HR: 1330h
AN: V52B-0437    [PDF]
TI: Andesitic Magmatic Water Which Generated Matsushiro Earthquake Swarm And S Wave Reflector
AU: * Yoshida, N
EM: yoshida@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba-shi, Ibaraki-ken, 305-0006, Japan, Tsukuba, 305-0006 Japan
AU: Tsukahara, H
EM: htsukah@gipac.shinshu-u.ac.jp
AF: Shinshu University, Asahi 3-1-1, Matsumoto-shi, Nagano-ken, 390-8621, Japan, Matsumoto, 390-8621 Japan
AU: Okusawa, T
AF: Shinshu University, Asahi 3-1-1, Matsumoto-shi, Nagano-ken, 390-8621, Japan, Matsumoto, 390-8621 Japan
AB: When the Matsushiro earthquake swarm occurred between August 1965 and October 1967, large amounts of ground water were released. Even at present, ground water being released in this area, accompanied by the free gases. We measured the oxygen/hydrogen isotope ratio in ground water, the CO2/CH4 carbon isotope ratio contained in free gases, and the CH4/C2H6 ratio. The results of these measurements indicate that CO2, which was the major component of the gas, originated from the mantle, and that CH4, another major component, originated from thermal decomposition of organic materials contained in sedimentary rocks. We plotted our ground water data on the Giggebach d18O vs. dD diagram (The composition of the isotopes in the water, released when the magma below the island arc cooled and hardened, was estimated empirically on the basis of the data collected from many samples of volcanic gas and terrestrial heat water. He named this ground water "andesitic magmatic water", with d18O ranging between +5 and +10 permil and dD between -30 and -10 permil.). The linear regression line passes through the range for andesitic magmatic water. This means that this andesitic magmatic water was probably responsible for the earthquake swarm. It is known that there is a seismic wave reflective layer 15 km beneath the Matsushiro earthquake swarm area. It seems highly likely that this layer is made of aquifer. When the impermeable sheet broke and high-pressure water with CO2 rose into the upper crust, the crust was weakened, causing the Matsushiro earthquake. Hot spring waters from the Arima spa (Rokko Mountains) and Kashio spa (on the Median Tectonic Line, Nagano) have a similar characteristic to the Matsushiro ground water in chemical composition of concentration of Cl- and Na+ ion, and isotope ratio of heavy d18O and dD. The S wave reflector has been observed under the Arima spa but not under the Kashio spa. Earthquake swarm has been observed under the Arima spa but not under the Kashio spa. These mean that the S wave reflector is closely related to earthquake swarm.
DE: 1040 Isotopic composition/chemistry
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting