Integrated geodetic and seismic constraints on deformation at Nevados de Chillán, 2015–2025

The Nevados de Chillán Volcanic Complex in the Southern Volcanic Zone (SVZ) of the Andes is one of the most active volcanic systems in Chile and has recently hosted one of the longest and best-instrumented eruptive cycles documented in the country. Here, we integrate satellite-geodetic and seismic time series spanning December 2015 to late 2025 to characterize the full eruptive cycle. We identify three main periods: (I) slight subsidence from 2015 to 2019, (II) marked inflation from 2019 to 2022 with a maximum LOS displacement of ~ ∼ 25 cm, and (III) deflation from late 2022 onward. Inflation and deflation produced a similar spatial pattern with opposite sign and a cumulative LOS range change of ~ ∼ 20 cm. We modeled the 2019–2022 inflation using a pressurized prolate spheroid and found that deformation throughout all three periods was mainly associated with a shallow source region centered at ~ ∼ 5.4 km depth. The source aligns with the NW-SE-oriented Cortaderas Lineament and volcano-tectonic seismicity at 3–6 km depth. The volcano continued to inflate during the period of highest effusive output, while erupted lava volumes remained modest relative to the modeled source volume change, suggesting continued pressurization. Seismicity migrated northeastward during Period II, indicating structural control on shallow magma storage and transport. Through late 2025, deformation and seismicity declined toward pre-eruptive background conditions.