The role of dehydration reaction in seismogenic processes at subduction plate boundaries

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A popular hypothesis for the occurrence of double seismic zones, observed at the intermediate depth of about 50-200km, is dehydration embrittlement of the serpentinized slab mantle. The weakening of serpentinite was attributed to excess pore fluid pressure caused by dehydration reaction of serpentine. However, the confining pressure of these experiments using gas-medium apparatus was limited up to 500MPa (corresponding to 15km depth). It is questionable if the same mechanism could be effective in subducting slabs at higher pressures. We conducted constant strain-rate experiments of serpentinite using a solid-medium deformation apparatus and found that the yield strength of serpentinites drastically decreased when dehydration reaction proceeded. This phenomena, named "dehydration weakening", would be a key to understand seismogenic processes in the serpentinized mantle slabs.