The effects of post-seismic motions on the moment of inertia of a stratified viscoelastic earth with an asthenosphere |
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| Authors: | R Sabadini D A Yuen E Boschi |
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| Institution: | Istituto di Geofisica, Universitàdegli Studi di Bologna, Viale Berti Pichat 8, 40127 Bologna and Istituto Nazionale di Fisica Nucleare, Bologna, Italy;Department of Geology, Arizona Stata University, Tempe, AZ 85287, USA;Istituto di Geofisica, Universitàdegli Studi di Bologna, Viale Berti Pichat 8, 40127 Bologna and Istituto Nazionale di Geofisica, Via Ruggero Bonghi 11/B, 00184, Roma and Istituto Nazionale di Fisica Nucleare, Bologna, Italy |
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| Abstract: | Summary. We give the analytical formulation for calculating the transient displacement of fields produced by earthquakes in a stratified, selfgravitating, incompressible, viscoelastic earth. We have evaluated the potential of viscous creep in the asthenosphere in exciting the Chandler wobble by a four-layer model consisting of an elastic lithosphere, a two-layer Maxwell viscoelastic mantle, and an inviscid core. The seismic source is modelled as an inhomogeneous boundary condition, which involves a jump condition of the displacement fields across the fault in the lithosphere. The response fields are derived from the solution of a two-point boundary value problem, using analytical propagator matrices in the Laplace-transformed domain. Transient flows produced by post-seismic rebound are found to be confined within the asthenosphere for local viscosity values less than 1020P. The viscosity of the mantle below the low-viscosity channel is kept at 1022P. For low-viscosity zones with widths greater than about 100 km and asthenospheric viscosities less than 1018P, we find that viscoelasticity can amplify the perturbations in the moment of inertia by a factor of 4–5 above the elastic contribution within the time span of the wobble period. We have carried out a comparative study on the changes of the inertia tensor from forcings due to surface loading and to faulting. In general the global responses from faulting are found to be much more sensitive to the viscosity structure of the asthenosphere than those produced from surface loading. |
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