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Home , Lascar (volcano), Lastarria

TOMO 2 - Volcanismo y Magmatismo

MEASURING AND MODELING VOLCANIC DEFORMATION IN THE :

RESULTS AND PERSPECTIVES FROM INSAR AND GEODETIC SURVEYS.

S. Bonvalot1,2,4,6, JL. Froger1,3,5, D. Remy1,2,4, F. Bondoux1,2, G. Gabalda1,2,4, D. Legrand4, A. Pavez6, C. Robin1,3,5. (1) IRD (Institut de Recherche pour le Développement) – ([email protected], [email protected] , [email protected]) ; (2) UMR5563 Toulouse, France ; (3) UMR6524 Clermont-Ferrand, France ; (4) Facultad de Ciensas y Matematicas, Universidad de ; (5) Departamento de Geología, Universidad de Chile ; (6) Institut de Physique du Globe de Paris, France.

With the aim (i) to improve our knowledge of causes and processes originating volcanic deformation in the Andes and (ii) to better monitor active volcanoes closed inhabited areas, we carried out various InSAR data acquisition in the frame of two research projects supported by the European Space Agency (ESA). New ASAR-ENVISAT radar data are for instance collected for systematic surveying of the most active volcanic centers within the Andean , where ground deformation signals have been clearly evidenced or are highly suspected.

Figure 1 : Location of active volcanoes selected for repeated ASAR satellite surveying

417 XI CONGRESO GEOLOGICO CHILENO

We present here the current status of this on-going project in light of results obtained on various volcanoes from the analysis of a first set of over 100 ERS/ASAR radar images. Clear fringe patterns corresponding to ground deformations related to present-day volcanic activity have been detected on several volcanic centers: Uturuncu (,) (Ecuador), (Chile), -Azufre (Chile-) : see for instance :(Bonvalot et al., 2005 ; Froger et al., 2005 ; Pavez, 2005 ; Remy, 2005 ; Froger et al., 2006 ; Pavez et al., 2006). We will focus here on the results obtained on the Lastarria-Cordon del Azufre complex (Chile-Argentina) where high resolution time series of deformation processes are enhanced (see below) and we will discuss the implication of such results to the understanding of volcanological processes and to the improvement of monitoring strategies in the Andes. Example of temporal evolution of volcanic deformation (Lastarria-Azufre, Chile-Argentina) ASAR-ENVISAT data collected over the Lastarria-Cordon del Azufre complex between March 2003 and May 2005 show the persistence of the large wavelength ground inflation revealed by Pritchard and Simons in 2002 from the analysis of ERS InSAR data (Pritchard et Simons, 2004). After reducing the tropospheric contribution in the interferograms using a combination of data network adjustment and analysis of MODIS images, we produced an accurate interferometric time series showing a two years long temporal evolution of the ground displacements patterns (Figure 2). Two distinct inflating signals are detected. The main signal covers an elliptical area with a 45 km NNE-SSW major axis and a 37 km minor axis. It is correlated with a regional topographic dome. We estimated its maximum inflation rate to ~2.5 cm.yr-1. We thus inverted the InSAR data for a range of source geometries (spherical, prolate ellipsoids, penny-shaped cracks). The inferred source parameters for 2003-2005 period are consistent with an over-pressured reservoir at shallow to intermediate crustal depths (7-15 km), with an average volumetric rate of inflation of about 14×106 m3.yr-1. In addition to this main signal a new feature highlighted by the ASAR data is a short wavelength inflation (6 km wide) at the location of Lastarria volcano on the northern margin of the large wavelength signal. We explain this short wavelength signal by a spherical over-pressured source lying 1000 m below the summit of Lastarria volcano. We estimate the average volumetric rate of inflation during the observation period to be ∼35×103m3yr-1. It is remarkable that both volumetric variations for the

418 TOMO 2 - Volcanismo y Magmatismo

large and small inflations exhibit the same evolution during 2003-2005, suggesting that both processes could be related. On the basis of the inversion results and of arguments provided by a morpho- structural analysis of the Digital Elevation Model of the area, we propose that both deep and shallow sources have a magmatic origin and that they are likely connected. In our interpretation, the on-going deformation processes observed at Lastarria-Cordon del Azufre volcanic complex could represent an evolving pre- silicic system. Further field geological and geophysical investigations will be required to confirm these hypotheses and refine the proposed model mostly based on satellite observations.

Figure 2. Inverted displacement time series for each of the 7 images ASAR relative to the reference image (mach 2003).

ACKNOWLEDGEMENTS

Study supported by IRD and by the European Space Agency (ENVISAT A0 #857, Category 1#2899) and FONDECYT 1061253. Contributing institutes: a) Chile: SERNAGEOMIN, Universidad de Concepción, Universidad de Chile, Universidad Católica del Norte ; b) Perú: Instituto Geofísico del Perú, CONIDA; c) Ecuador: IG-EPN Quito.

419 XI CONGRESO GEOLOGICO CHILENO

REFERENCES Bonvalot, S., Gabalda, G., Remy, D., Pavez, A., Diament, M. and Mothes, P., 2002. Assessment of ground deformation measurements on Andean volcanoes using SAR interferometry and GPS data, Proceedings of 5° ISAG, Toulouse, France, pp. 97-100. Bonvalot, S., Froger, J.L. and Remy, D., 2005. Application of INSAR Interferometry and geodetic surveys for monitoring Andean volcanic activity : first results from ASAR-ENVISAT data, 6th International Symposium on Andean Geodynamics (ISAG), Barcelone, Espagne. Froger, J.L., Remy, D., Bonvalot, S. and Gerra Franco, M., 2005. Application of ASAR-ENVISAT Data for Monitoring Andean Volcanic Activity : Results From Lastarria-Azufre Volcanic Complex (Chile-Argentina), AGU, San Francisco, USA. Froger, J.L., Remy, D., Bonvalot, S. and Legrand, D., 2006. Dynamic of long term multi-scale inflations at Lastarria- Cordon del Azufre volcanic complex, central Andes, revealed from ASAR-ENVISAT interferometric data. and Planetery Science Letters, submitted. Mothes, P., Garcia, A., Aguilar, J., Viracucha, D., Caceras, V., Jaramillo, R., Stix, J. and Bonvalot, S., 2000. Inflationary/ Deflationary Cycles Related to Repetitive dome growth and destruction as seen in tiltmeter and GPS data: Guagua volcano, Ecuador, Eos Trans. AGU, Fall Meet. Suppl. Pavez, A., 2005. Structure et déformations du volcan Lascar à partir d’observations par satellite et au sol, Thèse de doctorat (I.P.G.P), Paris, 380 pp. Pavez, A., Remy, D., Bonvalot, S., Diament, M., Gabalda, G., Froger, J.L., Julien, P., Legrand, D. and Moisset, D., 2006. Insight into ground deformation at lascar volcano (Chile) from SAR interferometry, photogrammetry and GPS data: Implication on volcano dynamics and future space monitoring. of Environment, 100: 307-320. Pritchard, M.E. and Simons, M., 2002. A satellite geodetic survey of large scale deformation of volcanic centres in the central Andes. Nature, 418: 167-170. Pritchard, M.E. and Simons, M., 2004. An InSAR-based survey of volcanic deformation in the central Andes. G3, 5(2): Q02002, doi:10.1029/2003GC000610. Remy, D., 2005. Analyse et inversion de séries interférométriques et microgravimétriques temporelles sur les volcans actifs : Apport à la quantification d’effets de sites et à la compréhension de la dynamique volcanique, These de Doctorat (I.P.G.P), Paris, 290 pp.

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