title: Asthenosphere rheology inferred from observations of the 2012 Indian Ocean earthquake.
abstract: The concept of a weak asthenospheric layer underlying Earth's mobile tectonic plates is fundamental to our understanding of mantle convection and plate tectonics. However, little is known about the mechanical properties of the asthenosphere (the part of the upper mantle below the lithosphere) underlying the oceanic crust, which covers about 60 per cent of Earth's surface. Great earthquakes cause large coseismic crustal deformation in areas hundreds of kilometres away from and below the rupture area. Subsequent relaxation of the earthquake-induced stresses in the viscoelastic upper mantle leads to prolonged postseismic crustal deformation that may last several decades and can be recorded with geodetic methods. The observed postseismic deformation helps us to understand the rheological properties of the upper mantle, but so far such measurements have been limited to continental-plate boundary zones. Here we consider the postseismic deformation of the very large (moment magnitude 8.6) 2012 Indian Ocean earthquake to provide by far the most direct constraint on the structure of oceanic mantle rheology. In the first three years after the Indian Ocean earthquake, 37 continuous Global Navigation Satellite Systems stations in the region underwent horizontal northeastward displacements of up to 17 centimetres in a direction similar to that of the coseismic offsets. However, a few stations close to the rupture area that had experienced subsidence of up to about 4 centimetres during the earthquake rose by nearly 7 centimetres after the earthquake. Our three-dimensional viscoelastic finite-element models of the post-earthquake deformation show that a thin (30-200 kilometres), low-viscosity (having a steady-state Maxwell viscosity of (0.5-10) × 1018pascal seconds) asthenospheric layer beneath the elastic oceanic lithosphere is required to produce the observed postseismic uplift.
authors.affiliation: Berkeley Seismological Laboratory and Department of Earth and Planetary Science, University of California Berkeley, Berkeley, California, USA.|Mengcheng National Geophysical Observatory, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China.
authors.forename: Yan
authors.initials: Y
authors.lastname: Hu
authors.affiliation: Berkeley Seismological Laboratory and Department of Earth and Planetary Science, University of California Berkeley, Berkeley, California, USA.
authors.forename: Roland
authors.initials: R
authors.lastname: Bürgmann
authors.affiliation: Earth Observatory of Singapore, Asian School of the Environment, Nanyang Technological University, Singapore.
authors.forename: Paramesh
authors.initials: P
authors.lastname: Banerjee
authors.affiliation: Earth Observatory of Singapore, Asian School of the Environment, Nanyang Technological University, Singapore.
authors.forename: Lujia
authors.initials: L
authors.lastname: Feng
authors.affiliation: Earth Observatory of Singapore, Asian School of the Environment, Nanyang Technological University, Singapore.
authors.forename: Emma M
authors.initials: EM
authors.lastname: Hill
authors.affiliation: Graduate School of Environmental Studies, Nagoya University, Aichi 464-8601, Japan.
authors.forename: Takeo
authors.initials: T
authors.lastname: Ito
authors.affiliation: Department of Applied Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japan.
authors.forename: Takao
authors.initials: T
authors.lastname: Tabei
authors.affiliation: Pacific Geoscience Centre, Geological Survey of Canada, Natural Resources Canada, Sidney, British Columbia, Canada.
authors.forename: Kelin
authors.initials: K
authors.lastname: Wang