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

Abstract

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 (A ac and A as ) and the snowfall effect (A s ) 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

“…While the modeled gravity was qualitatively consistent with the observed steep gravity increase during rainfall and the subsequent gradual decrease after rainfall, the amplitude of the modeled gravity was only 75% (about 12 μGal) of the observed gravity, mainly because of the coarse three‐dimensional water distribution model for the undulating volcano [see Kazama and Okubo , , Figure 5]. On the other hand, Kazama et al [] applied the modeling procedures of Kazama and Okubo [] to Isawa Fan, one of flat regions in Japan, and succeeded in reproducing the time variation of soil moisture within its observation error, and the resultant gravity disturbance to within 1 μGal root‐mean‐square (RMS). These studies showed that the hydrological disturbances observed at Asama Volcano can be reproduced more accurately, as long as the three‐dimensional water distributions in the volcano are modeled realistically at a higher resolution.…”

“…The hydrological model addresses the spatiotemporal distributions of soil water and unconfined groundwater, which are the most dominant factors in the hydrological gravity disturbances observed on the ground [e.g., Creutzfeldt et al , ; Kazama et al , ]. Soil water, which lies between the ground surface and the water table of unconfined groundwater (i.e., in the unsaturated layer or the vadose zone; Figure ), is expected to infiltrate vertically according to the following diffusion equation (the so‐called Richard's Equation) [e.g., Jury and Horton , ]: where θ = θ ( x , y , z , t ) is the volumetric water content (m 3 /m 3 ) at position ( x , y , z ) and time t , varying between a minimum ( θ min ) and a maximum value ( θ max ).…”

“…An adequate choice of soil parameters is needed to model realistic water distributions [e.g., Creutzfeldt et al, 2010b;Kazama et al, 2012]. Kazama and Okubo [2009] already adopted several soil parameters that were measured by soil tests for the soil cores sampled from the ground at AVO (i.e., the top of the unsaturated layer).…”

Continuous gravity observation with a superconducting gravimeter at the Tomakomai CCS demonstration site, Japan: applicability to ground‐based monitoring of offshore CO2 geological storage

“…The gravity changes attributable to perched water level changes are expressed as a linear function of the water level changes. In earlier studies, this simple approach was applied for data obtained using SGs . Although the groundwater level measured at a single location is not always representative at a field scale, the model simplicity produces some practical benefits.…”

Temporal gravity anomalies observed in the Tokai area and a possible relationship with slow slips

“…For the gravity data obtained after the above corrections, we estimated a groundwater noise with the GWA-TER1D (Kazama et al 2012), incorporating the daily weather data of the AMeDAS during the 20-year observation period (Japan Meteorological Agency, http://www. data.jma.go.jp/obd/stats/etrn/index.php).…”