Hydrothermal dynamics beneath Aso volcano, southwest Japan: View from 3D inversion model of 4-D gravity data

In more than 70 years, the eruption of Aso volcano has originated from Naka-dake crater. The intensive monitoring of volcanic activity considered gravity surveillance as one of indispensable components for monitoring strategy. In May 2010, we installed a new microgravity network at seven benchmarks using Absolute gravimetry, which we re-occupied in October 2010 and June 2011. Relative gravity measurements also were performed using Scintrex CG-5 and La-Coste Romberg type G gravimeter at 28 benchmarks in April 2011, August 2011, November 2011, and April 2012. It covered the area more than 60 km2 in the west side of Aso caldera. The distribution of gravity variation helps to give a picture of the mass movements as a volcanic activity. The variation between April and August 2011 significantly rise near to Naka-dake crater. About 60 μGal of increased gravity is located around the crater. This broad positive anomaly, which is reflecting the increased hot water mass in the vicinity, occurs around crater. The next period gravity monitoring from August to November 2011 shows the broad positive anomaly of about 80 μGal around Kusasenri. The smoothed contouring gravity changes between November 2011 and April 2012 show negative anomaly of about -80 μGal around Kusasenri. The most active area at Aso volcano is located at benchmarks in location around Kusasenri and crater area. The 3D inversion models of 4-D gravity data deduce density contrast or mass change distribution beneath Aso volcano. Hydrothermal dynamics in subsurface reservoirs are indicated by these inversion models. The inversion models of 4-D gravity data have good validation from Naka-dake dynamic crater during these periods.