This work addresses the study of fluid circulation of the Stromboli island using a dense coverage of self-potential (SP) and soil CO 2 data. A marked difference exists between the northern flank and the other flanks of the island. The northern flank exhibits (1) a typical negative SP/altitude gradient not observed on the other flanks, and (2) higher levels of CO 2. The general SP pattern suggests that the northern flank is composed of porous layers through which vadose water flows down to a basal water table, in contrast to the other flanks where impermeable layers impede the vertical flow of vadose water. In the Sciara del Fuoco and Rina Grande^Le Schicciole landslide complexes, breccias of shallow gliding planes may constitute such impermeable layers whereas elsewhere, poorly permeable, fine-grained pyroclastites or altered lava flows may be present. This general model of the flanks also explains the main CO 2 patterns: concentration of CO 2 at the surface is high on the porous north flank and lower on the other flanks where impermeable layers can block the upward CO 2 flux. The active upper part of the island is underlain by a well-defined hydrothermal system bounded by short-wavelength negative SP anomalies and high peaks of CO 2. These boundaries coincide with faults limiting ancient collapses of calderas, craters and flank landslides. The hydrothermal system is not homogeneous but composed of three main subsystems and of a fourth minor one and is not centered on the active craters. The latter are located near its border. This divergence between the location of the active craters and the extent of the hydrothermal system suggests that the internal heat sources may not be limited to sources below the active craters. If the heat source strictly corresponds to intrusions at depth around the active conduits, the geometry of the hydrothermal subsystems must be strongly controlled by heterogeneities within the edifice such as craters, caldera walls or gliding planes of flank collapse, as suggested by the correspondence between SP^CO 2 anomalies and structural limits. The inner zone of the hydrothermal subsystems is characterized by positive SP anomalies, indicating upward movements of fluids, and by very low values of CO 2 emanation. This pattern suggests that the hydrothermal zone becomes self-sealed at depth, thus creating a barrier to the CO 2 flux. In this hypothesis, the observed hydrothermal system is a shallow one and it involves mostly convection of infiltrated meteoric water above the sealed zone. Finally, on the base of CO 2 degassing measurements, we present evidence for the presence of two regional faults, oriented N41 ‡ and N64 ‡, and decoupled from the volcanic structures.

Accurate and precisely located self-potential (SP), temperature (T) and CO 2 measurements were carried out in the summit area of Stromboli along 72 straight profiles. SP data were acquired every metre and T data every 2.5 m. CO 2 concentrations were acquired with the same density as T, but only along seven profiles. The high density of data and the diversity of the measured parameters allows us to study structures and phenomena at a scale rarely investigated. The shallow summit hydrothermal activity (Pizzo-Fossa area) is indicated by large positive SP, T and CO 2 anomalies. These anomalies are focused on crater faults, suggesting that the fracture zones are more permeable than surrounding rocks at Stromboli. The analysis of the distribution of these linear anomalies, coupled with the examination of the geologic, photographic and topographic data, has led us to propose a new structural interpretation of the summit of Stromboli. This newly defined structural framework comprises (1) a large Pizzo circular crater, about 350 m in diameter;(2) a complex of two concealed craters nested within the Pizzo crater (the Large and the Small Fossa craters), thought to have formed during the eruption of the Pizzo pyroclastites unit; the Small Fossa crater is filled with highly impermeable material that totally impedes the upward flow of hydrothermal fluids; and (3) The present complex of active craters. On the floor of the Fossa, short wavelength SP lows are organized in drainage-like networks diverging from the main thermal anomalies and converging toward the topographic low in the Fossa area, inside the Small Fossa crater. They are interpreted as the subsurface downhill flow of water condensed above the thermal anomalies. We suspect that water accumulates below the Small Fossa crater as a perched water body, representing a high threat of strong phreatic and phreatomagmatic paroxysms. T and CO 2 anomalies are highly correlated. The two types of anomalies have very similar shapes, but the sensitivity of CO 2 measurements seems higher for lowest hydrothermal flux. Above T anomalies, a pronounced high frequency SP signal is observed. Isotopic analyses of the fluids show similar compositions between the gases rising through the faults of the Pizzo and Large Fossa craters. This suggests a common origin for gases emerging along different structural paths within the summit of Stromboli. A site was found along the Large Fossa crater fault where high gas flux and low air contamination made gas monitoring possible near the active vents using the alkaline bottle sampling technique.

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[1] Finding the geometry of aquifers in an active volcano is important for evaluating the hazards associated with phreatomagmatic phenomena and incidentally to address the problem of water supply. A combination of electrical resistivity tomography (ERT), self-potential, CO 2 , and temperature measurements provides insights about the location and pattern of ground water flow at Stromboli volcano. The measurements were conducted along a NE-SW profile across the island from Scari to Ginostra, crossing the summit (Pizzo) area. ERT data (electrode spacing 20 m, depth of penetration of $200 m) shows the shallow architecture through the distribution of the resistivities. The hydrothermal system is characterized by low values of the resistivity (<50 W m) while the surrounding rocks are resistive (>2000 W m) except on the North-East flank of the volcano where a cold aquifer is detected at a depth of $80 m (resistivity in the range 70-300 W m). CO 2 and temperature measurements corroborate the delineation of the hydrothermal body in the summit part of the volcano while a negative self-potential anomaly underlines the position of the cold aquifer. Citation:

S U M M A R YStromboli volcano (Italy) is characterized by a permanent mild explosive activity disrupted by major and paroxysmal eruptions. These strong eruptions could be triggered by phreatomagmatic processes. With the aim of obtaining a better understanding of ground water flow in the vicinity of the active vents, we carried out a set of geophysical measurements along two profiles crossing the Fossa area (through the Pizzo, the Large and the Small Fossa craters). These measurements include electrical resistivity, induced polarization, self-potential, temperature and CO 2 ground concentration. These methods are used in order to delineate the crater boundaries, which act as preferential fluid flow pathways for the upflow of hydrothermal fluids. The absence of fumarolic activity in the Fossa area and the ground temperature close to 100 • C at a depth of 30 cm indicate that the hydrothermal fluids condense close to the ground surface. Part of this condensed water forms a shallow drainage network (<20 m) in which groundwater flows downslope toward a perched aquifer. The piezometric surface of this aquifer is located ∼20 m below the topographic low of the Small Fossa crater and is close (<100 m) to the active vents. Electrical resistivity tomography, temperature and CO 2 measurements show that this shallow aquifer separates the underlying hydrothermal body from the ground surface. Further studies are needed to ascertain the size of this aquifer and to check its possible implications for the major and paroxysmal events observed at the Stromboli volcano.