HR: 0800h
AN: G11A-0777    [Abstracts]
TI: Hydrological Changes Induced by the 2003 Tokachi-oki Earthquake at Wells in Hokkaido, Japan
AU: * Matsumoto, N
EM: n.matsumoto@aist.go.jp
AF: National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Akita, F
EM: akita@gsh.pref.hokkaido.jp
AF: Geological Survey of Hokkaido, Kita 19, Nishi 12, Sapporo, 060-0819 Japan
AB: Thirty hydrological responses induced by the M8.0 Tokachi-oki earthquake in 2003 were observed at hot spring wells and an undersea coal mine in Hokkaido, Japan. The purpose of measuring the groundwater level or discharge rate of the wells is mainly to monitor hot spring resources. Nineteen wells are observation wells to monitor the static groundwater level. Groundwater of two wells flows out without pumping, and that of nine wells is pumped out and used. Twelve coseismic increases and 18 decreases in groundwater levels or discharge rates were observed in 29 wells and one coal mine in response to the Tokachi-oki earthquake. Twenty-seven of the 30 hydrological anomalies can be explained as a poroelastic responses to the earthquake-induced contractional or dilatational strain inferred from a fault model determined by dense static GPS observation. In five wells, changes in groundwater levels induced by four large earthquakes, the Kushiro-oki (Mw 7.7), Hokkaido-nansei-oki (Mw 7.7), Hokkaido-toho-oki (Mw 8.3) and Sanriku-haruka-oki (Mw 7.6) earthquakes in 1993 -- 1994 were proportional to the inferred volumetric strain steps. Observed groundwater-level changes and inferred volumetric strain steps induced by the Tokachi-oki earthquake are consistent with the proportionality. Strain sensitivities estimated by $M_2$ tidal constituent are consistent with those determined by coseismic responses in three of the five wells. The strain sensitivities estimated by $M_2$ at the three wells are plausible, because extracted phase shifts in groundwater level are almost the same as theoretical phase shifts in volumetric strain. We observed many increases in groundwater-levels at wells in eastern Hokkaido induced by the 2003 event, whereas many water-level decreases induced by the 1952 Tokachi-oki earthquake (Mw 8.2) were reported at wells in the same area. It suggests that strain induced by the 1952 event was dilatational but the strain induced by the 2003 event was contractional in eastern Hokkaido.
DE: 9320 Asia
DE: 1206 Crustal movements--interplate (8155)
DE: 1242 Seismic deformations (7205)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting