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Clarke D.,CNRS Paris Institute of Global Physics | Zaccarelli L.,CNRS Paris Institute of Global Physics | Shapiro N.M.,CNRS Paris Institute of Global Physics | Brenguier F.,Observatoire Volcanologique du Piton de la Fournaise
Geophysical Journal International

Temporal variations in the elastic behaviour of the Earth's crust can be monitored through the analysis of the Earth's seismic response and its evolution with time. This kind of analysis is particularly interesting when combined with the reconstruction of seismic Green's functions from the cross-correlation of ambient seismic noise, which circumvents the limitations imposed by a dependence on the occurrence of seismic events. In fact, because seismic noise is recorded continuously and does not depend on earthquake sources, these cross-correlation functions can be considered analogously to records from continuously repeating doublet sources placed at each station, and can be used to extract observations of variations in seismic velocities. These variations, however, are typically very small: of the order of 0.1 per cent. Such accuracy can be only achieved through the analysis of the full reconstructed waveforms, including later scattered arrivals. We focus on the method known as Moving-Window Cross-Spectral Analysis that has the advantage of operating in the frequency domain, where the bandwidth of coherent signal in the correlation function can be clearly defined. We investigate the sensitivity of this method by applying it to microseismic noise cross-correlations which have been perturbed by small synthetic velocity variations and which have been randomly contaminated. We propose threshold signal-to-noise ratios above which these perturbations can be reliably observed. Such values are a proxy for cross-correlation convergence, and so can be used as a guideline when determining the length of microseismic noise records that are required before they can be used for monitoring with the moving-window cross-spectral technique. © 2011 Institute de physique du Globe de Paris Geophysical Journal International © 2011 RAS. Source

Pioli L.,University of Geneva | Scalisi L.,University of Geneva | Costantini L.,University of Geneva | Di Muro A.,Observatoire Volcanologique du Piton de la Fournaise | And 2 more authors.
Bulletin of Volcanology

Villarrica (Chile) is a basaltic stratovolcano, currently in an open-conduit condition. It now has relatively frequent Strombolian and effusive eruptions, but it had large explosive eruptions in prehistoric times. Among them, the most recent eruption was Chaimilla, which occurred about 3100 years ago and produced deposits that indicate complex, multiphase eruptive dynamics. Significant differences in mineralogy and glass compositions of the erupted scoria suggest the eruption was fed by two distinct magma batches with similar bulk compositions but distinct crystallization and degassing histories. The lower sequence scoria has a complex crystal assemblage with several crystal populations produced by mixing between a relatively degassed magma containing Fo75-79 olivine, normally or reversely zoned plagioclase (An70-94) and augite (type 1 magma), and a subordinate volume of more-primitive and more volatile-rich magma rising from depth (type 2 magma) and carrying normally zoned plagioclase and higher-Mg (Fo81-85) olivine crystals. Type 2 magma was the main component emitted during the larger and more explosive eruptive phase that deposited the upper sequence. The Chaimilla eruption occurred under closed-vent conditions and was fed by water-rich magmas. When compared with the petrological features of the magma currently erupted at Villarrica, which has slightly more-evolved bulk compositions, lower crystal content and lower water content, these results suggest that the evolution in eruptive style of the volcano from highly explosive to a lava lake/Strombolian activity corresponds to significant changes in the shallow plumbing system (which is now at much shallower depths); these plumbing-system changes were not associated with significant changes in the parental magma compositions. © 2015, Springer-Verlag Berlin Heidelberg. Source

Coppola D.,University of Turin | James M.R.,Lancaster University | Staudacher T.,Observatoire Volcanologique du Piton de la Fournaise | Cigolini C.,University of Turin
Bulletin of Volcanology

We present thermal measurements made by high spatial resolution ground-based (a hand-held thermal camera) and low spatial resolution space-based (MODIS) instruments for a lava flow field active during the last phase of the May-July 2003 eruption at Piton de la Fournaise (La Réunion). Multiple oblique ground-based thermal images were merged to provide full coverage of the flow-field. These were then corrected for path length attenuation and orthorectified, allowing the at-surface radiance emitted by the flow-field to be estimated. Comparison with the radiance recorded by the MODIS sensors during the eruption reveals that, for clear-sky conditions and moderate-to-low viewing angles (satellite zenith < 40°), the satellite measurements represent ~90% of the at-surface radiance, and thus represent valuable data for quantifying volcanic thermal anomalies. Nevertheless, extreme viewing geometries and the presence of clouds strongly affect the radiance reaching the sensor and affected data from 94% of the overpasses. Ground-based thermal data were used to investigate an empirical relationship between the radiant heat flux and lava discharge rate during the emplacement of pahoehoe flows. While the average radiation temperature for flow surface that were 6-24 h old ranged between 500 K and 625 K, the ratio between radiative heat flux and Time-Averaged lava Discharge Rate (TADR) ranged between 1.5 × 108 J m-3 and 3.5 × 108 J m-3. This relationship was used to estimate TADR values from optimal MODIS data and produced results in line with those obtained from GPS surveys (Coppola et al. 2005). Our results underscore the importance of ground-based thermal analysis for the interpretation of satellite measurements, particularly in terms of calculating discharge rate trends. © Springer-Verlag 2009. Source

Froger J.-L.,CNRS Magmas and Volcanoes Laboratory | Souriot T.,CNRS Magmas and Volcanoes Laboratory | Villeneuve N.,University of Reunion Island | Villeneuve N.,Center Ird Of La Reunion | And 6 more authors.
Revue Francaise de Photogrammetrie et de Teledetection

We present preliminary results of an interferometric survey, based on TerraSAR-X data, of Piton de la Fournaise volcano (Reunion Island, Indian Ocean) for the period between September 2009 and January 2010. In a first part of this study, we analyse in detail the benefits of X-band in terms of interferometric coherence from a set of 57 TerraSAR-X interferograms. These benefits are compared with those of C-band, obtained from a set of 340 Envisat ASAR interferograms. As a result, we demonstrate that X-band coherence is significantly better in areas without vegetation than that of C-band and that X-band coherence declines quickly in areas covered with vegetation as expected. We also show that TerraSAR-X coherence can be used to map, with good accuracy, lava flows emplaced during the time spanned by the interferogram. In addition, TerraSAR-X interferograms provide new insight on displacements associated with an intrusion which happened on 18-19 October 2009 and with three eruptions which happened on 5-6 November 2009, 14 December 2009 and 2-12 January 2010. A qualitative analysis of fringe patterns, visible on interferograms, indicates that each period of activity is associated with an inflation of the Central Cone. The sources of inflation are located within the external southern flank of the Dolomieu crater and are coeval with short wavelength deflation of the southern margin of the Dolomieu crater. Such deflations are predicted by theoretical models above dykes, but may also be related to sliding of the southern rim of the Dolomieu crater and therefore require attention in the future. Source

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