Permalink : http://hdl.handle.net/2115/45144

JaLCDOI : 10.14943/gbhu.74.1

We have performed repeat surveys of self-potential (SP) in Tokachidake Volcano since 1995, in which some elevated volcanic activity followed the appearance of fumaroles at 62-0 Crater. Since thermal anomalies had always preceded to the eruptions at Tokachidake in the last 100 years, we expected some temporal and spatial variations of SP anomalies accompanying the thermal activity beneath active craters. Topography-related SP distribution was seen on the northwestern flank in the surveys of 1995, 2008 and 2010. This characteristic feature with a gradient of -0.3 mV/m was independent of the survey year and thus thought to be generated from stable flow of ground water. We assumed that the gradient is uniform over the survey area and corrected the topographic effect prior to the following analyzes in order to emphasize the SP of volcanic origins. We found positive SP anomalies on 62-Craters and Taisho Crater where thermal anomalies have been seen throughout our SP surveys. These SP anomalies are probably related to the hydrothermal activity beneath the craters but the corresponding electrokinetic current sources should not be very deep since spatial size of these SP anomalies are smaller than a few hundred meters. We also detected temporal variations of SP anomalies corresponding to the thermal activity at 62-Craters. The positive SP anomaly on 62-Craters gradually developed toward 1998 up to +100 mV, being accompanied by an increase of thermal activity. The anomaly showed a subsequent attenuation down to -50 mV by 2001 in accordance with a decrease of thermal activity. In addition, another large positive SP anomaly with an amplitude of approximately +200mV appeared near the active fumaroles on Taisho Crater where fuming was enhanced in 2010. The result of our SP surveys implies that main thermal activity in the near future may develop at Taisho Crater rather than at 62-Craters. We point that attention should be paid not only to 62-Craters but also to Taisho Crater in monitoring the volcanic activity until the next eruption at Tokachidake.