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) × 10 pascal seconds) asthenospheric layer beneath the elastic oceanic lithosphere is required to produce the observed postseismic uplift.

Importance of rheological heterogeneity for interpreting viscoelastic relaxation caused by the 2011 Tohoku-Oki earthquake

“…One of the best locations for studying oceanic mantle rheology is the Indian Ocean, where a large M w 8.6 earthquake occurred in 2012 (e.g., Meng et al 2012). Two recently published papers regarding oceanic mantle rheology analyze the postseismic deformation following the 2012 Indian Ocean earthquake (Hu et al 2016b;Masuti et al 2016). …”

Dominant Controls of Downdip Afterslip and Viscous Relaxation on the Postseismic Displacements Following the Mw7.9 Gorkha, Nepal, Earthquake

“…In the multiple‐mechanism models we consider below, we will further examine the trade‐off between D and . For the sake of simplicity, we assume the ratio of the steady state transient viscosity to the transient viscosity in the Burgers body is a constant and follow Hu, Bürgmann, Banerjee, et al () and Hu, Bürgmann, Uchida, et al () and use = 10, which is also the same value we obtain for the upper mantle beneath Tibet.…”

Geodetic Observations of Time‐Variable Glacial Isostatic Adjustment in Southeast Alaska and Its Implications for Earth Rheology

“…Hu, Bürgmann, Uchida, et al () reported a mantle wedge viscosity of 3 × 10 19 Pa s for Japan, the same as in this study, through modeling the viscoelastic postseismic deformation of the 2011 Mw9.0 Tohoku earthquake. They found that the oceanic upper mantle may include an 80‐km thick top layer with a viscosity of 2 × 10 18 Pa s overlying a more rigid layer with a viscosity of 10 20 Pa s Hu, Bürgmann, Banerjee, et al (). The asthenospheric properties obtained in this work thus may be representative of other tectonically active regions.…”