a collocated commercial broadband seismometer for periods up to 50 s. This opens promising perspectives for large‐scale ocean bottom instrumentation with up to 50‐kilometer‐long optical lines; an installation is planned for 2020, off Guadeloupe, with a 5‐kilometer‐long fiber cable. It may also prove useful for installations in other challenging and exposed environments, such as deep hot boreholes, active volcanoes, unstable landslides, for real‐time monitoring in regions with high natural hazard, but also for seismic monitoring of geoindustries.

The evolution of intra-plate orogens is still poorly understood. Yet, this is of major importance for understanding the Earth and plate dynamic, as well as the link between surface and deep geodynamic processes. The French Massif Central is an intraplate orogen with a mean elevation of 1000 m, with the highest peak elevations ranging from 1500 m to 1885 m. However, active deformation of the region is still debated due to scarce evidence either from geomorphological or geophysical (i.e. geodesy and seismology) data. Because the Cévennes margin allows the use of karst sediments geochronology and morphometrical analysis, we study the vertical displacements of that region: the southern part of the French Massif-Central. Geochronology and morphometrical results, helped with lithospheric-scale numerical modelling, allow, then, a better understanding of this intraplate-orogen evolution and dynamic. Using the ability of the karst to durably record morphological evolution, we first quantify the incision rates. We then investigate tilting of geomorphological benchmarks by means of a high-resolution DEM. We finally use the newly quantified incision rates to constrain numerical models and compare the results with the geomorphometric study. We show that absolute burial age (10Be/26Al on quartz cobbles) and the paleomagnetic analysis of karstic clay deposits for multiple cave system over a large elevation range correlate consistently. This correlation indicates a regional incision rate of 83.4 +17.3/−5.4 m Ma−1 during the last ca 4 Myrs (Plio-Quaternary). Moreover, we point out through the analysis of 55 morphological benchmarks that the studied region has undergone a regional southward tilting. This tilting is expected as being due to a differential vertical motion between the north and southern part of the studied area. Numerical models show that erosion-induced isostatic rebound can explain up to two-thirds of the regional uplift deduced from dating technics and are consistent with the southward tilting obtain from morphological analysis. We presume that the remaining part is related to dynamic topography or thermal isostasy due to the Massif Central plio-quaternary magmatism.

International audience; Borehole tiltmeters are widely used to continuously record small surface deformation of reservoirs and volcanoes. Because these instruments display unknown long-term drift, only short-term tilt signal can be used for monitoring purpose. We propose a method to invert long-term time series of tilt data induced by strain variations at depth. The assumption that tiltmeter drift is linear over time is on its own insufficient to remove the drift and uniquely determine the deformation source parameters. To overcome this problem, we first invert the data with no constrain on the drift to obtain one particular solution among all admissible. Then, using the linearity of the forward model, we use the statistical properties of the drift distributions to restore the uniqueness of the solution. We illustrate our approach with four synthetic cases simulating volume changes of a reservoir. We demonstrate the efficiency of our method and show that the accuracy of estimated volume variation dramatically improves if low drift tiltmeters are used.

In this paper, we investigate the impact of ambient temperature changes on the gravity reading of spring-based relative gravimeters. Controlled heating experiments using two Scintrex CG5 gravimeters allowed us to determine a linear correlation (R $$^{2}>$$ 0.9) between ambient temperature and gravity variations. The relation is stable and constant for the two CG5 we used: −5 nm/s $$^{2}/^\circ $$ C. A linear relation is also seen between gravity and residual sensor temperature variations (R $$^{2}>$$ 0.75), but contrary to ambient temperature, this relation is neither constant over time nor similar between the two instruments. The linear correction of ambient temperature on the controlled heating time series reduced the standard deviation at least by a factor of 2, to less than 10 nm/s $$^{2}$$ . The laboratory results allowed for reprocessing the data gathered on a field survey that originally aimed to characterize local hydrological heterogeneities on a karstic area. The correction of two years of monthly CG5 measurements from ambient temperature variations halved the standard deviation (from 62 to 32 nm/s $$^{2}$$ ) and led us to a better hydrological interpretation. Although the origin of this effect is uncertain, we suggest that an imperfect control of the sensor temperature may be involved, as well as a change of the properties of an electronic component.

International audience; Deep-seated gravitational spreading (DSGS) affects the slopes of formerly glaciated mountain ridges. On Mars, DSGS has played a key role in shaping the landforms of the giant Valles Marineris troughs. Though less spectacular, DSGS is common in terrestrial orogens, where understanding its mechanics is critical in the light of the ongoing climate change because it is a potential source of catastrophic landslides in deglaciated valleys. We conducted parametric numerical studies in order to identify important factors responsible for DSGS initiation. DSGS models are computed using an elastoviscoplastic finite element code. Using ADELI's software, we reproduce topographic ridge spreading under the effect of valley unloading. Two types of spreading topographic ridges are investigated, homogeneous or with horizontal rheological layering. We find that gravitational instabilities are enhanced by high slopes, which increase gravitational stress, and low friction and cohesion, which decrease yield stress. In the unlayered ridge, instability is triggered by glacial unloading with plastic strain concentration inside the ridge and at the base of the high slopes. Vertical fractures develop in the upper part of the slope, potentially leading to fault scarps. Ridge homogeneity promotes a deformation mode controlled by uphill-facing normal faulting and basal bulging. In the second case, the ridge encompasses horizontal geological discontinuities that induce rock mass anisotropy. Discontinuity located at the base of the slope accumulates plastic strain, leading to the formation of a sliding plane evolving into a landslide. The presence of a weak layer at ridge base therefore promotes another slope deformation mode ending up with catastrophic failure. Mechanical conditions and slope height being equal, these conclusions can probably be extrapolated to Earth. Compared with Mars, DSGS on Earth is inhibited because terrestrial topographic gradients are lower than in Valles Marineris, an effect counterbalanced by increased gravitational stress, where the intensity of deformation is enhanced because of the Earth gravity potential.

The magnitude of epikarstic water storage variation is evaluated in various karst settings using a relative spring gravimeter. Gravity measurements are performed over a 1.5-year period at the surface and inside caves at different depths on three karst hydro-systems in southern France: two limestone karst systems and one dolomite karst system. We find that significant water storage variations occur in the first 10 m of karst unsaturated zone. The subsurface water storage is also evidenced by complementary magnetic resonance sounding. The comparison between sites of the depth gravity measurements with respect to net water inflow suggests that seasonal water storage depends on the lithology. The transmissive function of the epikarst on the seasonal scale has been deduced from the water storage change estimation. Long (> 6 months) and short (

International audience; While vertical motion induced by long-term geological loads is often used to estimate the flexural rigidity of the lithosphere, we intend to evaluate the shear rigidity of the lithosphere using horizontal motion. Our approach considers that the rigidity of the lithosphere may be defined as its resistance to horizontal tectonic lateral forces. In this case, a spatial distribution of an effective shear rigidity can be estimated from the analysis of the interseismic velocity fields. We consider the Western United States zone where weakly strained areas (e.g., the Sierra Nevada) are connected with areas of large strain rate (e.g. San Andreas Fault system). By inverting interseismic strain distribution measured by geodetic methods, we infer the effective shear rigidity of the lithosphere. The forward problem is defined using the equations of linear elasticity. The inversion relies on the minimization of the sum of a quadratic measure of the differences between measured and modelled velocity fields. The functional also includes regularization terms for the parameters of the model. The gradient of the functional with respect to the minimization parameters is computed using an adjoint formulation. This permits the treatment of large dimensional minimization problems. Finally, a measure of the uncertainty of our inversion is illustrated through the covariance matrix of the parameters at the optimum. The optimization chart is validated on two synthetic velocity distributions. Then, the effective shear rigidity variations of the Western United States are estimated using the CMM3 interseismic velocities. The inversion displays low effective rigidities along the San Andreas Fault system, the Mojave Desert and in the Eastern California Shear Zone, while rigid areas are found in the Sierra Nevada and in the South Basin and Range. Finally, we discuss the differences between our strain rate and rigidity maps with previously published results for the Western United States.

International audience; The Pyrenean mountain range is a slowly deforming belt with continuous and moderate seismic activity. To quantify its deformation field, we present the velocity field estimated from a GPS survey of the Pyrenees spanning 18 yr. The PotSis and ResPyr networks, including a total of 85 GPS sites, were installed and first measured in 1992 and 1995–1997, respectively, and remeasured in 2008 and 2010. We obtain a deformation field with velocities less than 1 mm yr−1 across the range. The estimated velocities for individual stations do not differ significantly from zero with 95 per cent confidence. Even so, we estimate a maximum extensional horizontal strain rate of 2.0 ± 1.7 nanostrain per year in a N–S direction in the western part of the range. We do not interpret the vertical displacements due to their large uncertainties. In order to compare the horizontal strain rates with the seismic activity, we analyse a set of 194 focal mechanisms using three methods: (i) the ‘r’ factor relating their P and T axes, (ii) the stress tensors obtained by fault slip inversion and (iii) the strain-rate tensors. Stress and strain-rate tensors are estimated for: (i) the whole data set, (ii) the eastern and western parts of the range separately, and (iii) eight zones, which are defined based on the seismicity and the tectonic patterns of the Pyrenees. Each of these analyses reveals a lateral variation of the deformation style from compression and extension in the east to extension and strike-slip in the west of the range. Although the horizontal components of the strain-rate tensors estimated from the seismic data are slightly smaller in magnitude than those computed from the GPS velocity field, they are consistent within the 2σ uncertainties. Furthermore, the orientations of their principal axes agree with the mapped active faults.

In this study we attempt to evaluate the magnitude of epikarstic water storage variation in various karst settings using a relative spring gravimeter. Gravity measurements are performed two times a year at the surface and inside caves at different depths on three karst aquifers in southern France: two limestone karst systems and one dolomite karst system. We find that water storage variations occur mainly in the first ten meters of karst unsaturated zone. Afterward, surface to depth gravity measurements are compared between the sites with respect of net water inflow. A difference of seasonal water storage is evidenced probably associated with the lithology. The transmissive function of the epikarst has been partially deduced from the water storage change estimation. Long (> 6 months) and short (

International audience; Optimizing the resource extraction and limiting the environmental constraint motivate to accurately characterize reservoirs. The extraction of resources induces volume change of the reservoir causing displacements in the surrounding medium up to the surface. The resulting surface deformation is usually significant enough to be measurable by geodetic instruments (GPS, satellites, tiltmeters, strainmeters). However, the data needs to be associated to a reservoir model using an inverse scheme to allow the estimation of physical properties of the reservoir. Complete geodetic data including InSAR, GPS, tilt, strain and gravimetric data are uncommon for conventional, unconventional or mining exploitations. In this study, we work on the one site that gathers a wide range of geodetic data: the salt exploitation of Vauvert, France, operated by KemOne. Our approach aims at estimating the volume changes of the cavities created by the extraction combining all the available geodetic data. To do so, we minimize a global functional including weighting factors for each data type. The weighting factors are optimally estimated for our study and can be adapted depending on the issue. Hence, an optimal distribution of data can be suggested for a specific reservoir characterization.

We present here the performance of an innovative, high resolution, robust, and low-cost optical seismometer. The instrument was developed within the LINES project ( ANR, 2009-2012), and tested at the low noise underground laboratory of LSBB (France). It is based on a Fabry-Perot interferometry between the extremity of a laser fiber and a reflecting mirror fixed on the mobile mass of a passive geophone (2 Hz ). At the other extremity of the fiber, 3 km long, an optical system includes a laser diode (1310 nm) and the recording and control systems. A double modulation (at 50 KHz and 1 KHz ) of the intensity of the diode current produces a wavelength modulation, allowing to calculate in real-time the position on the mass, thanks to fast and innovative numerical processing. It ran more than two years, recording many teleseismic earthquakes and a few regional ones. It resolves the low seismic noise of the Earth for periods as low as 5 s, and presents an acceleration floor noise lower than 1 ng/sqrt(Hz) in the 0.2-5 Hz range. The LINEs optical seismometer is thus a promising instrument for the microseismic monitoring of deep and/or hot industrial boreholes.

International audience; Tilt fluctuations can potentially reflect the response of hydrosystems to important rainfall. In this context, long baseline tiltmeters have been installed in an underground tunnel penetrating the Fontaine de Vaucluse karst to study the medium deformation related to solicitations exerted by water infiltrating the hydrosystem. The instruments monitor the tilt as well as its spatial variation. Northward tilts reaching a 1 μrad amplitude are observed consecutively to rainfalls. The tilt amplitude is highly correlated with the Fontaine de Vaucluse outlet flow fluctuations. The measured tilt signal is also relatively homogeneous over a 150 m length. Different types of structure likely to produce such observations are tested in order to identify their location with respect to the tiltmeters, their dimension as well as the amount of water level variation in the structure. Following rainfalls, the infiltration of water modifies the pore pressure, inducing a medium deformation. The hypothesis of an homogeneous surface loading on the Vaucluse plateau is first refuted since the related tilt is much lower than the one measured. The water supplied by rainfalls has to accumulate in discontinuities in order to generate a higher tilt. So, the deformation related to a pressure exerted on a fracture filled by water is assessed. A first study reveals the interest of the tilt homogeneity information that constrains strongly the fracture properties. Thus, the fracture must be located at a distance more than a few hundreds metres from the tiltmeters in order to produce a tilt homogeneous in space. If the fracture is initially dry, it must also be filled on a height higher than 150 m consecutive to a rainfall in order to generate a tilt amplitude in the same magnitude as the one measured. Then, we explore the influence of water level variations on the tilt produced by a fracture located at the interface between the saturated and unsaturated zones, which are thereby permanently flooded. Since several parameters of that model satisfactorily explain the field observations, we discuss how simultaneous geodetical observations could provide complementary information that would further constrain the geometry of the structure at the origin of the medium deformation.

In this paper we present the potential of a new compact superconducting gravimeter (GWR iGrav) designed for groundwater monitoring. At first, 3 yr of continuous gravity data are evaluated and the performance of the instrument is investigated. With repeated absolute gravity measurements using a Micro-g Lacoste FG5, the calibration factor (−894.8 nm s−2 V−1) and the long-term drift of this instrument (45 nm s−2 yr−1) are estimated for the first time with a high precision and found to be respectively constant and linear for this particular iGrav. The low noise level performance is found similar to those of previous superconducting gravimeters and leads to gravity residuals coherent with local hydrology. The iGrav is located in a fully instrumented hydrogeophysical observatory on the Durzon karstic basin (Larzac plateau, south of France). Rain gauges and a flux tower (evapo-transpiration measurements) are used to evaluate the groundwater mass balance at the local scale. Water mass balance demonstrates that the karst is only capacitive: all the rainwater is temporarily stored in the matrix and fast transfers to the spring through fractures are insignificant in this area. Moreover, the upper part of the karst around the observatory appears to be representative of slow transfer of the whole catchment. Indeed, slow transfer estimated on the site fully supports the low-flow discharge at the only spring which represents all groundwater outflows from the catchment. In the last part of the paper, reservoir models are used to characterize the water transfer and storage processes. Particular highlights are done on the advantages of continuous gravity data (compared to repeated campaigns) and on the importance of local accurate meteorological data to limit misinterpretation of the gravity observations. The results are complementary with previous studies at the basin scale and show a clear potential for continuous gravity time-series assimilation in hydrological simulations, even on heterogeneous karstic systems.

Previous estimates of the geodetic and geologic slip rates of the 1500 km long Altyn Tagh fault bordering the northern edge of the Tibetan plateau vary by a factor of five. Proposed reasons for these discrepancies include poor GPS geometry, interpretative errors in terrace morphology, and changes in fault slip rate over time. Here we present results from a new dense GPS array orthogonal to the fault at ~86.2°E that indicates a velocity of 9.0−3.2/+4.4 mm/yr, in close agreement with geomorphologic estimates at the same location. Our estimated geodetic slip rate is consistent with recent geological slip rates based on terrace offsets. The resulting mean combined geological and geodetic slip rate (9.0 ± 4.0 mm/yr) is remarkably uniform for the central ~800 km of the Altyn Tagh fault, significantly lower than early kinematic estimates and consistent with deformation elsewhere in Tibet and central Asia.

International audience; The Betic-Rif orocline surrounding the Alboran Sea, the westernmost tip of the Mediterranean Sea, accommodates the NW–SE convergence between the Nubia and Eurasia plates. Recent GPS observations indicate a ∼4 mm/yr SW motion of the Rif Mountains, relative to stable Nubia, incompatible with a simple two-plate model. New gravity data acquired in this study define a pronounced negative Bouguer anomaly south of the Rif, interpreted as a ∼40 km-thick crust in a state of non-isostatic equilibrium. We study the correlation between these present-day kinematic and geodynamic processes using a finite-element code to model in 2-D the first-order behavior of a lithosphere affected by a downward normal traction (representing the pull of a high-density body in the upper mantle). We show that intermediate viscosities for the lower crust and uppermost mantle (View the MathML source1021–1022Pas) allow an efficient coupling between the mantle and the base of the brittle crust, thus enabling (1) the conversion of vertical movement, resulting from the downward traction, to horizontal movement and (2) shortening in the brittle upper crust. Our results show that incipient delamination of the Nubian continental lithosphere, linked to slab pull, can explain the present-day abnormal tectonics, contribute to the gravity anomaly observed in northern Morocco, and give insight into recent tectonics in the Western Mediterranean region.

International audience; More than 10 years of geodetic measurements demonstrate an uplift rate of 1–3 mm/yr of the high topography region of the Western Alps. By contrast, no significant horizontal motion has been detected. Two uplift mechanisms have been proposed: (1) the isostatic response to denudation responsible for only a fraction of the observed uplift and (2) the rebound induced by the Wurmian ice cap melting which predicts a broader uplifting region than the one evidenced by geodetic observations. Using a numerical model to fit the geodetic data, we show that a crustal viscosity contrast between the foreland and the central part of the Alps, the latter being weaker with a viscosity of 1021 Pa s, is needed. The vertical rates are enhanced if the strong uppermost mantle beneath the Moho is interrupted across the Alps, therefore allowing a weak vertical rheological anomaly over the entire lithosphere.

SUMMARY This paper presents an inversion method using the interseismic velocity field to determine effective rigidity of the lithosphere. The method is based on the minimization of a cost function defined as the quadratic measure of the difference between measured and modelled velocity fields on a discrete set of points. The continuous mapping of the rigidity is fulfilled with a limited set of parameters and the forward solution is achieved using a plane stress finite element code. The computation of the cost function gradient in the parameters’ space allows one to iteratively find the best parameters set through a suitable optimization algorithm. We first design a benchmark including an abrupt rigidity variation that cannot be described by a continuous function. For such a case, we show that increasing the number of parameters is a way to accurately describe sharp variations of the rigidity map. Then, we use a dense GPS velocity field over the southwestern United States to estimate the corresponding rigidity variations for different spatial resolutions of the parameters’ grid. We analyse the conceptual and practical difficulties associated with our methodology. Finally, rigidity maps obtained by our inversion method in southwestern United States and particularly across the San Andreas Fault System are reviewed and compared to current plate rigidity estimates and geophysical data over this area.

International audience; We present the results of dynamic modeling of the western Mediterranean that accounts for observed global positioning system (GPS) surface deformation of the Alboran Sea and surrounding Rif and Betic Mountains as the result of the combined effects of relative motion of the Eurasian and Nubian plates, low strength in the Alboran Sea region and sub-lithospheric processes occurring beneath the External Rif domain. Assuming that the lithosphere behaves elastically over the short time period of the GPS observations, an elastic plate model is considered in our study, including an area of weak lithosphere (factor of 10) centered on the Alboran Sea and in which lateral boundary conditions consist of the Nubia-Eurasia oblique convergence. Sub-crustal processes are modeled by application of a horizontal traction on a small area (patch) at the base of the elastic plate. Our modeling studies demonstrate the need for sub-crustal or sub-lithospheric, southwestward-directed forcing to account for observed southwestward motion of the Rif and Betic domains. Based on the location, orientation, and small area of the traction patch, we hypothesize that forcing is associated with delamination and rollback of the subducted African continental lithospheric mantle beneath the External Rif zone, due to the pull of the oceanic part of the Western Mediterranean slab, a dynamic process that may be similar to that where the over-riding plate is driven toward the subduction zone during slab rollback.

International audience; In this study we attempt to understand the water storage variations in a karst aquifer on the Larzac Plateau (South of France) using ground-based gravimetry. Surface to 60 m depth gravity measurements are performed three times a year since 2006 down a pothole, in complement to monthly absolute gravity (AG) measurements at three sites. The time variations of the surface to depth (STD) gravity differences are compared with the AG variations. Using a simple Bouguer plate model, we find that the STD gravity differences are very similar to the AG variations. The STD gravity differences are then used to determine apparent density values. These integrative density values are compared with measured grain densities from core samples to obtain an apparent porosity and saturation change representative of the investigated depth. The apparent porosity ranges from 4.8 to 7.3 per cent. We then discuss on the repartition of the apparent physical properties with respect to the epikarst and infiltration zone karst structures. We argue that AG and STD differences monitor epikarst water storage variations. Within this scope, we discuss the fact that seasonal scale water storage variation occurs predominantly in the epikarst.

International audience; We use 3-D mechanical modeling representing faults as planar surfaces with frictional properties that obey Coulomb-failure process to explore the long-term slip rates of the Altyn Tagh fault and Kunlun faults in the north Tibetan plateau. Crustal theology is simplified as an elastoplastic upper crust and a viscoelastic lower crust. Far-field GPS velocities and late Quaternary fault slip rates are used to constrain the model results. Rheological tests show that effective fault friction lower than 0.1-0.08 leads to high slip rates that fit with geologically and geodetically determined slip rates of the Kunlun fault (10-11.7 +/- 1.5 mm/yr). Meanwhile, the modeled Altyn Tagh fault reaches slip rates similar to 13.7 mm/yr to similar to 7.8 mm/yr in its central portion, between ranges of the geological slip rates. Associated with high slip rates, our model predicts that central Tibet (similar to 84 degrees E-95 degrees E) from the Altyn Tagh fault to the north of the Himalayan arc accommodates north-south shortening and east-west extension rates of similar to 10-12 mm/yr and similar to 8-10 mm/yr, respectively. We also question the widely accepted idea that interseismic strain is driven at the base of the seismogenic zone by a screw dislocation. If this assumption fails, the presented model implies that interseismic strain around large strike-slip faults could be distributed in a much broader way if the lithosphere deforms as a thin elastic plate rather than an elastic half-space with an embedded dislocation. If this distributed deformation is ignored, and the instantaneous surface deformation field modeled as that resulting from slip on a dislocation below a specified depth embedded in an elastic half-space, the estimated slip rate will inevitably be lower than the true long-term slip rate. This appears to explain why geodetic slip rates proposed for the Altyn Tagh fault (510 mm/yr) are lower than some of the geological slip rates. (C) 2008 Elsevier B.V. All rights reserved.

International audience; The interseismic velocity field provided by geodetic methods is generally interpreted in the framework of a thick elastic lithosphere with a slipping fault at depth. Because lateral variations of lithospheric rheology play a key role in determining the geological strain distribution, I examine the idea that interseismic strain rate variations also occur in response to lateral variations in the elastic thickness of the lithosphere. Using a stress balance principle and some simplifying assumptions, I show using a 1D model that elastic thickness is inversely proportional to strain rate for the simple case of pure strike-slip faulting. Elastic thickness computed on three profiles crossing the San Andreas fault system (SAFS) suggests that the distribution of interseismic strain rate is compatible with a thick elastic lithosphere in the Great Basin-Sierra Nevada province and on the Pacific plate. Conversely, a thin plate with a shallow asthenosphere is needed on the SAFS to explain its high strain rate. A 2.5D Finite Element model of interseismic strain in the Carrizo Plain region in Central California shows how known vertical and horizontal variations of elastic properties refine 1D model predictions. In a case of a multiple fault system, I point out that the interseismic velocity is not causally tied to faults slip rate. Therefore, analysing the velocity field across the SAFS cannot reliably provide faults slip rate distribution as previously claimed. Rather, the apparent correlation between geologic slip rate and interseismic strain may only indicate that the elastic thickness plays a dominant role in controlling fault strength. Finally, I suggest that interseismic geodetic strain could be a new way to infer effective elastic plate thickness on the continents.

International audience; Sensitive instruments like strainmeters and tiltmeters are necessary for measuring slowly varying low amplitude Earth deformations. Nonetheless, laser and fibre interferometers are particularly suitable for interrogating such instruments due to their extreme precision and accuracy. In this paper, a practical design of a simple pendulum borehole tiltmeter based on laser fibre interferometric displacement sensors is presented. A prototype instrument has been constructed using welded borosilicate with a pendulum length of 0.85 m resulting in a main resonance frequency of 0.6 Hz. By implementing three coplanar extrinsic fibre Fabry-Perot in-terferometric probes and appropriate signal filtering, our instrument provides tilt measurements that are insensitive to parasitic deformations caused by temperature and pressure variations. This prototype has been installed in an underground facility (Rustrel, France) where results show accurate measurements of Earth strains derived from Earth and ocean tides, local hydro-logic effects, as well as local and remote earthquakes. The large dynamic range and the high sensitivity of this tiltmeter render it an invaluable tool for numerous geophysical applications such as transient fault motion, volcanic strain and reservoir monitoring.

International audience; A fiber interferometer interfaced to 3 geo-mechanical elements is presented for applications in geophysics. The fiber sensor is based on an extrinsic fiber Fabry-Perot interferometer (EFFPI) which incorporates a modulation scheme to lock the interferometer at quadrature and to enable displacement measurements below a quarter of the interrogating wavelength. It operates over a relatively large frequency dynamic of ~500000 with a precision better than 2 nm. The fiber interferometer is next interfaced to a differential hydrostatic long baseline inclinometer, a 3-axis borehole tiltmeter and a single-axis seismometer, respectively. Results obtained demonstrate that the fiber interferometrically-interrogated instruments exhibit performances equivalent to or even surpassing those of the reference instruments employed for comparison during their deployment to an underground test site since March 2012. Keywords—modulation-based extrinsic fiber Fabry-Perot interferometer; fiber optique hydrostatic inclinometer; fiber optique borehole tiltmeter; fiber optique seismometer

International audience; Measurements on either side of the Kazerun fault system in the Zagros Mountain Belt, Iran, show that the accommodation of the convergence of the Arabian and Eurasian Plates differs across the region. In northwest Zagros, the deformation is partitioned as 3–6 mm yr−1 of shortening perpendicular to the axis of the mountain belt, and 4–6 mm yr−1 of dextral strike-slip motion on northwest–southeast trending faults. No individual strike-slip fault seems to slip at a rate higher than ~2 mm yr−1. In southeast Zagros, the deformation is pure shortening of 8 ± 2 mm yr−1 occurring perpendicular to the simple folded belt and restricted to the Persian Gulf shore. The fact that most of the deformation is located in front of the simple folded belt, close to the Persian Gulf, while seismicity is more widely spread across the mountain belt, confirms the decoupling of the surface sedimentary layers from the seismogenic basement. A comparison with the folding and topography corroborates a southwestward propagation of the surface deformation. The difference in deformation between the two regions suggests that right-lateral shear cumulates on the north–south trending Kazerun strike-slip fault system to 6 ± 2 mm yr−1.

Contrasted tectonic styles occur in northern Iran. West of Kopet Dagh, the south Caspian basin is an aseismic and possibly rigid blockinvolved inthecollisionzonebetweenEurasiaandArabia.TotheNW,largescalevelocityfield ofIrangivenbyrecentrepeated GPS surveys suggests ∼8 mm/yr of right-lateral displacement for the WNW–ESE faults in the Tabriz region. More surprisingly, geodetic motion also suggests a prominent N 30° extension north to the Tabriz fault where compression would be expected. In order to quantify more precisely the right-lateral movement of the Tabriz region, the extension of NW Iran and the motion of the south Caspian basin, we deployed a dense GPS network in NW Iran, from Central Iran to the Turkish, Armenian and Azerbaijan borders. Three repeated surveys from 2002 to 2004 assess that the Tabriz fault concentrates entirely the ∼8 mm/yr of right-lateral movement observed in NW Iran. This rate is in good agreement with a recurrence interval time of 250–300 yr proposed from historical seismicity studies. We found also two zones of extension, one just north of the Tabriz fault in the south of the Talesh plateau and another close from the Azerbaijan border north of the Talesh plateau. We suggest that the existence of a northward subduction of the south Caspian Basin could explain such an extension in the core of the Arabia–Eurasia collision. Increasing evidences of quaternary extensional tectonics in the region from Armenia to Alborz may lead to significantly alter our understanding of the northern Iran tectonics and challenge the traditional Arabian indenter plate tectonics.

The GPS velocity field of the present-day deformation in Iran is modeled using a 3-dimensional (3D) finite element approach. The deformation can be accommodated either by a continuum medium or by faults which are modeled using discontinuities with Coulomb-failure criteria. Depending on the fault friction, the deformation will be accommodated by the continuum medium or by the faults. Therefore, no a priori hypothesis on continuum or microplate behavior is assumed. In addition, geological fault slip rates are used to better determine the optimum model. The best model fitting both GPS and geological data shows quasi-rigid blocks within the deformation zone and low effective fault friction for the main Iranian strike slip faults. The mechanical behavior of the Iranian lithosphere seems to be partly controlled by the large strike slip faults. However, some deformation is still taken up by the continuum medium, suggesting a compromise between the microplate and continuum descriptions. Results also suggest localized shear zones in the mantle underneath the crustal faults bordering the quasi-rigid blocks. Lastly, the Arabia push relative to Eurasia explains most of the kinematics in Iran, but the complex velocity field of the surrounding South Caspian basin cannot be fitted by this model. Therefore, slab pull due to a remnant oceanic crust may occur in the Caucasus–Caspian region.

SUMMARY Recent GPS surveys indicate that the Zagros kinematics corresponds to an oblique convergence between a rigid central Iranian plateau and the Arabian plate at ∼ 7m m yr −1 at the longitude of the Persian Gulf. Convergence is almost frontal in the SE Zagros and oblique (45 ◦ )i n the NW part of the range. It has been proposed that internal deformation of the NW Zagros occurs in a partitioned mode. In such a view, the Main Recent Fault (MRF) bordering the Iranian plateau accommodates all the tangential motion, while shortening happens by pure thrusting within the fold and thrust belt as suggested by the focal mechanisms within the range. We use a 2.5-D mechanical finite element model of the Zagros to understand the influence on the Zagros deformation of (1) the obliquity of convergence, (2) the rheological layering of the lithosphere (strong upper crust, weak lower crust, strong or weak uppermost mantle) and (3) a possible weakness of the MRF. Surprisingly, a fully partitioned mode occurs only when the collision is very oblique. In the case of the NW Zagros, we find that the MRF can accommodate only ∼25 per cent of the whole tangential motion. As the GPS inferred range parallel motion across the NW Zagros is ∼ 5m m yr −1 , the present day slip rate of this fault could be as low as 1.2 mm yr −1 . In agreement with the observed strain in SE Zagros, our model predicts that no strike-slip activity takes place on a vertical fault when the direction of convergence is more frontal. We discuss the implications of our model on the seismicity and kinematics of oblique collision zones by comparing our results with the available data in the Zagros and other oblique convergence domains.

Based on the agreement between geodetic and geological plate velocities, interplate fault slip rates are usually considered constant over long periods of time. However, measurements made at different time scales on intracontinental faults suggest that slip rate evolves with time. We examine the slip evolution of a fault embedded in an elastic lithosphere loaded by plate motion. We first assume that the fault friction varies due to a climatic cause. Then we show that high fault stress and low lithospheric stiffness favour large variations of slip rate. In the case where fault weakening is controlled by slip rate, we find that high loading velocity leads to a low stress, constant slip rate, while low loading velocity drives the fault slip rate to cycle between high and low values. This suggests that paleoseismic slip rate could overpass the loading velocity but also fall to zero for some period of time.

SUMMARY A combined analysis of the geodetic strain-rate field and the strain-rate field deduced from the seismicity allows us to define the style of deformation and to distinguish seismic from aseismic deformation. We perform this analysis in Iran where the present-day tectonics results from the north‐south convergence between the plates of Arabia to the south and Eurasia to the north. The data consist of velocities measured with a GPS network of 28 benchmarks and of instrumental and historical earthquake catalogues. The axes of the seismic strain-rate tensor have similar orientations to those deduced from the GPS velocity field. This indicates that the seismicity can be used to improve GPS information on the style and the orientation of the deformation. Comparison of seismic and geodetic strain rates indicates that highly strained zones experience mainly aseismic deformation in southern Iran and seismic deformation in northern Iran. A large contrast is observed between the Zagros (less than 5 per cent seismic deformation) and the Alborz‐Kopet-Dag regions (more than 30‐100 per cent seismic deformation). The distribution of the seismic/geodetic ratio correlates with the distribution of large earthquakes: intensive, low-magnitude seismicity is observed in the Zagros whereas the largest earthquakes occur in northern Iran. The contrast of seismic deformation between the Zagros and peri-Caspian mountains is confirmed considering 300 or 1000 yr of seismicity rather than 100 or 200 yr.

The Alborz is a narrow (100 km) and elevated (3000 m) mountain belt which accommodates the differential motion between the Sanandaj–Sirjan zone in central Iran and the South Caspian basin. GPS measurements of 12 geodetic sites in Central Alborz between 2000 and 2002 allow to constrain the motion of the belt with respect to western Eurasia. One site velocity on the Caspian shoreline suggests that the South Caspian basin moves northwest at a rate of 6±2 mm/year with respect to western Eurasia. North–South shortening across the Alborz occurs at 5±2 mm/year. To the South, deformation seems to extend beyond the piedmont area, probably due to active thrusting on the Pishva fault. We also observe a left-lateral shear of the overall belt at a rate of 4±2 mm/year, consistent with the geological motion observed along E–W active strike-slip faults inside the belt (e.g., the Mosha fault).

SUMMARY A network of 27 GPS sites was implemented in Iran and northern Oman to measure displacements in this part of the Alpine‐Himalayan mountain belt. We present and interpret the results of two surveys performed in 1999 September and 2001 October. GPS sites in Oman show northward motion of the Arabian Plate relative to Eurasia slower than the NUVEL-1A estimates (e.g. 22 ± 2m m yr −1 at N8 ◦ ± 5 ◦ E instead of 30.5 mm yr −1 at N6 ◦ E at Bahrain longitude). We define a GPS Arabia‐Eurasia Euler vector of 27.9 ◦ ± 0.5 ◦ N, 19.5 ◦ ± 1.4 ◦ E, 0.41 ◦ ± 0.1 ◦ Myr −1 . The Arabia‐Eurasia convergence is accommodated differently in eastern and western Iran. East of 58 ◦ E, most of the shortening is accommodated by the Makran subduction zone (19.5 ± 2m m yr −1 ) and less by the Kopet-Dag (6.5 ± 2m m yr −1 ). West of 58 ◦ E, the deformation is distributed in separate fold and thrust belts. At the longitude of Tehran, the Zagros and the Alborz mountain ranges accommodate 6.5 ± 2m m yr −1 and 8 ± 2m m yr −1 respectively. The right-lateral displacement along the Main Recent Fault in the northern Zagros is about 3 ± 2m m yr −1 , smaller than what was generally expected. By contrast, large rightlateral displacement takes place in northwestern Iran (up to 8 ± mm yr −1 ). The Central Iranian Block is characterized by coherent plate motion (internal deformation < 2m m yr −1 ). Sites east of 61 ◦ E show very low displacements relative to Eurasia. The kinematic contrast between eastern and western Iran is accommodated by strike-slip motions along the Lut Block. To the south, the transition zone between Zagros and Makran is under transpression with right-lateral displacements of 11 ± 2m m yr −1 .

The rate of crustal deformation in Iran due to the Arabia–Eurasia collision is estimated. The results are based on new global positioning system (GPS) data. In order to address the problem of the distribution of the deformation in Iran, Iranian and French research organizations have carried out the first large-scale GPS survey of Iran. A GPS network of 28 sites (25 in Iran, two in Oman and one in Uzbekistan) has been installed and surveyed twice, in September 1999 and October 2001. Each site has been surveyed for a minimum observation of 4 days. GPS data processing has been done using the GAMIT-GLOBK software package. The solution displays horizontal repeatabilities of about 1.2 mm in 1999 and 2001. The resulting velocities allow us to constrain the kinematics of the Iranian tectonic blocks. These velocities are given in ITRF2000 and also relative to Eurasia. This last kinematic model demonstrates that (1) the north–south shortening from Arabia to Eurasia is 2–2.5 cm/year, less than previously estimated, and (2) the transition from subduction (Makran) to collision (Zagros) is very sharp and governs the different styles of deformation observed in Iran. In the eastern part of Iran, most of the shortening is accommodated in the Gulf of Oman, while in the western part the shortening is more distributed from south to north. The large faults surrounding the Lut block accommodate most of the subduction–collision transition.

The North Anatolian fault (NAF) extends over 1500 km in a complex tectonic setting. In this region of the eastern Mediterranean, collision of the Arabian, African and Eurasian plates resulted in creation of mountain ranges (i.e. Zagros, Caucasus) and the westward extrusion of the Anatolian block. In this study we investigate the effects of crustal rheology on the long-term displacement rate along the NAF. Heat flow and geodetic data are used to constrain our mechanical model, built with the three-dimensional finite element code ADELI. The fault motion occurs on a material discontinuity of the model which is controlled by a Coulomb-type friction. The rheology of the lithosphere is composed of a frictional upper crust and a viscoelastic lower crust. The lithosphere is supported by a hydrostatic pressure at its base (representing the asthenospheric mantle). We model the long-term deformation of the surroundings of the NAF by adjusting the effective fault friction and also the geometry of the surface fault trace. To do so, we used a frictional range of 0.0^0.2 for the fault, and a viscosity varying between 10 19 and 10 21 Pa s. One of the most striking results of our rheological tests is that the upper part of the fault is locked if the friction exceeds 0.2. By comparing our results with geodetic measurements [McClusky et al., J. Geophys. Res. B 105 (2000) 5695^5719] and tectonic observations, we have defined a realistic model in which the displacement rate on the NAF reaches V17 mm/yr for a viscosity of 10 19 Pa s and a fault friction of 0.05. This strongly suggests that the NAF is a weak fault like the San Andreas fault in California. Adding topography with its corresponding crustal root does not induce gravity flow of Anatolia. Rather, it has the counter-intuitive effect of decreasing the westward Anatolian escape. We find a poor agreement between our calculated velocity field and what is observed with GPS in the Marmara and the Aegean regions. We suspect that the simple lithosphere model is responsible for this discrepancy. Taking into account the weaknesses of these deforming regions should allow us to build a more realistic model that would match ground observations more appropriately. On the other hand, our results fit well GPS measurements in central Anatolia, setting the basis of modeling crustal strain in Turkey. B 2003 Elsevier Science B.V. All rights reserved.

International audience; The Western Alps are among the best studied collisional belts with both detailed structural mapping and also crustal geophysical investigations such as the ECORS and EGT seismic profile. By contrast, the present-day kinematics of the belt is still largely unknown due to small relative motions and the insufficient accuracy of the triangulation data. As a consequence, several tectonic problems still remain to be solved, such as the amount of N-S convergence in the Occidental Alps, the repartition of the deformation between the Alpine tectonic units, and the relation between deformation and rotation across the Alpine arc. In order to address these problems, the GPS ALPES group, made up of French, Swiss and Italian research organizations, has achieved the first large-scale GPS surveys of the Western Alps. More than 60 sites were surveyed in 1993 and 1998 with a minimum observation of 3 days at each site. GPS data processing has been done by three independent teams using different software. The different solutions have horizontal repeatabilities (N-E) of 4-7 mm in 1993 and 2-3 mm in 1998 and compare at the 3-5-mm level in position and 2-mm/ yr level in velocity. A comparison of 1993 and 1998 coordinates shows that residual velocities of the GPS marks are generally smaller than 2 mm/yr, precluding a detailed tectonic interpretation of the differential motions. However, these data seem to suggest that the N-S compression of the Western Alps is quite mild (less than 2 mm/yr) compared to the global convergence between the African and Eurasian plate (6 mm/yr). This implies that the shortening must be accomodated elsewhere by the deformation of the Maghrebids and/ or by rotations of Mediterranean microplates. Also, E-W velocity components analysis supports the idea that E-W extension exists, as already suggested by recent structural and seismotectonic data interpretation.

Three M>8 earthquakes have occurred in Mongolia during a 52-year period (1905–1957). Since these earthquakes were well separated in space (400 km), the coseismic stress change is far too low (0.001 bar) to explain mutual earthquake triggering. By contrast, postseismic relaxation gradually causes a significant stress change (0.1–0.9 bar) over large distances. Using a spring-slider model to simulate earthquake interaction, we find that viscoelastic stress transfer may be responsible for the earthquake time clustering observed in active tectonic areas. Therefore, revealing postseismic strain by satellite geodesy and modeling earthquake clusters could improve our understanding of earthquake occurrence, especially in zones where large earthquakes have already struck in past decades.

We develop a theory that links stress interaction between earthquakes and the occurrence of temporal clustering. Coseismic static stress change in the vicinity (50 km) of large earthquakes suggests that perturbations of 0.1 to 1 bars may affect the occurrence of other earthquakes. At larger distances, interactions also seem to exist: four M 8 earthquakes have occurred in Mongolia on distant faults (400 km) during the last century. Also, paleoseismic observations documenting much longer time periods display a time clustering of major events. We demonstrate with simple mechanical concepts that postseismic stress relaxation magnifies the coseismic stress change and has a major effect on fault interaction during the seismic cycle. In the simple case where two distant faults are coupled, the probabilistic occurrence of triggered earthquakes may increase dramatically due to long range postseismic coupling.

Several lines of evidence support the general view of the San Andreas fault system (SAFS) as a major lithospheric weakness in a generally transpressive plate margin setting. However, the influence of the weakness of the SAFS on the observed stress and deformation fields is not straightforward because factors such as interactions between the brittle upper crust and ductile lower crust, lateral fault strength variation, and the amount of convergence may all be important. The goal of this study is to model steady state deformation, relative fault-parallel velocity, and crustal stress orientations in central and northern California using realistic rheologies and boundary conditions. Using a simplified three-dimensional (3-D) finite element analysis in large strain, we model the SAFS in 2-D cross sections with no stress or strain variations along strike. We investigate the influence of different parameters such as the frictional properties of the fault zone and the adjacent crust, the viscous properties of the lower crust as determined by its thermal structure, and the thermal structure of the lithosphere. Our model appears to provide a good conceptual framework for some first-order aspects of actively deforming plate margins such as the SAFS. The following findings emerge from a variety of numerical experiments: (1) The difference in the manner of transpressive strain partitioning in central and northern California can be explained by different fault strengths and the manner in which heat flow varies with distance from the fault. (2) Only the combination of a weak fault (with an effective friction coefficient of ≈0.1) and a strong lateral heat flow variation predicts approximately correct stress directions in the crust adjacent to the SAFS.

The kinematics of the present-day deformation in the western Alps is still poorly known, mostly because of a lack of direct measurements of block motion and internal deformation. Geodetic measurements have the potential to provide quantitative estimates of crustal strain and block motion in the Alps, but the low expected rates, close to the accuracy of the geodetic techniques, make such measurements challenging. Indeed, an analysis of 2.5 years of continuous GPS data at Torino (Italy), Grasse (France), and Zimmerwald (Switzerland), showed that the present-day differential motion across the western Alps does not exceed 3 mm/yr [Calais, 1999]. Continuous measurements performed at permanent GPS stations provide unique data sets for rigorously assessing crustal deformation in regions of low strain rates by reducing the amount of time necessary to detect a significant strain signal, minimizing systematic errors, providing continuous position time series, and possibly capturing co- and post-seismic motion. In 1997, we started the implementation of a network of permanent GPS stations in the western Alps and their surroundings (REGAL network). The REGAL network mostly operates dual frequency Ashtech Z12 CGRS GPS stations with choke-ring antennae. In most cases, the GPS antenna is installed on top of a 1.5 to 2.5 m high concrete pilar directly anchored into the bedrock. The data are currently downloaded once daily and sent to a data center located at Geosciences Azur, Sophia Antipolis where they are converted into RINEX format, quality checked, archived, and made available to users. Data are freely available in raw and RINEX format at http://kreiz.unice.fr/regal/. The GPS data from the REGAL network are routinely processed with the GAMIT software, together with 10 global IGS stations (KOSG, WZTR, NOTO, MATE, GRAZ, EBRE, VILL, CAGL, MEDI, UPAD) that serve as ties with the ITRF97. We also include the stations ZIMM, TORI, GRAS, TOUL, GENO, HFLK, OBER because of their tectonic interest. We obtain long term repeatabilities on the order of 2-3 mm for the horizontal components, 8-10 mm for the vertical component. Using a noise model that combines white and coloured noise (flicker noise, spectral index 1), we find uncertainties on the velocities ranging from 1 mm/yr for the oldest stations (ZIMM, GRAS, TOUL, TORI, SJDV) to 4-5 mm/yr for the most recently installed (CHAT, MTPL). Station velocities obtained in ITRF97 are rotated into a Eurasian reference by substracting the rigid rotation computed from ITRF97 velocities at 11 central European sites located away from major active tectonic structures (GOPE, JOZE, BOR1, LAMA, ZWEN, POTS, WETT, GRAZ, PENC, Effelsberg, ONSA). The resulting velocity field shows residual motions with respect to Eurasia lower than 3 mm/yr. We obtain at TORI, in the Po plain, a residual velocity of 2.3+ or -0.8 mm/yr to the SSW and a velocity of 1.9+ or -1.1 mm/yr at SJDV, on the Alpine foreland. These results indicate that the current kinematic boundary conditions across the western Alps are extensional, as also shown by the SJDV-TORI baseline time series. We obtain at MODA (internal zones) a residual velocity of 1.2+ or -1.2 mm/yr to the SSE. The MODA-FCLZ baseline show lengthening at a rate of 1.6+ or -0.8 mm/yr. These results are still marginally significant but suggest that the current deformation regime along the Lyon-Torino transect is extension, as also indicated by from recent seismotectonic data. It is in qualitative agreement with local geodetic measurements in the internal zones (Briancon area) but excludes more than 2.4 mm/yr of extension (FCLZ-MODA baseline, upper uncertainty limit at 95% confidence). Our results indicate a different tectonic regime in the southern part of the western Alps and Provence, with NW-SE to N-S compression. The GRAS-TORI baseline, for instance, shows shortening at a rate of 1.4+ or -1.0 mm/an. This result is consistent with seismotectonic data and local geodetic measurements in these areas. The Middle Durance fault zone, one of the main active faults in this area, is crossed by the GINA-MICH baseline, which shows shortening at a rate of 1.0+ or -0.8 mm/an. This result is only marginally significant, but confirms the upper bound of 2 mm/yr obtained from triangulation-GPS comparisons. The REGAL permanent GPS network has been operating since the end of 1997 for the oldest stations and will continue to be densified. Although they are still close to or within their associated uncertainties, preliminary results provide, for the first time, a direct estimate of crustal deformation across and within the western Alps.

The REGAL permanent GPS network currently consists of 12 GPS stations covering the Western Alps and their foreland. Preliminary results indicate ( i ) residual velocities with respect to Eurasia lower than 3 mm·yr –1 , ( ii ) extensional kinematic boundary conditions along the Lyons–Turin transect and extensional strain regime in the central part of the Western Alps, and ( iii ) north–south to NW–SE compression in the southern Alps and Provence. These results are still preliminary and marginally significant, given the associated uncertainties. They are however consistent with the most recent seismotectonic results published in these areas.

We model the recent deformation along a lithospheric-scale cross section perpendicular to the northern part of the Baikal rift zone (BRZ) with two-dimensional finite element models. Using realistic lithospheric structure and rheological properties and imposing extension as a far-field boundary condition, we find a model that matches reasonably well the topography, observed deformation pattern, gravity anomalies, and age of formation of the northern BRZ. Our results suggest that (1) extensional strain can occur away from the main rift basin in the Sayan Baikal range and create, depending on the rheological properties of the lithosphere, a large “off-rift” basin analogous to the Barguzin basin, or a series of basins and ranges, as observed in the southern part of the Sayan Baikal range, (2) anelasticity plays a major role by participating in the uplift of rift shoulders and hanging wall deformation, and allowing the subsidence and tilting of the hanging wall along high-angle planar normal faults, (3) the lower crust accommodates differential strain between the brittle upper crust and upper mantle by horizontal shear and lateral flow towards the regions of crustal and mantle thinning, (4) far-field extensional stress and a lithospheric discontinuity inherited from Paleozoic tectonic events are sufficient to initiate rifting and basin subsidence, and (5) there is no, or very little, dynamic contribution of a hypothetic asthenospheric plume under the BRZ, at least in the recent phase of its evolution (“fast rifting stage”, 3.5 Myr).

Using geophysical observations and some basic geological ideas on the present structure of the Ventura basin, we model its possible mechanical evolution during the last 4 million years. We use a two-dimensional finite element model with frictional and elastoviscoplastic rheologies that allow strain localization. This allows us to study the mechanical origin of the San Cayetano thrust fault (SCF) and its interaction with the Oak Ridge fault (ORF). The results of our modeling indicate that the geometry and activity of a shear zone which mimics the San Cayetano fault are conditioned both by the preexistence of the Oak Ridge fault and by the shape of the brittle-ductile transition. Furthermore, the presence of a decollement level is shown to favor the development of the San Cayetano fault shear zone as a low-angle structure. The value of the effective friction on the Oak Ridge fault is evaluated from this model. Possible values of the viscosity of the sediments are also estimated; it is shown that low sediment viscosity may completely hinder the activity of such a fault at the surface, while slip at depth continues. The stratigraphic profile of the basin is also shown to depend on the relative strengths of the ORF and SCF, which also provide an explanation for the shortening and rapid subsidence of the basin.

A finite-element code is used to model the mechanical behaviour of elastoplastic sedimentary layers that are deformed by the movement of a faulted, deformable basement. The propagation of the blind thrust fault is analysed in terms of the evolution of strain localization in the elastoplastic layers of strain-softening and strain-hardening materials. The role of layer thickness, bedding-parallel slip, and fault dip are also studied. We found that, if no interlayer slip is allowed, blind thrusts tend to propagate with constant dip, regardless of the rheological properties of the sediment layers; however, if bedding-parallel slip is present, a mechanical decoupling appears between layers, allowing stress transfers along large distances and favouring the formation of backthrusts. Fault dip also has significant influence on the qualitative behaviour; when interlayer slip is possible, a steep fault will tend to split.

Precise estimation of the vertical deformation is a challenge for understanding geodynamical processes. In mountainous areas, the main limitation of precise GPS vertical measurement lies in the difficulty of estimating tropospheric delay due to high differential elevation and large weather variability. We carried out two GPS campaigns in July 1994 (8 days) and March 1996 (11 days), during extreme weather conditions, on a 1 390 m height difference baseline. The zenith delay was calibrated with a standard atmospheric model (STD), Water Vapor Radiometric measurements (WVR) and surface pressure measurements, or computed through a least squares estimation (LSE). Half daily repeatability of the vertical LSE solutions has a precision of 11–14 mm with a small difference of 1 mm between the two expeditions.