十勝岳における自然電位繰り返し測定 | Repeat Surveys of Self-potential in Tokachidake Volcano

We have performed repeat surveys of self-potential (SP) in Tokachidake Volcano since1995, in which some elevated volcanic activity followed the appearance of fumaroles at 62-0Crater. Since thermal anomalies had always preceded to the eruptions at Tokachidake in the last100 years, we expected some temporal and spatial variations of SP anomalies accompanying thethermal activity beneath active craters.Topography-related SP distribution was seen on the northwestern flank in the surveys of1995, 2008 and 2010. This characteristic feature with a gradient of -0.3 mV/m was independentof the survey year and thus thought to be generated from stable flow of ground water. Weassumed that the gradient is uniform over the survey area and corrected the topographic effectprior 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 anomalieshave been seen throughout our SP surveys. These SP anomalies are probably related to thehydrothermal activity beneath the craters but the corresponding electrokinetic current sources shouldnot 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 at62-Craters. The positive SP anomaly on 62-Craters gradually developed toward 1998 up to +100mV, being accompanied by an increase of thermal activity. The anomaly showed a subsequentattenuation down to -50 mV by 2001 in accordance with a decrease of thermal activity. Inaddition, another large positive SP anomaly with an amplitude of approximately +200mV appearednear the active fumaroles on Taisho Crater where fuming was enhanced in 2010. The result ofour SP surveys implies that main thermal activity in the near future may develop at Taisho Craterrather than at 62-Craters. We point that attention should be paid not only to 62-Craters but alsoto Taisho Crater in monitoring the volcanic activity until the next eruption at Tokachidake.