Temperature changes at depths to 150 metres near the active crater of Aso Volcano: preliminary analysis of seasonal and volcanic effects

Abstract

Two holes were drilled to depths of 150 m and 70 m from the surface about 200 m from the active crater of Aso Volcano and quartz thermometers were installed in the holes at depth intervals of 30 m and 35 m, respectively. This series of observations is one of the first measurements of temperature at depth so close to an active crater. The ground temperature at a depth of 2 m had an annual variation with a range of about 10°C, as expected, clearly corresponding to the atmospheric temperature variation, but delayed by about 1 month. Temperatures measured at depths from 30 m to 70 m had very small annual temperature variations. The range of temperature at 30 m depth was about 0.04°C. The temperature at a depth of 60 m, however, was particularly stable, probably because at this depth the hole is in the middle of a massive lava flow. At depths of 70 m or more, small (less than 0.2°C) annual temperature variations were again observed. These variations are probably due to the effects of surface water descending to these levels through cracks and fissures. At 120 m depth, the average temperature is about 17.5°C, over 5°C above the surface average temperature, and the annual temperature variation has a range of about 2°C, out of phase with the atmospheric changes. This is probably due to the interaction of rainfall descending from the surface with convecting hotter fluids from below. The temperature gradient below 100 m depth is very high, with the average temperature at a depth of 150 m being about 31°C. The temperature variations at this depth are dominated by long-period variations, with a steady decline after a peak in November–December 1989, overlain by a rather irregular seasonal variation, with a range of about 0.5°C. October 1989 was the time of most active volcanic activity, with Strombolian eruptions depositing ash to a distance of 50 km from the crater, accompanied by very high amplitude volcanic tremor. So the temperature changes at 150 m seem to be mainly the result of volcanic activity. The maximum temperature at this depth occurred 1 or 2 months after the peak of observed volcanic activity. So it is likely that the temperature variations show a delayed influence of the level of volcanic activity. These results show that in the unconsolidated materials often found by active volcanic craters, the effects of seasonal atmospheric variations are carried to substantial depths by groundwater flows, so that at Aso Volcano, only at the maximum depth of 150 m are the temperature variations clearly dominated by the level of volcanic activity.