Applications and implications of monitoring surface hydrothermal deposits at Lastarria Volcano, Chile, using multispectral satellite data and cloud computing

Studies of hydrothermal alteration involve the effects of circulating hot and aggressive fluids in volcanic environments, which are crucial for understanding volcanic hazards, slope instability, and steam-driven explosions. Visible hydrothermal deposits at the surface provide direct evidence of subsurface hydrothermal systems or volcanic unrest and can be detected by remote sensing tools. Here, we introduce the Hydrothermal Deposit Index (HDI), a remote sensing-based index derived from the Ultra Blue, Red, SWIR 1, and SWIR 2 bands of multispectral satellite data that allows spatiotemporal analysis of surface hydrothermal deposits. We apply the HDI approach to Lastarria, a stratovolcano on the border between Chile and Argentina that shows vigorous fumarole activity. With the support of Google Earth Engine (GEE), we mitigate environmental interferences like steam plumes and snow, thereby guaranteeing the precision of findings. Our HDI results identify three main depositional zones on the Lastarria Volcano, covering approximately 600,000 m 2 , and are validated against independent field surveys. Time series analysis reveals three distinct patterns of HDI variation and dynamic shifts in hydrothermal activity within the summit crater and flank regions. Furthermore, we demonstrate that activity at the summit and flanks occurs in succession and that an increase in HDI concurs with the appearance of new sulphur flows. This research contributes to the advancement of remote sensing methodologies for volcano monitoring and emphasizes the importance of spatiotemporal dynamics in hazard assessment. • A semi-automated method monitors hydrothermal deposits using Google Earth Engine. • The method is robust to environmental factors and suitable for time-series analysis. • Three hydrothermal anomalies were identified, and their causes were examined. • New insights into the importance of investigating hydrothermal deposits are presented.