Upper Mantle Water Stratification Inferred From the 2012 Mw 8.6 Wharton Basin Earthquake

Water, the most abundant volatile in Earth’s interior, preserves the young face of our planet by catalyzing mantle convection, lubricating plate tectonics, and feeding arc volcanism. Since planetary accretion, water has been exchanged between the hydrosphere and the geosphere, but its depth distribution in the mantle remains elusive. Here, we exploit the water-sensitivity of the strength of olivine, the weakest and most abundant mineral in the upper mantle, and the unique constraints brought by the exceptionally large 2012 Mw8.6 Wharton Basin earthquake in the Indian Ocean to evidence stratification of water content in the upper mantle. We explain the transient deformation in the aftermath of the earthquake that was recorded by continuous geodetic stations along Sumatra by the wet dislocation creep of olivine, indicating a minimum water content of 0.01wt% or 1,600 H/10Si in the asthenosphere, considering a wide range of temperature conditions and including the effect of transient creep of olivine. The earthquake ruptured conjugate faults throughout the entire oceanic lithosphere down to great depths3, compatible with dry olivine in the thick, brittle, layer. We explain the steep rheological contrast by dehydration across the lithosphere-asthenosphere boundary, presumably by buoyant melt migration to form the oceanic crust.

Funding Sources: 

National Research Foundation