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.

A visual detection and monitoring of volcanic eruptions is the most essential information. In February 2, 2009, Asama volcano, Japan erupted and a large amount of volcanic ash was ejected from the vent. We have observed the activity at Asama since October 12, 2008. For eruption monitoring we used cosmic‐ray muon radiography (muography), a new volcano monitoring system recently developed by Tanaka et al. (2009). We measured a quantitative mass loss inside the crater during the eruption event although no changes were found below the crater. The measured value of 30,780 tons is consistent with a model calculation of volcanic ash flow as observed on February 2, 2009. The obtained radiographic image suggests that a “boiling liquid expanding vapor explosion” occurred and a part of an old lava mound was exploded. This picture is consistent with the analytical result of the volcanic ash ejected on February 2, 2009.

A time-dependent model for volume changes in pressure sources at Asama volcano is developed from precise leveling data collected since 1902. The optimal source model is determined by comparing five different models (a model with three types of spherical sources, single dike, and dike with a spherical source) during three periods: 1935 to 1939, 1939 to 1950/1953, and 1990 to November 2004. The optimal parameters for each model are estimated by employing a genetic algorithm (GA). The model with two spherical sources was selected as the optimal model. In this model, one of the sources was estimated to be located at a depth of approximately 6 km beneath Kurofu volcano; the other source was estimated to be located close to Maekake volcano at a depth of approximately 2 km. The volume changes in the two spherical sources are calculated using the Akaike's Bayesian information criterion (ABIC) for the period from 1902 to 2005. During the period from 1902 to 1943, a large inflation was estimated in a source at a depth of approximately 6 km beneath Kurofu volcano, which is an older volcano at the base of Asama. After 1943, a rapid deflation continued until 1967. This rapid deflation changed to a marginal inflation that continued from 1967 until 2005. The temporal change in the pressure source beneath Kurofu volcano exhibits a strong positive correlation with the eruption frequency. The leveling data does not suggest a significant volume change in the source beneath Kurofu volcano during the quiet period between 1962 and 2005. During the 20th century, magma appears to have been episodically supplied beneath Asama volcano. In particular, the inflation of the source beneath Kurofu volcano, and probably the magma supply, reached a peak from 1930 to 1940. (c) 2007 Elsevier B.V. All rights reserved.

We have developed a novel radiographic imaging method to survey the inhomogeneous structure of the crust. As an example, we performed measurements at Mt. Asama volcano, and studied the feasibility of using an azimuthally isotropic flux of cosmic-ray muons in the energy range up to a few TeV. The principle of the technique is that by measuring muon absorption along different nearly horizontal paths through a solid body, one can deduce the density distribution in the interior of the object. A moon detector with an area of 4000 Cr-2 was installed in a 1-m deep instrument vault located about 1 km from the summit crater of Mt. Asama. Muon tracks within emulsion layers in the detector were analyzed by 3d image processing to determine the level of energy absorption along different ray paths through the summit crater region. A typical angular resolution of the ration detector of 10 milliradians (mrad) corresponds to a spatial resolution of 10 m at a distance of 1 km. The measurements would be ideal for studying the shallow structure of the crust at sites which cannot be well resolved because of their strong structural heterogeneity and potential difficulty to be accessed, and which therefore cannot have their structure determined by conventional electromagnetic or seismic techniques. The present method can also provide three dimensional images of the subsurface by making measurements from two or more different points. In this work, we have radiographically imaged a few hundred meters below the crater floor of Mt. Asama, Japan, and have detected a dense region, which corresponds to the position and shape of a lava mound created during the last eruption (Urabe, B., Watanabe, N., Murakami, M., Topographic change of the summit crater of Asama Volcano during the 2004 eruption derived from Airborne Synthetic Aperture Radar (SAR) measurements, Bulletin of Geographical Survey Institute, 53, 1-6 (2006).). Right below the lava mound we found a low density region that suggests a drain-back-induced porous conduit (Urabe, B., Watanabe, N., Murakami, M., Topographic change of the summit crater of Asama Volcano during the 2004 eruption derived from Airborne Synthetic Aperture Radar (SAR) measurements, Bulletin of Geographical Survey Institute, 53, 1-6 (2006).). The density contrast was resolved with a precision of 1-3%. This method provides a resolution of the shallow density structure that is significantly higher than is possible with conventional geophysical measurements. (c) 2007 Elsevier B.V. All rights reserved.

On September 1, 2004, Mt. Asama in central Japan erupted for the first time in 21 years. Between this moderate eruption and mid-November of the same year, 4 additional moderate eruptions occurred. We installed 8 broadband seismic stations in addition to the short period seismic network around the volcano and succeeded in recording the near-field seismic signals associated with the summit eruptions. The results of the waveform inversions clearly show that the force system exerted at the source region is dominated by vertical single force components. The source depths of the single force are shallower than 200 m from the bottom of the summit crater, and the order of magnitude of the single force is 10 10 –10 11 N. The source time history of each vertical single force component consists of two downward forces and one upward force. The initial downward force probably corresponds to the sudden removal of a lid capping the pressurized conduit. The drag force due to viscous magma moving upward in the conduit can explain the upward force. The correlation between the single force amplitudes and the amounts of volcanic deposits emitted from the summit crater are not necessarily positive, suggesting that the amount of deposits remaining within the summit crater may have played an important role in the excitation of the single force.

Repeated precise leveling in the earthquake swarm area of Ontake, central Japan has revealed uplift of 3–6 mm in proximity to the epicentral region of the most active earthquake cluster in 2002–2004. Although the uplift is small, the vertical displacement is significant even considering leveling error. This uplift is associated with increases in 3He/4He ratios and CO2 δ13C values at a mineral spring in the region, indicating an upper mantle contribution. A region of low resistivity at a depth of 2 km beneath the uplift area has also been inferred, suggesting that the observed uplift is related to changes in a shallow seismogenic layer due to increased hydrothermal input from the earthquake swarm area.