Absolute gravity change associated with magma mass movement in the conduit of Asama Volcano (Central Japan), revealed by physical modeling of hydrological gravity disturbances

• Published in: Journal of geophysical research. Solid earth 2015-02, Vol.120 (2), p.1263-1287
• Main Authors: Kazama, Takahito, Okubo, Shuhei, Sugano, Takayuki, Matsumoto, Shigeo, Sun, Wenke, Tanaka, Yoshiyuki, Koyama, Etsuro
• Format: Article
• Language: English
• Subjects: absolute gravimeter; Altitude; Amplitude; Amplitudes; Area; Asama Volcano; Attraction; Boundary conditions; Conduits; Density; Discharge; Disturbances; Earthquakes; Emissions; eruption; Eruptions; Geophysics; Gravimeters; Gravimetry; Gravitation; Gravity; Gravity data; Gravity meters; Groundwater; Groundwater discharge; Hydrologic models; hydrological modeling; Hydrology; Lava; Magma; magma mass; Mass; Mass movement; Meteorological data; Modelling; Parameter estimation; Parameters; Real time; Seismic activity; Soil; Soil conditions; Synchronization; Three dimensional models; Volcanic activity; volcanic conduit; Volcanic eruptions; Volcanic gases; Volcanoes
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2014JB011563

The gravity signal originating from magma mass movement in a volcanic conduit is retrieved from the hydrologically disturbed absolute gravity data obtained at Asama Volcano (Central Japan) in 2004, using a three‐dimensional hydrological model. We improve the hydrological model of the previous study using realistic soil parameters and boundary conditions, to better estimate the spatiotemporal land‐water distributions and the consequent hydrological gravity disturbances. The newly estimated gravity disturbances agree with the absolute gravity values observed by FG5 gravimeters in 2004–2009 within about 2.6 μGal, by additionally accounting for the excess discharge of groundwater mass associated with a sloping impermeable surface below the discharge area. After the gravity disturbance of 20 μGal amplitude is subtracted from the absolute gravity data observed during the 2004 eruptive event, the gravity residual of 5 μGal amplitude shows a significant decrease in synchronization with eruptions, because the ascending magma mass in the conduit affects the upward attraction force to the gravimeters installed on the flank of Asama Volcano. The magma head altitude, to which the residual gravity is converted assuming a homogeneous linear density in the conduit, shows a comprehensive agreement of the time variation in the magma head with those in other volcanic observations, such as gas emission rate and earthquake frequency. By correcting the hydrological gravity disturbances using this hydrological model and simultaneously obtained meteorological data in real time, spatiotemporal variations in the magma mass can be instantaneously monitored at Asama Volcano, even before eruptions during future volcanic events. Key Points Hydrological gravity disturbances are reproduced within about 2.6 μGal RMS A 5 μGal gravity signal is retrieved from the observed absolute gravity data Magma movement in the volcanic conduit is responsible for the residual gravity