HELIUM DEGASSING RELATED TO THE KOBE EARTHQUAKE

The He-3/He-4 ratios of groundwaters at Nishinomiya city close to the epicenter decreased immediately after the 1995 Kobe Earthquake in Japa n, while the He-4/Ne-20 ratios increased. The delta D and delta(18)O v alues of the groundwaters were identical before and after the earthqua ke. Apparent groundwater ages calculated by tritium-He-3 concentration s also did not change, if we correct the addition of radiogenic helium into the samples by the earthquake. These observations may be attribu ted to following phenomenon; (1) release of radiogenic He as a result of micro-fracturing of rocks during the earthquake followed by degassi ng from the aquifer system, in the amount of about 4.0 +/- 1.3 x 10(-9 ) cm(3) STP/cm(3)-rock, (2) simple mixing of low-tritium water with hi gh radiogenic helium in the clay layer and high-tritium water with atm ospheric helium in the aquifer, and (3) flow path mixing in the region of the sample screen. If we take the (1) degassing hypothesis as a fu nction of micro-fracturing, it is possible to calculate the total He d egassing related to the Kobe earthquake, 4 x 10(11) cm(3) STP, which i s about 3% of the steady-state global He flux of 1.2 x 10(13) cm(3) ST P/year. Thus, earthquake may play an important role in He degassing fr om the solid earth. (C) 1998 Elsevier Science B.V. All rights reserved .