One of the seven potentially active andesite stratovolcanoes in southern Peru, Misti (5822 m), located 17 km northeast and 3.5 km above Arequipa, represents a major threat to the population (∼900,000 inhabitants). Our recent geophysical and geochemical research comprises an extensive self-potential (SP) data set, an audio-magnetotelluric (AMT) profile across the volcano and CO2 concentrations in the soil along a radial profile. The SP survey is the first of its kind in providing a complete mapping of a large andesitic stratovolcano 20 km in diameter. The SP mapping enables us to analyze the SP signature associated with a subduction-related active volcano.The general SP pattern of Misti is similar to that of most volcanoes with a hydrogeologic zone in the lower flanks and a hydrothermal zone in the upper central area. A quasi-systematic relationship exists between SP and elevation. Zones with constant SP/altitude gradients (Ce) are observed in both hydrogeologic (negative Ce) and hydrothermal (positive Ce) zones. Transition zones between the different Ce zones, which form a concentric pattern around the summit, have been interpreted in terms of lateral heterogeneities in the lithology. The highest amplitudes of SP anomalies seem to coincide with highly resistive zones. The hydrothermal system 6 km in diameter, which extends over an area much larger than the summit caldera, may be constrained by an older, concealed collapse caldera. A sealed zone has apparently developed through alteration in the hydrothermal system, blocking the migration of CO2 upward. Significant CO2 emanations are thus observed on the lower flanks but are absent above the hydrothermal zone.
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We thank Jaime Suni, Rachel Gusset, Pedro Navarro, Katherine Gonzales, Vicentina Cruz, Señor Uribe, and Victor Montesinos for their help during the 1997 and 1998 field campaigns. This work was supported by the Institut de Recherche pour le Développement (IRD) and the Instituto Geofı́sico del Perú (IGP). Reviews by H.M. Bibby and an anonymous reader have considerably helped to improve the manuscript. We also thank Laurence Jouniaux, Xavier Guichet, and André Revil for interesting discussions and remarks. A.F. acknowledges Jean-Luc Froger in providing the DEM of Misti volcano improved by SAR-interferometry, Benjamin Van Wyk de Vries for the Landsat image of Misti area, and the “Société de Secours des Amis des Sciences” for a research grant.