[S-CG55] 地殻流体と地殻変動

[SCG55-02] 2018年北海道胆振東部地震における地下水位の変化

★招待講演

*柴田 智郎¹、高橋 良²、高橋 浩晃³、鹿児島 渉悟⁴、佐野 有司⁴、Pinti Daniele⁵ (1.京都大学大学院理学研究科、2.北海道立総合研究機構　地質研究所、3.北海道大学大学院理学研究科、4.東京大学大気海洋研究所、5.GEOTOP, Université du Québec)

キーワード：地下水位、多孔質弾性的挙動、地震、北海道

Groundwater table level in wells can fluctuate under stress provoked by earthquakes located even thousands of kilometers away. The mechanisms of changes in groundwater level can be explained by hydrological changes such as pore-pressure response to coseismic static elastic strain, permeability changes caused by seismic waves and fluid migration along seismogenic dilatant cracks or crustal ruptures. The Geological Survey of Hokkaido, Hokkaido Research Organization (HRO), has been monitoring groundwater tables in several wells after the 2018 Hokkaido Eastern Iburi earthquake. The wells were drilled for exploiting hot spring water but have not been used in the last 20 years. The groundwater level was measured by pressure gauges with resolution about 5-10 mm in 10-minute intervals in each monitored well. After the Hokkaido Eastern Iburi earthquake, we were able to find a step-like decrease in groundwater levels in three wells. These wells are located within the region where tensile changes of the quake-induced volumetric strain can be calculated by using the fault model, suggesting that the changes in groundwater levels are qualitatively consistent with those in volumetric strain. The observed groundwater level changes are also affected by tidal and barometric effects, and we can estimate the response of groundwater level for tidal components. The tidal responses in all the wells have not changed before and after the earthquake.