Papers - TSUNOGAI Urumu

Anomalous changes in groundwater chemistry - Possible precursors of the 1995 Hyogo-ken Nanbu earthquake, Japan Reviewed

Tsunogai, U and Wakita, H

JOURNAL OF PHYSICS OF THE EARTH   Vol. 44 ( 4 ) page： 381-390   1996

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Language：English   Publishing type：Research paper (scientific journal)  

A post-earthquake investigation revealed statistically significant changes in the chemical composition of groundwater pumped from wells located about 20 km east of the epicenter within the aftershock region, preceding the disastrous M7.2 Hyogo-ken Nanbu earthquake in southwestern Japan on January 17, 1995. The chloride ion concentration had increased steadily after August 1994, compared to the constant level since July 1990, and reached an excess of about 10 % just before the earthquake. Groundwater sampled after the earthquake showed much higher Cl- concentrations. Together with the increase in Cl-, sulfate and alkaline earth metal ions (Mg2+, Ca2+, Sr2+, Ba2+) showed similar increases in their concentrations, while sodium ion and silica contents showed decreases. These changes may be the result of shallow groundwater inflow, enriched in chloride, sulfate and alkaline earth metal ions, to the artesian layer of the wells, due to permeability enhancement in the shallow stratum. The observed changes in groundwater chemistry before and after the earthquake are useful clues to understanding the mechanism of the present earthquake.

Precursory chemical changes in ground water - Kobe Earthquake, Japan Reviewed

Tsunogai, U. and Wakita, H.

SCIENCE   Vol. 269 ( 5220 ) page： 61-63   1995.7

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Chloride (Cl-) and sulfate (SO42-) ion concentrations of ground water issuing from two wells located near the epicenter of the Kobe earthquake in Japan fluctuated before the disastrous magnitude 7.2 event on 17 January 1995. The samples measured were pumped ground water packed in bottles and distributed in the domestic market as drinking water from 1993 to April 1995. Analytical results demonstrate that Cl- and SO42- concentrations increased steadily from August 1994 to just before the earthquake. Water sampled after the earthquake showed much higher Cl- and SO42- concentrations. The precursory changes in chemical composition may reflect the preparation stage of a large earthquake.

DOI： 10.1126/science.269.5220.61

Peculiar features of Suiyo seamount hydrothermal fluids, Izu-Bonin arc: Differences from subaerial volcanism Reviewed

Tsunogai, U, Ishibashi, J, Wakita, H, Gamo, T, Watanabe, K, Kajimura, T, Kanayama, S, Sakai, H

EARTH AND PLANETARY SCIENCE LETTERS   Vol. 126 ( 4 ) page： 289-301   1994.9

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This is the first report of the dissolved gas composition of a deep-sea venting hydrothermal system originating from are volcanism; the origin of dissolved gases in hydrothermal fluids from the Suiyo seamount in the southern Izu-Bonin (Ogasawara) Are is discussed on the basis of chemical and isotopic compositions. Twelve high-temperature (up to 310 degrees C) fluid samples were collected from three vents of the hydrothermal system using the submersible Shinkai 2000. The average He-3/He-4 ratio of the dissolved He is 8.1 (R/R(atm)), which is within the range of values reported for N-MORB. The CO2/He-3 ratio is 12 x 10(9), which is within the range of are volcanic gas values. The delta¹³C values of CO2 and CH₄ are -1 and -8.5 parts per thousand respectively, which is quite high compared with other subaerial are volcanic gases and those from mid-ocean ridges. These results, which do not accord with those from subaerial volcanism, are unlikely to result from fractionation or addition of carbon sources during the fluid circulation, but are regarded as characteristic of the magma source of Suiyo Seamount. A simple mixing of 97 % MORB mantle with 3 % subducting slab explains well the composition of the helium and carbon in the parent magma.

DOI： 10.1016/0012-821X(94)90113-9