Prospeccion geofisica del misti

Journal of Volcanology and Geothermal Research 135 (2004) 343 – 360
www.elsevier.com/locate/jvolgeores
Fluid circulation and structural discontinuities inside Misti volcano
(Peru) inferred from self-potential measurements

Anthony Finizola a,*, Jean-Fran cois Le´nat a, Orlando Macedo b, Domingo Ramos b, Jean-Claude Thouret a, Francesco Sortino c

a Laboratoire Magmas et Volcans,Universite´ Blaise Pascal et CNRS, OPGC, IRD, 5 rue Kessler, 63038 Clermont-Ferrand, France
b Instituto Geof´ısico del Peru´ , Urbanizacio´ n La Marina B19, Cayma, Arequipa, Peru
c Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Via Ugo La Malfa, 153, 90144 Palermo, Italy

Received 24 June 2002; accepted 9 March 2004
Abstract
One of the seven potentially active andesite stratovolcanoesin southern Peru, Misti (5822 m), located 17 km northeast and
3.5 km above Arequipa, represents a major threat to the population ( f 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 thefirst 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-systematicrelationship 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 tocoincide 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 areabsent above the hydrothermal zone.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Misti volcano; self-potential; audio – magnetotelluric; electrical resistivity; structural discontinuity; hydrothermal system; Peru
and Kauahikaua, 1987), Piton de la Fournaise (Le´nat,
1987; Malengreau et al., 1994, Michel and Zlotnicki,
1998), Karthala (Le´nat et al., 1998), or on stratovol- canoes, suchas Vesuvius (Di Maio et al., 1998) or Merapi, Lamongan (Aubert and Dana, 1994), show positive anomalies, a few hundreds to thousands
millivolts in amplitude, associated with hydrothermal or active zones. These surface electric fields reflects
1. Introduction
Self-potential (SP) surveys carried out on shield
volcanoes, such as Kilauea (Zablocki, 1976; Jackson
* Corresponding author.Osservatorio Vesuviano, INGV-
Napoli, via Diocleziano 328, 80124 Napoli, Italy.
E-mail address: finizola@ov.ingv.it (A. Finizola).
0377-0273/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2004.03.009

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A. Finizola et al. / Journal of Volcanology and Geothermal Research 135 (2004) 343–360
some polarization mechanisms occurring at depth.
Two kindsof mechanisms are assumed to create the anomalies on active volcanoes: electrokinetic and thermoelectric couplings (Corwin and Hoover,
1979). Electrokinetic coupling (or streaming poten- tial) results from a fluid pressure gradient whereas thermoelectric coupling results from a temperature gradient. Theoretical considerations show that elec- trokinetic potentials, significantly larger than...