Abstract:
Seismic tomography has proven to be a powerful method for studying three-dimensional properties of the Earth's interior. Using seismic waves, however, limits the principally accessible period range to periods less than 1 h. Furthermore, density is not among the parameters that can be derived unambiguously from seismic tomography. The redistribution of mass on the Earth's surface due to a wide variety of meteorological processes provides a continuous source of deformations of the Earth, and these deformations depend on the mechanical properties of the Earth. The increasing capabilities of space-geodetic methods and the availability of global meteorological data open up new perspectives for utilizing these broadband exogenic deformations to study the mechanical properties of the Earth in a period range not accessible by seismic techniques. To develop an inversion method based on exogenic deformations, several problems have to be solved: (1) the direct modelling of exogenic deformations has to be formulated for laterally heterogeneous Earth models; (2) the requirements for geodetic networks to continuously monitor exogenic deformations have to be specified in terms of spatial resolution; (3) a time-domain inversion method has to be established; and (4) for regional studies, a working model of the far-field contribution has to be set up. In the present paper, the theoretical problems involved in (1) are discussed. The effect of lateral heterogeneities on exogenic deformations due to meteorological loading are discussed, and found to be of an order sufficient to allow for an inversion. The explicit expressions relating the Green's functions for surface loading on a laterally heterogeneous Earth to the mechanical properties of the model are given. These expressions are based on an expansion in spherical harmonics. Due to computational limits, it is concluded, that this approach will be feasible for global inversions with a resolution considerably higher than present-day global seismic tomography. For regional inversions, however, another approach will be required, using the global spherical harmonic solution to model the far-field contribution.