Temperature profile indicating a newly developed hydrothermal system formed during the 2000 eruption of Usu volcano, Japan

Abstract

Depth profiles of temperature were collected in a borehole drilled at a new crater zone that formed during the 2000 eruption of Usu volcano, Japan. The profile data revealed rapid increases and decreases in temperature over several hundred meters. The increases occurred near intervals with relatively rapid heating which reveal the location of permeable feed zones, and the temperatures were gradually restored to the level of the regional background linear conductive profile away from the intervals. The profile data showed that there were three steep increases in temperature in the ranges of 100–220 m, 400–490 m, and 640–1280 m. In first two and a part of the third increases, the well fluids are heated by conduction behind the casing, and the other is located in the reservoirs, where reservoir fluids have access into the well. The large increase over the 640–1280 m interval consists of four individual feed zones, and the maximum temperature of 186 °C is uniform in the range of 760–840 m, which is indicative of a lateral flow. There is thermal diffusion of heat from this reservoir into its upper and lower confining layers.