The precursory seismic champagne effect: a physicochemical framework for potential short-term indicators of megathrust earthquakes

Laboratory experiments show that pore-pressure reduction can cause dissolved gases to exsolve from groundwater. The Precursory Seismic Champagne Effect (P-SCE) is proposed, whereby preslip-related depressurisation preceding some megathrust earthquakes may lead to supersaturation of dissolved gases, such as methane, followed by bubble nucleation, growth, upward migration, and discharge. Historical records of phenomena potentially relevant to the P-SCE preceding the 1923 Great Kanto, 1944 Showa Tonankai, 1946 Showa Nankai, and 2011 Tohoku-Oki earthquakes were compiled and compared using boxplot analysis. Across the historical cases, reported groundwater anomalies generally preceded possible combustion-related earthquake lights, although the available observations differed among events. For the Tohoku-Oki earthquake, poroelastic deformation combined with depth-dependent methane-solubility and bubble-growth modelling indicates that comparable pressure reductions promote greater methane exsolution and faster bubble growth at shallower crustal levels. These results provide a physical explanation for the preferential occurrence of anomalies in shallow groundwater systems. P-SCE thus offers a testable framework linking preseismic deformation, groundwater anomalies, and methane exsolution, to be evaluated through prospective multiparameter monitoring before future megathrust earthquakes.