PROCEEDINGS, TOUGH Symposium 2006 Lawrence Berkeley National Laboratory, Berkeley, California, May 15–17, 2006 - 1 - MODELING OF GAS COMPOSITION AND GRAVITY SIGNALS AT THE PHLEGREAN FIELDS CALDERA Micol Todesco 1 , Giovanni Chiodini 2 , Giovanna Berrino 2 1 Istituto Nazionale di Geofisica e Vulcanologia, Bologna Via D. Creti, 12 - Bologna, 40128, Italy e-mail: todesco@bo.ingv.it 1 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano Via Diocleziano, 328 - Napoli, 80124, Italy ABSTRACT Hydrothermal systems are known to play an important role in the evolution of active calderas: these volcanic systems periodically undergo dramatic unrest crises, commonly involving ground deformation, seismic activity and important changes in several geophysical and geochemical parameters monitored at the surface. These unrest crises may, or may not, culminate with a renewal of the eruptive activity, but in any case they bear important consequences in densely populated regions. Early warning and a prompt evaluation of the state of evolution of the volcanic system are therefore essential to ensure proper mitigation measures. A proper interpretation of monitoring data, however, is only achieved within the framework of a robust conceptual model of the system. Recent research work carried out at the Phlegrean Fields shows that the recent evolution of the caldera is consistent with the presence of a pulsating magmatic source, periodically discharging CO 2 -enriched fluids into a shallow hydrothermal system. Such pulsating degassing affects the amount of heat and fluids entering the hydrothermal system, the distribution of fluid phases throughout the system, and their composition. As a consequence, degassing controls not only the composition of fluids discharged at the surface, but also ground displacement and gravity residuals. In this work, the TOUGH2 code has been applied to study how different degassing scenarios could affect the composition of discharged fluids and the gravity signals recorded at the surface. UNRESTS AT THE PHLEGREAN FIELDS CALDERA The Phlegrean Fields (Figure 1) caldera is an active volcanic center located in the urban area of Naples (Italy). The last eruptive event (Monte Nuovo) occurred in 1538, following several decades of ground deformation, culminating with a 7 m uplift, a few days before the eruption (Di Vito et al., 1987; Rosi and Sbrana, 1987; Orsi et al., 1999). More recently, two non-eruptive unrest crises occurred in 1969 and 1982. Each time, remarkable ground uplift (up to 1.8 m in Pozzuoli) was reached within a couple of years, and was accompanied by seismic activity, gravity changes, as well as geochemical variations at Solfatara fumaroles. Since 1985, a slow subsidence is taking place, periodically interrupted by minor, short- lasting uplift episodes (Barberi and Carapezza, 1996, Orsi et al., 1999; and references therein). Figure 2 illustrates some typical changes in geophysical and geochemical parameters recorded since 1982. Figure 1. The Phlegrean Fields caldera and the Solfatara crater. Blue triangles indicate the location of gravity stations. Blue numbers refer to the stations considered in the present study (10: Serapeo; 11: Arco Felice; 13: Miseno; 17: Solfatara). Radiocarbon dating of fossil marine organisms, found within Roman ruins in Pozzuoli, revealed that submersion of the famous Roman pillars was interrupted three times by uplift episodes, thelast of which led to the 1538 eruption. These new data imply that non-eruptive unrest episodes are rather the rule than the exception in the complex deformation history of the caldera (Morhange et al., 2006). Unrest crises of active volcanoes are commonly ascribed to magma ascent toward the surface, or to its emplacement at shallow crustal level. Recent research, however, emphasizes the role of hydrothermal fluids in the evolution of quiescent