Takahito Kazama

Gravity changes associated with variations in local land-water distributions: Observations and hydrological modeling at Isawa Fan, northern Japan

Gravity changes associated with variations in local land-water distributions have been observed at Isawa Fan in northern Japan, and modeled by hydrological equations. We solve the Richards equation numerically for the time variation in the vertical soil moisture distribution, which is then spatially integrated to estimate gravity changes due to the soil moisture distribution. In modeling Isawa Fan, we assume a simple hydrological model: a horizontally homogeneous soil in an infinite half-space. The estimated gravity is consistent with the observed gravity during a 50-day period within about 0.4 µgal root mean square, owing to both observed soil parameter values and the observation building geometry being incorporated into the hydrological model. However, the estimated gravity cannot fully reproduce annual gravity changes observed during a 2-year time frame, because the boundary conditions in the modeling determine only local water distributions and the resultant short-period gravity changes. Instead, the observed gravity over these 2 years can be reproduced within about 1.0 µgal root mean square, if the additional parameters of the annual gravity change (Aac and Aas) and the snowfall effect (As) are calculated by the function regression to the observed gravity with the least-squares method. The hydrological modeling techniques presented here can be utilized at all gravity sites in flat areas similar to Isawa Fan, such that hydrological effects in gravity data can be corrected and mass transfers associated with earthquakes and volcanoes can be monitored.

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

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.

New Absolute Gravity Measurements in New Zealand

To enhance and extend the absolute gravity (AG) measurements in New Zealand, we conducted new measurements using a FG5 (#210 of Kyoto University) in January and March 2016. The measurements in the North Island were made at two existing points (the Warkworth Radio Astronomical Observatory and Wellington A) and at one newly established point at the Wairakei Research Centre, Taupo. The gravity measurements in the South Island were made at five existing AG points; Godley Head, Mt John, the University of Otago, Helipad and Bealey Hotel. At each point more than 4,000 drops were made and AG values were determined with measurement uncertainties better than 3 μGal (mostly better than 2 μGal) at 130 cm instrument height. The values are compared with those of the 2015 campaigns. Although the differences of about 10 μGal were observed at Wellington A and Godley Head, those at the other points were within 5 μGal. At points in the Southern Alps we combined AG with relative gravity measurements and achieved good agreement with the 2015 results. Definite values for long-term gravity trends at the points in Southern Alps and Christchurch could not be obtained from the survey, but the results are consistent with those of the previous studies. Further measurements are planned to accurately determine these gravity changes.