Geothermal monitoring of Kueishantao island offshore of Northeastern Taiwan

Abstract

To understand the geothermal variation of the active volcano, Kueishantao island, we conducted continuous monitoring of borehole temperatures beginning in July 2006. Fifteen miniature temperature loggers with resolution better than 0.0010C were deployed at different depths in a 290-m well. The borehole temperatures ranged between 22 and 34°C. No significant annual variation was found below 30 m depth. We divided the geotherm into three sections. The shallow one, at a depth less than 110 m, showed an extreme negative geothermal gradient of -0.6°C (100 m)-1, which may be strongly affected by the convection of shallow water. The middle section, at a depth between 110 and 250 m, had a geotherm of 9.7°C (100 m)1. The heat flow was estimated to be 153 mW nr-2, on the same order as that acquired on the seafloor around Kueishantao island. The lower portion, below 250 m depth, showed a geotherm of -8.5°C (100 m)-1 which may be effected by a fracture zone identified by the drilling data. The average geotherm for the whole hole is 4.8°C (100 m)-1. Several thermal events with amplitudes of -0.023 - 0.015°C and durations of tens of hours were found in different depths in the well. These events have been identified to correlate with the occurrences of typhoons. During the study period there were 17 earthquakes greater than 3.4 in magnitude and within 50 km radius of Kueishantao. However, we did not find any significant relationships between earthquake occurrence and temperature variations in the borehole. More observation data are needed to discuss the mechanism of temperature variation.