Time Variations of the Vertical Component in Some of Japanese GEONET GNSS Sites

Abstract

We analyze the vertical component of GEONET GNSS measurements in Central Japan and clarify in some of the sites the origin of large annual time variations, as well as the secular variations. Many of these vertical movements may be attributable to the use of groundwater for agriculture, for snow melting, industrial, and hospital usages, etc. and the pumping up of the groundwater mining for refining natural gas and iodine at the production area of natural gas dissolved in water. For this reason, highly accurate monitoring of vertical variations by GNSS observations can provide new observation methods for understanding of not only geodynamics but also hydrology through monitoring groundwater fluctuation, and natural gas and oil resource development through monitoring ground movements caused by mining.

Seasonal Modulation of Crustal Seismicity in Northeastern Japan Driven by Snow Load

“…Thus, the atmospheric load is too small to explain the discrepancy between observed vertical displacements and expected displacements due to snow. Geodetic investigations of groundwater from snow melt along roads in the Niigata area have also indicated significant subsidence (Morishita et al., 2020; Sato et al., 2003; Shimada et al., 2021). Additionally, the impoundment of dams might affect vertical displacement, but its contribution is much less than snow and atmosphere (Heki, 2004).…”

“…We note that any seasonal changes in the time‐varying amplitudes (Cleveland et al., 1990; Köhne et al., 2023) should be examined in a future study. Furthermore, careful consideration must be taken to exclude the effects of both groundwater (Morishita et al., 2020; Sato et al., 2003; Shimada et al., 2021) and snow accretion on the GPS antenna (Heki & Jin, 2023; Larson, 2013; Larson et al., 2015) from the vertical displacements when estimating the spatiotemporal distribution of the surface load. Note that Heki and Jin (2023) proposed a method to discriminate fake signals due to snow accretion from real subsidence.…”

“…We exclude the vertical displacements from 10 stations in this analysis because the displacements at five stations are closely related to groundwater‐level changes due to local irrigation (blue triangles in Figure 1a; Munekane et al., 2004; Tobita et al., 2004) and the displacements at another five stations contain high seasonal amplitudes which are more than three standard deviation larger than the average of the seasonal amplitudes of the yellow and green stations (green triangles in Figure 1a). These high amplitudes are interpreted as possibly due to the groundwater extraction for snow melting (e.g., Shimada et al., 2021) or more snow locally than in the surrounding area, and do not reflect the appropriate amount of snow at that location.…”

Seasonal modulation of crustal seismicity in northeastern Japan driven by snow load

“…This suggests that the observed GNSS vertical subsidence includes loading sources other than the snow mass, which is obtained from the snow depth. For example, geodetic investigations of groundwater from snow melt along roads in the Niigata area have indicated significant subsidence (Morishita et al, 2020;Sato et al, 2003;Shimada et al, 2021). Or the displacement measurements may be incorrect because of the accumulation of snow on the GNSS pillar and/or radome covering the antenna, thereby impeding the GNSS signals and enhancing signal scattering effects (Jaldehag et al, 1996).…”

“…We note that any seasonal changes in the timevarying amplitudes (Cleveland et al, 1990;Köhne et al, 2023) should be examined in a future study. Furthermore, careful consideration must be taken to exclude the effects of both groundwater (Morishita et al, 2020;Sato et al, 2003;Shimada et al, 2021) and snow accumulation on the GPS antenna (Larson, 2013;Larson et al, 2015) from the vertical displacements when estimating the spatiotemporal distribution of the surface load.…”