Analysis of thermal anomalies at Copahue Volcano between October 2011 and the December 2012 eruption with MODIS

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Copahue volcano is an active stratovolcano located in the Southwest of the Agrio-Caviahue depression in the Province of Neuquén at the border between Argentina and Chile, and the most recent eruptive activity involved five cycles between 1992 and 2016. The Moderate Resolution Imaging Spectroradiometer (MODIS) is an instrument onboard the satellites Terra and Aqua that provides, on average, four images per day and has channels centered at 4 μm and 12 μm that are useful for detecting volcano temperature anomalies. In this work, we evaluated the normalized temperature index (NTI) as implemented by MODVOLC and the radiance recorded by MODIS at 4 μm as measures of precursory volcanic activity between October 2011 and January 2013, with a special focus on the December 2012 eruption. We evaluated the thermal anomalies identified by MODVOLC based on a global threshold of −0.8 and observed that they were namely syn- or post-eruptive and mostly correlated with the presence of incandescence material related to Strombolian activity. The global nighttime threshold was adapted to the local context of Copahue and we observed that a threshold of −0.875 is a more suitable value for the examined period. An analysis of time series of the NTI and of the radiance at 4 μm based on 116 MODIS images, resulted in that the adapted threshold allowed the detection of anomalies undetected by the global value. The anomalies were in either case unrelated to precursory activity but correlated with surface expressions of activity such as the emission of volcanic ash, volcano degassing, and the ejection of ballistic and incandescent material. The knowledge on the chronology of activity prior to and during the eruption could be improved, as regards the thermal behavior of the volcano in relation also to geochemical, deformation, and seismic signals. The replication of this kind of analysis in other periods of activity at Copahue and other volcanoes, the evaluation of other algorithms and the incorporation of data acquired by other satellite instruments are necessary to improve our knowledge on the temperature behavior of volcanoes before and during periods of unrest. This would ultimately contribute towards the improvement of volcano monitoring capabilities in volcanic areas.