Applying local Green's functions to study the influence of the crustal structure on hydrological loading displacements

Abstract

The influence of the elastic Earth properties on seasonal or shorter periodic surface mass loads due to atmospheric surface pressure and terrestrial water storage variations is usually modeled by applying a local isostatic model like a homogeneous half-space model, or by a one dimensional spherical Earth model like PREM from which a unique set of elastic load Love numbers, or alternatively, elastic Green's functions are derived. The drawbacks of these strategies are that, in the first case, the response according to the local Earth structure is valid only if load and observer almost coincide, or that, in the second case, only the response of an average Earth structure is considered. However, for surface loads with horizontal scales less than 2500 km 2 , as for instance, for strong localized hydrological signals associated with heavy precipitation events and river floods, the Earth elastic response becomes very sensitive to inhomogeneities in the Earth crustal structure.We derive a set of local Green's functions defined for every global 1• × 1• gridcell for the 3-layer crustal structure TEA12. Local Green's functions show standard deviations of ±12% in the vertical and ±21% in the horizontal directions for distances in the range from 0.1• to 0.5• . The application of local Green's functions introduces a variability of 0.5−1.0 mm into the hydrological loading displacements, both in vertical and in horizontal directions. Maximum changes due to the local crustal structures are from −25% to +26% in the vertical and −91% to +55% in the horizontal displacements. In addition, the horizontal displacement changes its direction significantly, even to the opposite. The modeling of a site-dependent crustal response to surface loads provides an alternative way to probe the density and elastic structure of the Earth's crust and mantle by means of observed surface deformations caused by mass re-distributions. In addition, realistic loading models allow the monitoring of mass variations of the hydrosphere and cryosphere in the spatial range between satellite resolution and in-situ observations by the analysis of geodetically measured surface displacements.

The sensitivity of surface mass loading displacement response to perturbations in the elastic structure of the crust and mantle

“…Even small perturbations to upper crustal layers, however, can yield large variations in the LGFs at short observer‐to‐load angular distances [cf. Wang et al , ; Dill et al , ; Bos et al , ], as exemplified by the high‐degree LLNs shown in Figure , the LGF differences between 1066A and PREM, and the LGF differences between the region‐specific (CR and SNA) and globally averaged Earth models (PREM, STW105, and AK135f).…”

Elastic deformation of the Australian continent induced by seasonal water cycles and the 2010–2011 La Niña determined using GPS and GRACE

“…The La Niña subsidence is consistent between GPS measurements and GRACE predictions in spatial pattern (Figure f). The GRACE magnitude is found to be generally smaller than GPS, possibly indicating inefficacy of the 1‐D elastic Earth model used to compute the deformation [ Dill et al ., ]. The calculated trend after the La Niña from GRACE confirms the GPS observations in most places (Figure g).…”

Annual variations in GPS‐measured vertical displacements near Upernavik Isstrøm (Greenland) and contributions from surface mass loading

“…However, according to Dill et al . [], such effects cause a maximum of 10% in the annual vertical displacements over western Greenland.…”

“…We note that the analysis described above does not quantify the effects of lateral variations of crustal structure on the modeled loading displacements. However, according to Dill et al [2015], such effects cause a maximum of 10% in the annual vertical displacements over western Greenland.…”