Preseismic hydrogen gas anomalies caused by stress‐corrosion process preceding earthquakes

Abstract

Simultaneous monitoring of He, H 2 , Ar, N 2 , CH 4 and Rn in gas bubbles of groundwater from the 1,500 m deep Nagashima well in the active Yoro fault zone, central Japan, reveals that three microearthquakes with M=1.6–2.6 in the fault zone are accompanied and preceded by increases of H 2 , as is a moderately large event with M=5.4 in the same fault zone. All the focal distances of the earthquakes are within 25 km. No comparable anomalies were seen in the gas species other than H 2 . We suggest that the selective and preseimic increases of H 2 in groundwater gas bubbles are geochemical signals of the earthquake nucleation process of stress‐corrosion mechanism. Aseismic slips preceding earthquakes in the active fault zone are likely to have enhanced H 2 production by mechanochemical reaction of fractured rock surface with water in pre‐existing fault planes as groundwater conduits.