Jean-Luc Froger

Cointrusive shear displacement by sill intrusion in a detachment: A numerical approach

Sheared sills are observed at Piton des Neiges (the eroded basaltic volcano of La Reunion) and are a suspected cause of the coeruptive flank displacement of Piton de la Fournaise in 2007. We performed a 2-D numerical study to quantify the perturbation induced by sill injection within a volcanic edifice. Magma is considered as an inviscid and pressurized fluid injected in an initially stable edifice under gravity-controlled extension. Two cases of injection are tested, in an elastic homogeneous edifice or along a detachment controlled by a friction law. We show that sill injection induces tangential displacements. This effect is strongly increased when sills are emplaced along a detachment, producing extension behind the injection and potentially ending in a large-scale flank collapse. Sill injections can thus explain the cointrusive shear deformation observed at Piton des Neiges and the tangential displacements measured at Piton de la Fournaise in 2007.

Persistent uplift of the Lazufre volcanic complex (Central Andes): New insights from PCAIM inversion of InSAR time series and GPS data

We reanalyzed the surface displacements observed at the Lazufre volcanic complex in the Southern Andean Central Volcanic Zone using GPS measurements made between 2006 and 2008 and a large InSAR data set. We performed a detailed spatiotemporal analysis of the displacements using a principal component analysis inversion method (PCAIM). The PCAIM reveals a source with no significant changes in shape and dimension and with a remarkably linear strength increase over the whole period of observation (i.e., 2003–2010). Then we used a three-dimensional mixed boundary element method (MBEM) to invert the first component of surface displacement as obtained from PCAIM. We explored a continuum of geometries from a shallow elliptic crack to a deep massive truncated elliptical cone that could represent a sill or a large magma chamber, respectively. The best models indicate a large flat-topped source with a roof area between 40 and 670 km2 and a depth of between 2 and 14 km below ground surface. Lastly, on the basis of the limited data available for the thermomechanical structure of the crust in the Southern Andean Central Volcanic Zone, we consider some possible scenarios to explain the spatial and temporal pattern of displacements at Lazufre.

Revised interpretation of recent InSAR signals observed at Llaima volcano (Chile)

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

Monitoring an effusive eruption at Piton de la Fournaise using radar and thermal infrared remote sensing data: insights into the October 2010 eruption and its lava flows

Abstract Accurate and fast delivery of information about recent lava flows is important for near-real-time monitoring of eruptions. Here, we have characterized the October 2010 lava flow at Piton de la Fournaise using various InSAR datasets. We first produced a map of the area covered by the lava flow (i.e. Arealava=0.71–0.75 km2) using the coherence of two syn-eruptive interferograms. Then we analysed two post-eruptive InSAR datasets (i.e. monostatic and bistatic data). The monostatic database provided us simultaneously with the displacement rates, lava thickness, volume and volume flux. We found that the lava flow was subsiding and moving eastward at maximum rates of 13±0.3 and 4±0.2 cm a−1, respectively. Also, it had a mean thickness of Zmean=5.85 m, VolDRE=1.77±0.75×106 m3 (1σ) and MOR=1.25±0.53 m3 s−1. The bistatic database provided us only with the thickness and volume information (i.e. Zmean=6.00 m, VolDRE=1.83±0.65×106 m3 and MOR=1.29±0.46 m3 s−1). Finally, we used a thermal remote sensing technique to verify the InSAR-derived measurements. Results show that the monostatic and bistatic datasets were both well within the range for the DRE volume obtained from MODIS data (2.44–4.40×106 m3). Supplementary material: Tables A1 and A2 give satellite images used in this study. Table A3 gives the parameters used for the calculation of the effusion rates. The figures give the data processing of the post-eruptive radar images. These are available at https://doi.org/10.6084/m9.figshare.c.2213563

Inversion of coeval shear and normal stress of Piton de la Fournaise flank displacement

The April 2007 eruption of Piton de la Fournaise was the biggest volcano eruptive crisis of the 20th and 21st centuries. Interferometric synthetic aperture radar (InSAR) captured a large coeruptive seaward displacement on the volcanos eastern flank, which continued for more than a year at a decreasing rate. Coeruptive uplift and posteruptive subsidence were also observed. While it is generally agreed that flank displacement is induced by fault slip, we suggest that this flank displacement might have been induced by a sheared sill, based on observations of sheared sills at Piton des Neiges. To test this hypothesis, we develop a new method to invert a quadrangular curved source submitted to simultaneous pressure and shear stress changes. This method, based on boundary elements, is applied to data acquired along six Envisat orbits covering a 14 month period subsequent to the April 2007 eruption. Posteruptive displacement is well explained by closure and slip of a large (5 km by 8 km) and shallow (500 m) trapezoidal fracture parallel to the flank and probably coincident with a lithological discontinuity. We investigate whether thermal contraction or degassing of a coeruptive sill can explain the displacement. Such a sill would have to be 10 times thicker than inferred from the coeruptive uplift and solidification time 10 times shorter (~20 days) than the duration of the posteruptive subsidence (24 to 33 months). Instead, we propose that the posteruptive eastern flank displacement is due to the compaction and ongoing slow slip on a shallow detachment fault.