| Title | Monitoring Geothermal Activity at Aso Volcano, Japan, After Small Eruption in May 2011 |
|---|---|
| Authors | Yayan SOFYAN, Jun NISHIJIMA, Yasuhiro FUJIMITSU, Shin YOSHIKAWA, Tsuneomi KAGIYAMA, Takahiro OHKURA |
| Year | 2014 |
| Conference | Stanford Geothermal Workshop |
| Keywords | Aso volcano, gravity change, mass variation |
| Abstract | The small eruption in May 2011 has occurred following intense volcanic activity in Aso volcano since the end of 2010. An eruptive pattern should defined a calm period after the eruption. The volcanic activity subsides to calm period levels and crater bottom are filled again with hot water and the water level increases in the Nakadake crater. This Crater Lake dynamic phenomenon has relation to hydrothermal dynamics in the subsurface of Aso volcano. We monitor the geothermal activity using repeated gravity measurement in the western part of Aso volcano, which has 4 hot springs, during this period. The repeated gravity measurement for monitoring hydrothermal dynamics beneath Aso volcano was initiated using A10-017 portable absolute gravimeter in 2010 and ScintrexCG-5 (549) relative gravimeter in April 2011. Relative gravity measurements were performed at 28 benchmarks in every three to five months. It covered the area more than 60 km2 in the west side of Aso caldera. A new gravity network was also installed at seven benchmarks using an absolute gravimeter on May 2010, which re-occupied in October 2010, and June 2011. As a result, the gravity changes detect hydrothermal flow in the subsurface which has a correlation to water level fluctuation in the crater. The 3D inversion models of 4-D gravity data deduce the density contrast distribution beneath Aso volcano. The mass changes are quantitatively estimated using two methods, which is Gaussian from gravity data and density contrast from the simulation result. The largest increased mass about 21 MTon by density contrast or 30 MTon by Gaussian method occurred between April and August 2011. This is the calm period, a 6 month after the eruption in May 2011. The largest decreased mass about -36 MTon by density contrast or -35 MTon by Gaussian method occurred between April 2011 and May 2013, about 2 years after the eruption. This result will contribute to understanding the process of eruption. |