Inferring phases of thermal unrest at Mt. Asama (Japan) from infrared satellite observations

On 1 September 2004 a vulcanian eruption, the strongest since 1983, occurred at Mt. Asama, one of the most active volcanoes in Japan. An ash plume that rose 2km above the crater was emitted, together with incandescent blocks ejected several kilometers away from the summit crater. Subsequently, after a short quiescence period of about 2weeks, a series of minor eruptions took place from 14 September. They were considered unusual for Mt. Asama for the presence of juvenile material dominant among fallout. Small and moderate eruptions continued even in the following months to end in December 2004. On the basis of GPS observations, a dike intrusion occurred under the western flank of Mt. Asama about 6weeks before 1 September eruption, triggering the magma ascension towards the surface. This phenomenon produced many signs of unrest including a thermal activity observed on the ground from the end of July 2004. The retrospective analysis presented in this paper, performed applying the RST VOLC algorithm to Moderate Resolution Imaging Spectroradiometer (MODIS) data, shows the occurrence of pre-eruptive hot spots at Mt. Asama during 1-31 August and 11-12 September 2004. These results extend observations of a previous MODIS-based independent study and show the occurrence of a distinct phase of thermal unrest, of short-time duration, a few days before the 14 September minor eruption. Moreover, by the temporal trend of radiative power from infrared data, time of occurrence of dome extrusion is also estimated, in good agreement with independent radar observations. These outcomes seem to be compatible with different mechanisms of upward propagation of magma speculated by previous authors, confirming the potential of infrared satellite observations in identifying thermal precursory signals of volcanic eruptions, provided that proper detection algorithms are used.