Satellite thermal analyses of lava dome effusion rates at Unzen Volcano, Japan

Abstract

This paper presents analyses of infrared remote sensing data of the 1992–1994 lava dome growth episode of Unzen Volcano, Japan. The primary aim is to determine whether such data can indicate trends in the rate of magma supply to the dome, since this is one of the principal factors controlling the frequency of pyroclastic flow. Spaceborne, low‐spatial‐resolution infrared radiance data correctly delimit the two discrete phases of magma supply to the dome, and during phase 1 the short‐wavelength data are found to be strongly correlated with ground‐based estimates of lava effusion rate. The correlation is, however, significantly weaker during phase 2. Analysis of high‐spatial‐resolution data from airborne and satellite based systems allows us to determine the principal source of shortwave infrared flux as fumarolically heated dome surfaces, these being significantly hotter than the actively growing lava lobes. Variations in the low‐spatial‐resolution shortwave infrared signal are therefore directly attributed to fluctuations in the size and temperature of the fumarolically heated area. These fumarolic variations are largely dependent upon the flux rate of magmatic gas, which in turn is partly dependent upon the rate of magma supply. Gas fluxes measured using correlation spectrometer (COSPEC) measurements of SO2 concentration indicate that the relationship between fumarolic discharge and the effusion rate of lava was relatively stable during phase 1 but highly variable during phase 2. We suggest these variations as an explanation for the differing correlations observed between the emitted shortwave infrared flux and the effusion rate of lava during phases 1 and 2. We conclude that appropriate analysis of high‐ and low‐spatial‐resolution shortwave infrared remote sensing data may assist in elucidating trends in lava supply at volcanoes exhibiting similar dome growth episodes to that experienced at Unzen.