Fluid‐Induced Fault Reactivation Due To Brucite + Antigorite Dehydration Triggered the Mw7.1 September 19th Puebla‐Morelos (Mexico) Intermediate‐Depth Earthquake

The Puebla‐Morelos (Mexico) 2017 earthquake nucleated ∼250 km inland from the trench within the Cocos oceanic plate mantle. Here, we argue that the Puebla‐Morelos (Mexico) 2017 earthquake resulted from changes in effective stress due to the reaction brucite + antigorite = olivine + H2O leading to the reactivation of pre‐existing seafloor faults. Fluid release (∼185 kg of H2O per m3 of subducted hydrated harzburgite) and volume increase (ΔVr solid+fluid = ∼0.8%) likely occur along subducted seafloor‐inherited faults. The amount of H2O released, and magnitude of volume change depends on the degree of faults hydration; only highly hydrated (>40% of hydration) faults will stabilize brucite and experience this reaction at depth. The brucite + antigorite dehydration reaction may be key for intermediate seismicity worldwide.