[SCG55-P04] 震源断層周辺における地震前後のヘリウム同位体比の変化
キーワード：2011年長野県中部地震、ヘリウム同位体
Static stress change caused by the megathrust slip of the 2011 Mw 9.0 Tohoku-Oki earthquake considerably affected the seismicity pattern in inland areas, resulting in the occurrence of numerous earthquakes along several active faults in Japan. On June 30, 2011, the Mj 5.4 central Nagano earthquake occurred at a shallow depth of 5 km, indicating the reactivation of the Gofukuji fault in Central Japan. This study was undertaken to elucidate spatial and temporal changes of 3He/4He ratios around a source region before and after an inland earthquake using both new and existing helium isotope data from hot spring and drinking water wells. Most gas samples near the seismic source region are characterized by increase in postseismic 3He/4He ratios. In contrast, the postseismic ratios decrease by up to about 30 % away from the seismic source region. Episodic faulting could release accumulated crustal (radiogenic) helium from host rocks, or enhance the transfer of mantle volatiles through permeable fault zones, such that subsequent fluid flow proximal to the source region could then explain the spatio-temporal variations in 3He/4He ratios.