Release of mantle helium from seismicity regions induced by the 2011 Mw 9.0 Tohoku earthquake

Following the 2011 Mw 9.0 Tohoku earthquake, there occurred not only aftershocks near the source fault along the plate boundary, but also many inland earthquakes along several active faults in the overriding plate. This indicates that the static stress change caused by the fault slip of the earthquake considerably affected the seismicity pattern in inland areas. On the following day, March 12, 2011, northern Nagano was shook by the Mj 6.7 major earthquake near the Tokamachi fault. The focal mechanism of the mainshock indicates a reverse-fault type with a NE-SE compression axis, similar to previous earthquakes that occurred along the Tokamachi fault. On June 30, 2011, the Mj 5.4 earthquake occurred at shallow depths around 3~4 km in central Nagano. The focal mechanism shows a strike-slip fault with a compression axis in a WNW-ESE direction. According to GPS data, small crustal movements associated with this event were observed in the vicinity of the epicenter. To the east of these events is the Gofukuji fault which forms part of the Itoigawa-Shizuoka tectonic line. Since Kennedy et al. [1997] demonstrated that mantle derived helium emanates from the San Andreas Fault, analyses of the helium isotopic composition of fault-related fluids from numerous different faults has shown that volatiles escape the mantle via crustal pathways (faults) to the surface. However, there seems to be little evidence of changes in helium isotopes observed before and after faulting in any of these earlier studies. Helium isotope data had been observed from the hot spring and drinking wells near the Tokamachi fault and Itoigawa-Shizuoka tectonic line before the Tohoku earthquake occurred. In order to elucidate a significant emanation of mantle helium from the seismicity regions induced by the Tohoku earthquake, we measured helium isotopes from 20 wells around the Tokamachi fault and part of the Itoigawa-Shizuoka tectonic line. In the vicinity of the Tokamoachi fault, most gas samples showed changes in 3He/4He ratios within ± 10 %. The samples collected from the Senju and Tochikubo wells have 3He/4He ratios decreasing by about 20 %, and are characterized by 4He/20Ne ratios about ten times higher than previous values measured before the Mj 6.7 earthquake, indicating the addition of radiogenic helium derived from the crust. In contrast, the wells near the Itoigawa-Shizuoka tectonic line indicate no significant change in the 3He/4He and 4He/20Ne ratios.