Methane-hydrogen-rich fluid migration triggers seismic failure in subduction at forearc depths

Abstract Metamorphic fluids and deformation zones are fundamental carriers of carbon from the deep Earth to shallower reservoirs. It is well established that water weakens rocks and contributes to trigger seismicity. Conversely, the potential effects of carbon-rich aqueous fluids on deep-seated rocks remain little studied. Some of these fluids may be reduced and transport energy sources such as natural H 2 and CH 4 . Mechanisms and pathways capable to transport these deep energy sources towards shallower reservoirs, where they may be used by microbial life in the subsurface biosphere, remain unidentified. Here we present direct geological evidence of seismic failure of mechanically strong rock types due to the accumulation of CH 4 -H 2 -rich fluids at deep forearc depths, which ultimately reached supralithostatic pore fluid pressure. These fluids originated from adjacent reduction of carbonates by H 2 -rich fluids during serpentinization at eclogite-to-blueschist-facies conditions. Thermodynamic modelling predicts that the production and accumulation of carbon-rich fluids can produce overpressure much more easily than C-poor aqueous fluids, and that CH 4 -H 2 -rich fluids are more favourable to produce overpressure than CO 2 -rich ones. This study provides tangible evidence for the migration of deep Earth energy sources along tectonic discontinuities and suggests causal relationships with brittle failure of hard rock types that may trigger seismicity at forearc depths.