Possible magma intrusion revealed by temporal gravity, ground deformation and ground temperature observations at Mount Komagatake (Hokkaido) during the 1996-1998 crisis

Mount Komagatake (1131 m above sea level) is a subduction-related active vo lcano in Hokkaido (Japan), where two phreatic eruptions occurred in 1996 Ma rch and 1998 October, after 54 years of dormancy. We analyse four sessions of geodetic, microgravity and ground temperature observations made over a t wo-year period. From November 1996 to May 1997, neither significant gravity nor significant elevation changes (GPS or levelling) were observed. From M ay 1997 to November 1997, we observed a slight subsidence (1-2 cm) and cont raction (within 1 cm) of the edifice, a gravity increase (15-30 mu gal) at the benchmarks inside the 2 km wide summit crater, and a local temperature increase, of 15-20 degreesC, at the summit crater. From November 1997 to Ma y 1998, we observed an edifice-wide gravity increase of about 15 mu gal, wi th no significant elevation change for benchmarks outside the summit crater . Besides this edifice-scale variation, we recorded a subsidence of about 5 -6 cm and apparently no change of gravity inside the summit crater. We inverted our data using models of increasing complexity. Elastic models are able to explain our deformation observations, but they cannot explain e ither the gravity data or the temperature observations satisfactorily. We i ntroduced both an isothermal and a non-isothermal porous medium filled with fluids to model deformation and gravity variations, and we used a fissure model to invert the temperature anomaly. Our observations are consistent wi th a model of underground shallow-water evaporation occurring as a result o f heat rising from a hot and dense body (density contrast of 200 kg m(-3)) of about 10(11) kg at 4-5 km depth. This interpretation suggests a possible intrusion of a magmatic body within the edifice.