Publications

Theoretical models of the viscoelastic relaxation of a spherical earth are derived to model large-scale post- seismic deformation resulting from great earthquakes over decadal time scales. Most existing models of post- seismic deformation do not consider strong lateral heterogeneities in mantle viscosity, in particular in the subducting slab where such events occur. In addition, the self-gravitation effect is often treated only approximately. Both effects become important when observations from space geodetic techniques such as GPS and GRACE are interpreted. In this paper, we present a spectral finite-element approach that allows these two effects to be considered in a rigorous way. In this way, much larger lateral viscosity variations can be handled than by perturbation techniques. We derive interface conditions for an arbitrary shear fault in the form of double-couple forces that are equivalent to a prescribed dislocation, and simulate a relaxation process for an incompressible Maxwell earth with a three-dimensional viscoelastic structure. Computational results are validated for a spherically symmetric model by an independent method based on the inverse Laplace integration, and good agreement is obtained. As an example, we apply this approach to the 2004 Sumatra-Andaman earthquake, and simulate a large-scale post-seismic gravity potential variation by a forward calculation. In the presence of a slab, the secular variation in geoid height change decreases by 30% for wavelengths longer than 500 km, with respect to the case excluding the slab. The effect of the slab can exceed 0.3 mm/yr for short-term variations when the asthenosphere viscosity is 10 to 19 Pa s, which are larger than the observation errors of GRACE. Lateral heterogeneities in viscosity due to a slab should therefore be considered for interpreting observed post-seismic relaxation due to a large thrust event in a subduction zone.

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Tanaka, Y.; Klemann, V.; Fleming, K.; Martinec, Z. (2008): Post-seismic gravity variation caused by a great earthquake in subduction zone -spectral finite-element approach to consider 3-D viscosity structure. AGU 2008 Fall Meeting (San Francisco, USA 2008).