Magma Storage Below Sabancaya Volcano (Southern Peru) Imaged by Broadband Magnetotellurics

Abstract

Sabancaya volcano is one of the most active volcanoes in the Central Andes. Its ongoing eruptive process is accompanied by large-scale deformation, with activation of the Huambo-Cabanaconde fault system, marked by intense seismicity over an area of about 50 × 30 km². We present a pilot magnetotelluric survey performed in 2022, covering the Ampato-Sabancaya complex, Hualca-Hualca volcano, as well as the related system of normal faults. Our three-dimensional electrical resistivity model reveals pronounced vertical gradients and lateral contrasts at elevations above sea level, along with generally low resistivity values at depth. Seismicity at depths >4 km below sea level predominantly occurs in a low resistivity environment: 90% of seismic events occur at resistivity values below 10 Ωm. Two prominent electrical conductors (<0.5 and 2–4 Ωm) are imaged at depths 11–18 km and 3–8 km, respectively. Using petrological constraints, we interpret them as the signature of the magmatic plumbing system, connecting the Hualca-Hualca and Ampato-Sabancaya volcanoes. The deeper conductor is inferred to represent a magma reservoir situated beneath the older Hualca-Hualca volcano, consistent with long-term deformation and seismicity. It is connected to the laterally offset shallow magma chamber below Sabancaya. At depth 2–10 km, a strong conductor (<0.1 Ωm) is imaged in the Huambo-Cabanaconde fault zone. The extremely high conductivity of this body is attributed to the abundance of ultra-saline brines, originating from the deep magma reservoir below. We speculate that the strong seismicity cluster detected in 2013 facilitated the passage of magmatic fluids exsolved from the magma reservoir, and replenished this ultra-conductive body.