Continuous monitoring of the 2015–2018 Nevado del Ruiz activity, Colombia, using satellite infrared images and local infrasound records

Nevado del Ruiz Volcano (NRV) had a phreatomagmatic eruption in 1985. The eruption partially melted the volcano’s ice cap leading to floods and lahars flowing down to nearby towns, which killed at least 25,000 people. This event has raised particular importance of monitoring activity including small eruptions at ice-capped high-altitude volcanoes. However, the high altitude makes it difficult to maintain monitoring stations near the summit crater. Moreover, the visibility of the summit area is frequently prevented by clouds. In this paper, we report the results of a feasibility study for detecting thermal anomalies and small eruptions using satellite thermal remote sensing and ground-based infrasound technique. We newly included South and Central America to the observation areas of the near-real-time monitoring system of the active volcanoes, which uses infrared images from satellites. We also operated three infrasound stations in the distances of 4–6 km from the active crater. Each of the stations consisted of a pair of infrasound sensors, and a cross-correlation technique was applied. The thermal and infrasound data acquisition started in August 2015 and December 2016, respectively, and recorded the recent dome-forming activity of NRV. We proposed parameters representing the visibility of the thermal anomalies and infrasound signals. These parameters are useful for monitoring because the severe weather condition at NRV frequently prevents signal detections. We discussed the detected thermal anomalies and infrasound signals in comparison with their visibilities and the changes in the volcanic activity of NRV reported by the local observatory. The thermal anomaly and infrasound detections were consistent with the high eruptive activity occurring at the NRV from October 2015 to May 2017 and its subsequent decline. Within the active period, there were breaks in the detections of thermal anomaly and infrasound. The visibility analyses allowed us to interpret the breaks as a result of bad weather conditions and to distinguish them from the confirmed low-activity periods after May 2017.