We next describe long-term gravity changes observed between February 2011 and March 2012.
These researchers reported a long-term gravity increase near the tip of the peninsula using relative observation data obtained in 1970 and 2007.
Therefore, the long-term gravity changes observed by Kusumoto et al. (2008) may be associated with fluid migrations caused by SSEs.
Also evident are significant long-term gravity changes arising principally from hydrological processes around the volcano, where annual precipitation exceeds 5,000 mm.
A similar unexplained positive long-term gravity trend of ~ 0.5 μGal/year is seen also in the Cascadia subduction zone (Mazzotti et al. 2007).
However, the long-term gravity changes Δg(t) do still exhibit seasonal variations of ±5 μgal (high in summer and low in winter), emphasizing the need to perform repeated relative gravity measurements in the same seasons to avoid hydrological effects.
The existence of such noise does not pose a fatal problem for the purpose of studying long-term gravity changes at this station, because we have information on exactly when the noise is generated.
From these results, it can be seen that, by adding both the annual gravity term and the snow effects (Eq. (31)), our proposed hydrological modeling can now reproduce long-term gravity changes within about 1 µgal RMS.
Combining robust absolute gravity measurements with less robust but easier relative measurements using portable L&R gravimeters helped to enhance the spatial resolution of the long-term gravity change estimates.
When a porosity of 10% is assumed (Torge, 1989), an increase of 10 cm is expected to bring about a gravity change of +0.4 μGal (+1 m of water height = +40 μGal), which could not have significantly affected the long-term gravity change observed at A1.
We have evaluated long-term gravity changes due to variations in sea-surface height for sites A1 and R3, which are located only a few hundred meters from the coastline, using local tide-gauge data (available on the Web site of the Coastal Movements Data Center, Geospatial Information Authority of Japan, http://cais.gsi.go.jp/cmdc/center/annualgra.html).html
