Publication Date:
2020-02-12
Description:
Postseismic crustal deformation and gravity change associated with great earthquakes have been detected with advanced space geodetic techniques such as GPS and GRACE. Viscoelastic relaxation is the dominant mechanism of postseismic deformation on decadal time scales. Several theories of global postseismic deformation of spherical Earth have been proposed so far to deal with the far-field deformation considering self-gravitation in a consistent way (Piersanti et al., 1995; Pollitz, 1997; Wang, 1999; Pollitz, 2003; Tanaka et al., 2006). In these models, however, lateral heterogeneity in viscosity due to a subducting plate and time variation in the gravity have not been considered, simultaneously. Therefore, for interpreting GPS and GRACE data, it is worth constructing a viscoelastic model that incorporates these two effects at the same time. In this study, we apply a spectral finite element method (Martinec, 2000), which was originally developed for surface loading of a self-gravitating spherical Earth, to the dislocation one. This method allows to consider much larger deviations in the viscosity profile in magnitude than those in perturbation methods, which makes it suitable for simulating relaxation process in presence of a subducting elastic plate. We employ 2-D (i.e. rotationally symmetric) viscoelastic structure with ring sources allocated along latitude lines. This corresponds to an infinitely long fault in a half-space model, which can be applied to deformations caused by the Sumatra-Andaman and the Chile events whose fault lengths reach approximately 1,000 km. We prescribe the boundary conditions at the source in form of double couple forces equivalent to a considered dislocation. To validate this formulation, we compare the computed results with those by Tanaka et al. (2006) for a special case of 1-D (i.e. spherically symmetric) viscoelastic structure. Good agreements are obtained for the fault mechanisms of strike-slip, vertical dip-slip and dip-slip with dip angle of 45 degrees. Using this method, we calculate the viscoelastic relaxation in the following two cases: (I) spherically symmetric viscosity structure with an elastic lithosphere of thickness 40 km and (II) a subducting plate with dip angle of 20 degrees and width of 100 km. The result for a ring source with an infinitesimal width at depth 30 km shows that by the presence of the subducting plate the vertical displacement rate is increased by 30 to 80 % and the horizontal displacement rate is decreased by 50 % in the epicentral distance from one to two degrees at the continental side. The spatial pattern of this increase in the vertical displacement is consistent with the known feature for a flat-Earth model (Suito and Hirahara, 1999).
Keywords:
550 - Earth sciences
Type:
info:eu-repo/semantics/conferenceObject
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