Helium degassing related to the Kobe earthquake

The 3He/4He ratios of groundwaters at Nishinomiya city close to the epicenter decreased immediately after the 1995 Kobe Earthquake in Japan, while the 4He/20Ne ratios increased. The δD and δ18O values of the groundwaters were identical before and after the earthquake. Apparent groundwater ages calculated by tritium-3He concentrations also did not change, if we correct the addition of radiogenic helium into the samples by the earthquake. These observations may be attributed to following phenomenon; (1) release of radiogenic He as a result of micro-fracturing of rocks during the earthquake followed by degassing from the aquifer system, in the amount of about 4.0±1.3×10−9 cm3 STP/cm3-rock, (2) simple mixing of low-tritium water with high radiogenic helium in the clay layer and high-tritium water with atmospheric 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 function of micro-fracturing, it is possible to calculate the total He degassing related to the Kobe earthquake, 4×1011 cm3 STP, which is about 3% of the steady-state global He flux of 1.2×1013 cm3 STP/year. Thus, earthquake may play an important role in He degassing from the solid earth.