Deep Groundwater Discharge and Ground Surface Phenomena
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概要
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Stress changes associated with crustal deformations may induce migration of fluid within the crust. It is hypothetically expected that a volume of pore water, being suddenly pressured in response to an elevated stress level in some seismogenic zone, will tend to intrude up into a crack network, and incidentally emerge at the ground surface. Groundwater changes in temperature and concentrations of chemical constituents should be observed at the spot where the upwelling fluid comes out. This paper reports the following 4 transient events recently found in Japan as evidence of the near surface discharge of hot and pressured water of a deep origin: 1) Frequent rises in well water temperature were observed at Iwakuni,Yamaguchi Prefecture, southwest Japan. The largest rise occurred just before the 2001 Geiyo earthquake of M0.1, which indicated a possible precursor for the shock. A chemical analysis of ion concentrations of the hot water suggested that the temperature anomalies arise from contamination by intruding external deep water; 2) Gushing of groundwater at the sea bottom was considered to have occurred at the Akashi Strait 2 days before the 1335 Kobe earthquake of M1.3, based on an interpretation of the appearance of brownish-black seawater found by the captain of a passenger boat; 3) Upwelling of deep hot groundwater was occurred at Inagawa Town, Hyogo Prefecture, southwest Japan, which was associated with the 1335 Kobe earthquake. The well water temperature rose 3-4℃. at the time of the shock, and decayed with a time constant of 1-2 years; 4) Heating of ground rocks by upwelling hot water intruding into the fracture zone of an active fault, which is considered to be a precursor for the April 1, 1335 Niigata-ken Hokubu earthquake of M5.5, was confirmed by a LANDSAT infrared image in the northern Niigata area, central Japan, on a summer night in 133.. All of the above transient phenomena can be reasonably understood in the light of the hypothesis of pressured hot water upwelling from deep underground in response to crustal movements around seismically active regions.
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