Landsat infrared analysis of fumarole activity at Unzen Volcano: time-series comparison with gas and magma fluxes

We analyse shortwave infrared thermal data of the phase 1 eruption of Unzen Volcano (Japan) extracted from eight nighttime Thematic Mapper (TM) images taken from the Landsat 5 satellite between October 1991 and November 1992, We identify two discrete regions of the dome that were heated to high temp erature by the ongoing eruptive activity; a fumarolically heated region and an area associated with the effusion of new lava. We concentrate analysis on the fumarolically heated region and investigate the relationships betwee n parameters derived from the infrared radiance data and the nature of the fumarolic gas and magma fluxes. Temporal variations in the parameters deriv ed from the radiance data closely follow those observed in the measured rat e of magma effusion. The positive correlation observed between the fumaroli cally driven shortwave infrared flux and the magma discharge rate (r(2) = 0 .64) indicates that degassing occurred efficiently and in proportion to the amount of magma supplied. Over our monitoring period, the data suggest tha t gas accumulation within the edifice did not occur, this conclusion agreei ng with a previous finding obtained using correlation spectrometer (COSPEC) analysis of SO2 flux rates. A positive correlation (r(2) = 0.56) was also found between the mean radiance of the pixels in the fumarolically heated r egion and the overall size of that region. This suggests a potential mechan ism whereby, when gas pressure within the edifice increased, excess gas esc aped through additional pathways to the surface as well through an increase d flux at the main fumarolic vents. (C) 1999 Elsevier Science B.V. All righ ts reserved.