Anthropogenic and natural ground deformation in the Hengill geothermal area, Iceland

We investigate crustal deformation due to the extraction of water and steam from a high-enthalpy geothermal reservoir; a common occurrence, yet not well understood. The cause of this deformation can be a change in pressure or in temperature in the reservoir, both of which can be caused by extraction or injection of geothermal fluids. Our study area, the Hengill mountains in SW Iceland, is an active volcanic center and a plate triple junction that hosts two power plants producing geothermal energy. This combination of natural and anthropogenic processes causes a complex displacement field at the surface. We analyze geodetic data—Global Navigation Satellite System and Interferometric Synthetic Aperture Radar—to obtain the surface velocity field, which we then simulate using an inverse modeling approach. We focus on the deformation around the geothermal power plants but need to model the regional tectonic and volcanic deformation as well, because the signals are overlapping. We find that plate motion and a deep contracting body can explain the broad scale signal in the area. Local deformation near the two power plants, Hellisheidi and Nesjavellir, can be explained by extraction of geothermal fluids. We estimate reservoirs extending from 0.6 to 3.0 km depth at Hellisheidi, and 1.0 to 3.0 km depth at Nesjavellir for observed pressure decrease rates of 0.25 MPa/yr and 0.1 MPa/yr, respectively. We find that the main cause for the subsidence in the geothermal area is the observed pressure drawdown

Repeated magmatic intrusions at El Hierro Island following the 2011–2012 submarine eruption

After more than 200 years of quiescence, in July 2011 an intense seismic swarm was detected beneath the center of El Hierro Island (Canary Islands), culminating on 10 October 2011 in a submarine eruption, 2 km off the southern coast. Although the eruption officially ended on 5 March 2012, magmatic activity continued in the area. From June 2012 to March 2014, six earthquake swarms, indicative of magmatic intrusions, were detected underneath the island. We have studied these post-eruption intrusive events using GPS and InSAR techniques to characterize the ground surface deformation produced by each of these intrusions, and to determine the optimal source parameters (geometry, location, depth, volume change). Source inversions provide insight into the depth of the intrusions (~ 11–16 km) and the volume change associated with each of them (between 0.02 and 0.13 km3). During this period, > 20 cm of uplift was detected in the central-western part of the island, corresponding to approximately 0.32–0.38 km3 of magma intruded beneath the volcano. We suggest that these intrusions result from deep magma migrating from the mantle, trapped at the mantle/lower crust discontinuity in the form of sill-like bodies. This study, using joint inversion of GPS and InSAR data in a post-eruption period, provides important insight into the characteristics of the magmatic plumbing system of El Hierro, an oceanic intraplate volcanic island

Limitations of rupture forecasting exposed by instantaneously triggered earthquake doublet

Earthquake hazard assessments and rupture forecasts are based on the potential length of seismic rupture and whether or not slip is arrested at fault segment boundaries. Such forecasts do not generally consider that one earthquake can trigger a second large event, near-instantaneously, at distances greater than a few kilometers. Here we present a geodetic and seismological analysis of a magnitude 7.1 intra-continental earthquake that occurred in Pakistan in 1997. We find that the earthquake, rather than a single event as hitherto assumed, was in fact an earthquake doublet: initial rupture on a shallow, blind 2 reverse fault was followed just 19 seconds later by a second rupture on a separate reverse fault 50 km away. Slip on the second fault increased the total seismic moment by half, and doubled both the combined event duration and the area of maximum ground shaking. We infer that static Coulomb stresses at the initiation location of the second earthquake were probably reduced as a result of the first. Instead, we suggest that a dynamic triggering mechanism is likely, although the responsible seismic wave phase is unclear. Our results expose a flaw in earthquake rupture forecasts that disregard cascading, multiple-fault ruptures of this type

Les sciences somnambules « Pratiques informationnelles et contextes d'influences autour de la culture scientifique chez les jeunes »

Cette étude se propose d’analyser les rapports du public jeune avec la culture scientifique. Nous avons réalisé une enquête auprès de 11 jeunes de 14-15 ans scolarisés dans un établissement français à l’étranger afin de décrire les usages et les représentations que pouvaient avoir un public adolescent par rapport aux pratiques d’information personnelle, dans un contexte lié au discours alarmiste sur le désintérêt des jeunes pour les sciences. En nous basant sur des travaux antérieurs, nous avons choisi de nous intéresser aux pratiques informationnelles informelles : nous avons pu constater l’importance du support numérique et notamment des informations visuelles, l’importance de la crédibilité de « surface » pour la validation des sources et l’imprégnation d’une culture scientifique populaire liée à la fiction. S’il semble que la culture scientifique ne s’intègre pas facilement dans la culture jeune dans une perspective socialisante, ces conclusions recouvrent en fait une réalité complexe où les contextes d’influence et les notions de crédibilité jouent un grand rôle dans la réception des informations scientifiques. Les principaux résultats de cette étude montrent l’existence d’une « mythologie scientifique » personnelle, éloigné d’un savoir scientifique traditionnel, mais aussi la vivacité des pratiques informationnelles et la curiosité des jeunes par rapport aux sciences

Crustal deformation from geodetic techniques: Earthquakes and plate movements in the South Iceland Seismic Zone

The South Iceland Seismic Zone (SISZ) is one of the most seismically active area in Iceland. The last two major events occured in the central and western parts of the SISZ in June 2000 and May 2008, respectively. In this thesis, I use geodetic methods to estimate the deformation due to earthquakes and plate spreading and derive models of the crustal processes. In the first paper we derive a uniform-slip and a variable-slip fault models for the two 29 May 2008 earthquakes based on GPS and InSAR observations.We account for the variation in the elastic parameters of the crust with depth. Our models indicate that the slip for the first event (Ingolfsfjall) was concentrated at 2-4 km depth, with a maximum of 1.9 m, whereas the slip on the second fault (Kross) was located deeper, at 3-6 km depth with up to 1.4 m of motion. Static Coulomb failure stress calculations indicate that the first event caused a stress increase in the area of the main asperity on the second fault. We estimated a composite moment that equals a Mw 6.1 for the doublet. We also find that the June 2000 - May 2008 sequence only released about half of the moment accumulated by plate motion since the previous earthquake sequence in the SISZ during 1896-1912. In the second paper we document the transient following the June 2000 earthquakes, extending a previous study with GPS data from 2004 to 2008. We explore whether viscoelastic models based on Maxwell and/or standard linear solid (SLS) rheologies can reproduce the surface velocities observed during 2000-2008 and calculate the corresponding stress changes. Our preferred model consists of a ~15 km thick elastic crust overlying a SLS upper mantle with a viscosity of 1-2*10E18 and relaxation strength between 0.1-0.25. The viscoelastic transient promotes failure in the area of the May 2008 earthquake. However, the Coulomb failure stress changes due to the viscoelastic relaxation are very small (0.05 MPa) compared to the static stress changes from the June 2000 main shocks. Viscoelastic loading may therefore not be the main trigger for the May 2008 events. In the third paper we study the deformation following the May 2008 earthquakes using GPS and InSAR observations. We test poroelastic, viscoelastic and afterslip models in order to determine the dominent mechanism driving the transient observed. Our best model involves viscoelastic relaxation and shallow afterslip to explain both the far field and near field displacements. We also propose that an impermeable structure disrupts fluid flow and causes pore pressure changes that could be an explanation for a sharp EW lineament highlighted in some interferograms in an area where a large number of aftershocks occured, west of the two mainshocks.This PhD was funded by grants from the Icelandic Research Council for the projects "High-rate continuous GPS observations in Iceland" (Grant of Excellence, nr. 060243013), and Recent earthquake sequences in the South Iceland Seismic Zone" (project grant nr. 090237021)." GPS fieldwork was funded by the Icelandic Research Fund, as well as the Icelandic Research Council, and the Nordic Volcanological Center. During the June 2008 fieldwork additional GPS equipment was supplied by UNAVCO and assistance by the National Land Survey of Iceland, the Icelandic Road Authority (Vegagerdin), and the National Power Company (Landsvirkjun). New CGPS stations were funded by the Icelandic Research Fund, United State National Science Foundation (grants number EAR-0711446 and EAR-0711456 ), the University of Arizona, Pennsylvania State University

1788/1789 en Artois : un candidat en campagne électorale, Maximilien de Robespierre

Toute élection provoque fièvre et polémique. Celle de Robespierre aux États généraux n'échappe pas à la règle. Ernest Hamel, le fougueux hagiographe de Robespierre, nous parle d'une élection presque assurée dès le début eu égard à la grande réputation de Robespierre. C'est loin d'être certain. Dans une biographie un peu datée, son biographe-psychologue Max Gallo nous semble beaucoup plus proche de la réalité lorsqu'il écrit : "Il lui faut réussir, il le sait. Il est des moments où un homme de..

Spinning particles in Saturn's C ring from mid-infrared observations: Pre-Cassini mission results

International audienceSaturn's C ring thermal emission has been observed in mid-infrared wavelengths, at three different epochs and solar phase angles, using ground based instruments (CFHT in 1999 and VLT/ESO in 2005) and the Infrared Radiometer Instrument Spectrometer (IRIS) onboard the Voyager 1 spacecraft in 1980. Azimuthal variations of temperature in the C ring's inner region, observed at several phase angles, have been analyzed using our new standard thermal model [Ferrari, C., Leyrat, C., 2006. Astron. Astrophys. 447, 745–760]. This model provides predicted ring temperatures for a monolayer ring composed of spinning icy spherical particles. We confirm the very low thermal inertia (on the order of 10 ) found previously by Ferrari et al. [Ferrari, C., Galdemard, P., Lagage, P.O., Pantin E., Quoirin, C., 2005. Astron. Astrophys. 441, 379–389] that reveals the very porous regolith at the surface of ring particles. We are able to explain both azimuthal variations of temperature and the strong asymmetry of the emission function between low and high phase angles. We show that large particles spinning almost synchronously might be present in the C ring to explain differences of temperature observed between low and high phase angle. Their cross section might represent about 45% of the total cross section. However, their numerical fraction is estimated to only ∼0.1% of all particles. Thermal behavior of other particles can be modeled as isothermal behavior. This work provides an indirect estimation of the particle's rotation rate in Saturn's rings from observations

A Preseismic Strain Anomaly Detected Before M6 Earthquakes in the South Iceland Seismic Zone From GPS Station Velocities

Publisher's version (útgefin grein)The South Iceland Seismic Zone (SISZ) accommodates E‐W translation due to oblique spreading between the North American and the Eurasian plates in South Iceland with ruptures on N‐S faults. Strain is released in earthquake sequences that last days to years, at average intervals of 80–100 years. Two M6.5 earthquakes struck in the SISZ in June 2000, and two M6 earthquakes in May 2008. These events released only half of the strain accumulated since the last earthquake sequence in 1896–1912. GPS station velocities are estimated from annual campaigns and continuous measurements in the SISZ during 2001–2015. The GPS station velocities are used to calculate strain rates from a new method using the vertical derivatives of horizontal stress. Our new strain rates are obtained by integrating vertical derivatives of horizontal stress rates, rather than differentiating interpolated GPS velocities, allowing finer resolution of strain. Estimating the strain rates for eight time intervals, we find high strain rates in the SISZ (0.5–1 microstrain per year) with significant temporal and spatial variation. The strain rates in the SISZ reveal contraction in the Hengill area and dilatation over a large area in eastern SISZ, indicating inflation of the magma system beneath Hekla volcano. A prominent strain anomaly is evident in the epicentral area prior to the May 2008 earthquakes. We suggest that this signal is primarily caused by plate motion and an increase in the rate of contraction in the Hengill area, located ∼10 km west of the epicentral area.This study was funded by grants from the Icelandic Research Fund (grants 0315900-03/04/05, 060243-011/012/013, 090237-021/022/023, and 130371-051/052/053), the Icelandic Equipment Fund (grant 061059), and the University of Iceland Research fund (grant HI-6489).Peer Reviewe