Geodetic and Seismic Analysis of Magmatic Processes During the 2016–2024 Eruptive Cycle of Nevados de Chillán Volcanic Complex, Southern Andes.

The Nevados de Chillán volcanic complex, located in the Southern Volcanic Zone of the Andes, is one of the most active volcanoes in Chile. Its most recent eruptive cycle began in 2016 and lasted seven years until January 2023, with a volcanic explosivity index (VEI) level 2. This sequence is the best instrumentally recorded eruption in Chile to date, encompassing small vulcanian eruptions, the formation of the new crater (called Nicanor), dome growth and collapse, effusion of lava flows, and seismic activity characterized primarily by long-period, tremor, and volcano-tectonic earthquakes.

To characterize the complete eruptive sequence, we integrated seismic, accurate position (GNSS) and radar interferometric (InSAR) observations. We calculated displacement time-series of the Sentinel-1 (C-band), ALOS-2, and SAOCOM-1 (both L-band) satellite missions, and identified three distinct phases of the eruptive cycle. The volcanic inflation caused a total of ~20 cm LOS range change between December 18, 2015, and March 1, 2024, that was preceded by a subtle deflation signal during 2017–2019. Maximum displacement of 25 cm observed between 2019 and 2022 in both InSAR and GNSS data coincided with an increased activity of long-period seismicity, and effusion of 0.01-0.02 km³ of andesitic lava flows. The volcano deflated afterwards with a rate of ~4 cm/yr between 2022 and 2024, marking the final stage of our observation cycle.

We inverted the InSAR data with a pressurized prolate spheroid spanning the inflation period and compared the results to the total GNSS displacement. Inversion results indicate that the deformation source is located at ~6.2 km depth. Due to the source depth, deformation is likely of magmatic origin, and only a fraction of the intruded volume fed the small lava flows, even accounting for magma compressibility. Post-eruptive deflation can also be explained by the same deformation source.

Our results indicate that the expected behavior for this eruption, such as large-scale ground deflation throughout the complete event, was not observed. Instead, the volcano inflated during 2019–2022, the period with the largest mass effusion, before decaying exponentially.