北海道内温泉井における4回のM7.5以上の地震直後の地下水位変化

Abstract

M7.6 Kushiro-Oki, M7.8 Hokkaido-Nansei-Oki, M8.1 Hokkaido-Toho-Oki and M7.5 Sanriku-Haruka-Oki earthquakes occurred around Hokkaido, northern Japan during the period from January, 1993 to December, 1994. We have collected groundwater-level records in 12 wells observed by local governments in Hokkaido to investigate the relationship between coseismic changes in water levels and volumetric strain induced by the four large earthquakes. Coseismic water level changes are found in many wells. The maximum coseismic change in water level was 2.5m, induced by the Hokkaido-Nansei-Oki earthquake. The volumetric strain fields induced by the four earthquakes have been calculated by using published parameters of dislocation models, and compared with the coseismic water level changes. Generally, the volumetric strain changes at wells where water level fell coseismically are dilatation, and those at wells where water level rose are contraction. As far as the relationship between water level changes and volumetric strain induced by the earthquakes in seven wells where water level data are available for at least three earthquakes, coseismic water level changes at all the seven wells are found to be proportional to the calculated volumetric strain. Furthermore, the coseismic water level changes are found to be proportional to not only the volumetric strain but also an amplitude of ground motion caused by the earthquakes in four wells where water level are available for all the four earthquakes by applying AIC (The Akaike Information Criterion) method. The strain sensitivities of the water level at the four wells, which are between 1.6 and 19.6mm/10-8 strain when the contribution of both the volumetric strain and the ground motion are taken into account, are as large as strain sensitivities inferred from the earth tides in 17 wells observed worldwide.