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

The Puebla-Morelos (Mexico) 2017 earthquake nucleated similar to 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 (similar to 185 kg of H2O per m(3) of subducted hydrated harzburgite) and volume increase (Delta V-r (solid)(+)(fluid) = similar to 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.