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TitleAsthenosphere rheology inferred from observations of the 2012 Indian Ocean earthquake
AuthorHu, Y; Bürgmann, R; Banerjee, P; Feng, L; Hill, E M; Ito, T; Tabei, T; Wang, KORCID logo
SourceNature 2016, 17 pages, Open Access logo Open Access
Alt SeriesEarth Sciences Sector, Contribution Series 20160077
PublisherSpringer Nature
Mediapaper; on-line; digital
File formatpdf; csv
AreaIndian Ocean; Sumatra; Indonesia
Lat/Long WENS 89.0000 108.0000 15.0000 -13.0000
Subjectstectonics; earthquakes; plate tectonics; subduction zones; crustal studies; oceanic crust; crustal uplift; deformation; stress analyses; mantle; mechanical analyses; rheology; viscosity; modelling; 2012 Indian Ocean earthquake
Illustrationssatellite imagery; models; 3-D models; graphs; geological sketch maps; time series; tables
ProgramPublic Safety Geoscience Western Canada Geohazards Project
Released2016 10 10
AbstractThe 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 1-3. 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 4-6 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) × 1018 pascal seconds) asthenospheric layer beneath the elastic oceanic lithosphere is required to produce the observed postseismic uplift.
Summary(Plain Language Summary, not published)
In the study of great earthquake cycles, especially those at subduction zones such as off the west coast of Canada, a fundamentally important parameter is the viscosity of the mantle material beneath the Earth's lithosphere. This paper presents results that provide by far the most direct constraints on this parameter beneath the oceanic lithosphere. The results are obtained by modeling geodetically observed transient crustal deformation following a magnitude 8.6 earthquake in the Indian Ocean that occurred in 2012 seaward of the Sumatra subduction zone. The results indicate the presence of a low-viscosity (soft) layer beneath the much more rigid oceanic lithosphere that produces earthquakes. The recognition of this soft layer affects our interpretation of earthquake cycle deformation and thus understanding of time-dependent seismic risk.

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