金曜セミナー(Sylvain Barbot氏)

Upper-mantle water stratification inferred from observations of the 2012 Indian Ocean earthquake

Prof. Sylvain Barbot (Earth Observatory of Singapore, Nanyang Tecnological University)

Water, the most abundant volatile in Earth’s interior, preserves the young
surface of our planet by catalysing 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. Water drastically reduces the
strength of olivine and this effect can be exploited to estimate the water
content of olivine from the mechanical response of the asthenosphere to
stress perturbations such as the ones following large earthquakes. Here, we
exploit the sensitivity to water of the strength of olivine, the weakest and
most abundant mineral in the upper mantle, and observations of the
exceptionally large (moment magnitude 8.6) 2012 Indian Ocean earthquake to
constrain the stratification of water content in the upper mantle. Taking
into account a wide range of temperature conditions and the transient creep
of olivine, we explain the transient deformation in the aftermath of the
earthquake that was recorded by continuous geodetic stations along Sumatra
as the result of water- and stress-activated creep of olivine. This implies
a minimum water content of about 0.01 per cent by weight-or 1,600 H atoms
per million Si atoms-in the asthenosphere (the part of the upper mantle
below the lithosphere). The earthquake ruptured conjugate faults down to
great depths, compatible with dry olivine in the oceanic lithosphere. We
attribute the steep rheological contrast to dehydration across the
lithosphere-asthenosphere boundary, presumably by buoyant melt migration to
form the oceanic crust.