1. CNRS/INSU—Institut des Sciences de la Terre d'Orléans, 1a rue de la Férollerie, 45071 Orléans, cedex 2, France
2. Université d'Orléans—Institut des Sciences de la Terre d'Orléans, 45071 Orléans, France
3. Dipartimento di Scienze della Terra, Via Trentino 51, 09127 Cagliari, Italy
4. INGV, sezione di Pisa, Via della Faggiola, 56100 Pisa, Italy

Correspondence to: B. Scaillet^1,2 Correspondence and requests for materials should be addressed to B.S. (Email: bscaille@cnrs-orleans.fr).

Forecasting future eruptions of Vesuvius is an important challenge for volcanologists, as its reawakening could threaten the lives of 700,000 people living near the volcano^{1, 2}. Critical to the evaluation of hazards associated with the next eruption is the estimation of the depth of the magma reservoir, one of the main parameters controlling magma properties and eruptive style. Petrological studies have indicated that during past activity, magma chambers were at depths between 3 and 16 km (refs 3–7). Geophysical surveys have imaged some levels of seismic attenuation, the shallowest of which lies at 8–9 km depth⁸, and these have been tentatively interpreted as levels of preferential magma accumulation. By using experimental phase equilibria, carried out on material from four main explosive events at Vesuvius, we show here that the reservoirs that fed the eruptive activity migrated from 7–8 km to 3–4 km depth between the ad 79 (Pompeii) and ad 472 (Pollena) events. If data from the Pomici di Base event 18.5 kyr ago⁹ and the 1944 Vesuvius eruption⁷ are included, the total upward migration of the reservoir amounts to 9–11 km. The change of preferential magma ponding levels in the upper crust can be attributed to differences in the volatile content and buoyancy of ascending magmas, as well as to changes in local stress field following either caldera formation¹⁰ or volcano spreading¹¹. Reservoir migration, and the possible influence on feeding rates¹², should be integrated into the parameters used for defining expected eruptive scenarios at Vesuvius.

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