Dix Seismo Lab Seminar

  The formation of surface oceans, plate tectonics, and the  origin of life can all be linked to the fate of water after planetary  formation, but how much water is now trapped in the upper mantle  remains elusive. Current estimates of water in olivine from  geochemistry (the mid-ocean ridge basalt source) are between 200 and  3,600 H/1E6 Si or 10 to 200 weight ppm. Here, we exploit the  water-sensitive rheology of olivine to estimate the water content of  the Wharton Basin asthenosphere in the Indian Ocean using geodetic  data. On April 2012, a sequence of ruptures cascading up to the  largest strike-slip earthquake ever recorded (Mw 8.6) struck the  oceanic upper mantle off-shore Sumatra. The event and its postseismic  relaxation were recorded by the Sumatra GPS Array. The deformation  time series provide unique constraints on asthenospheric flow in an  oceanic plate. The asthenosphere is a soft region below the  lithosphere defined by the competing effects of temperature weakening  and pressure hardening on the rheology of olivine. The steady-state  behavior of olivine is well understood, but it accommodates only a  fraction of the mineral deformation. To better describe the mechanics  of deformation, we propose a new rheological law for olivine where a  state variable is used to represent the transient effects and the  evolution of internal stress. We then estimate the rheological  parameters - modeling out the parasitic effect of rate-strengthening  afterslip - in a Bayesian framework. We find that the Wharton Basin  asthenosphere contains about 1000 H/ 1E6 Si, representing 10% of water  saturation of olivine at 100 km depth. Extrapolating this water  content to other locations, we expect a layer of partial melt to form  in younger oceanic plates as was found from marine electro-magnetic  studies offshore Nicaragua.