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^{1, 2, 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^{4,5, 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.

Yan Hu及同事使用了2012年印度洋地震震后形变来约束大洋地幔的流变学结构。在震后最初三年内，区域内的大部分GPS测站都发生了东北向的水平位移，与同震位移方向类似。但少数靠近断裂区、最多发生了4厘米同震下沉的测站在震后上升了近7厘米。作者利用震后形变的三维粘弹性有限元模型表明，弹性大洋岩石圈下存在相对较薄的低粘度软流层才能解释上述数据。

文章：http://www.nature.com/nature/journal/v538/n7625/full/nature19787.html