Investigating Phases of Thermal Unrest at Ambrym (Vanuatu) Volcano through the Normalized Hot Spot Indices Tool and the Integration with the MIROVA System

Ambrym is an active volcanic island, located in the Vanuatu archipelago, consisting of a 12 km-wide summit caldera. This open vent volcano is characterized by an almost persistent degassing activity which occurs in the Benbow and Marum craters, which were also the site of recent lava lakes. On 15 December 2018, about three years after an intense lava effusion, the first recorded since 1989, a small-scale intra-caldera fissure eruption occurred. On 16 December, the eruption stopped, and the lava lakes at the Benbow and Marum craters were drained. In this work, we investigated the thermal activity of the Ambrym volcano, before, during, and after the 15 December 2018 eruption, using daytime Sentinel-2 (S2) Multispectral Instruments (MSI) and Landsat-8 (L8) Operational Land Imager (OLI) data, at a mid-high spatial resolution. The results were integrated with Moderate Resolution Imaging Spectroradiometer (MODIS) observations. Outputs of the Normalized Hotspot Indices (NHI) tool, retrieved from S2-MSI and L8-OLI data, show that the thermal activity at the Ambrym craters increased about three weeks before the 15 December 2018 lava effusion. This information is consistent with the estimates of volcanic radiative power (VRP), which were performed by the Middle Infrared Observation of Volcanic Activity (MIROVA) system, by analyzing the nighttime MODIS data. The latter revealed a significant increase of VRP, with values above 700 MW at the end of the October-November 2018 period. Moreover, the drastic reduction of thermal emissions at the craters, marked by the NHI tool since the day of the fissure eruption, is consistent with the drop in the lava lake level that was independently suggested in a previous study. These results demonstrate that the S2-MSI and L8-OLI time series, combined with infrared MODIS observations, may contribute to detecting increasing trends in lava lake activity, which may precede effusive eruptions at the open vent volcanoes. This study addresses some challenging scenarios regarding the definition of possible threshold levels (e.g., in terms of VRP and total Short Wave Infrared radiance) from the NHI and MIROVA datasets, which could require special attention from local authorities in terms of the occurrence of possible future eruptions.