Modeling deformation induced by seasonal variations of continental water in the Himalaya region: sensitivity to Earth elastic structure

The seasonal modulation of seismicity observed in Nepal is given to be related to subtle stress changes, induced by strong seasonal variations of ground deformation. Horizontal and vertical seasonal variations are observed on GPS times series from stations located in Nepal, India and Tibet (China). We demonstrate that this geodetic deformation is induced by seasonal variations of continental water storage driven by the Monsoon. We use data from the Gravity Recovery and Climate Experiment (GRACE) to determine the time evolution of surface loading. We compute the expected geodetic deformation assuming a perfectly elastic Earth model. We consider Green's functions, describing the surface deformation response to a point load, for an elastic homogeneous half-space model and for a layered non-rotating spherical Earth model based on the Preliminary Reference Earth Model (PREM) and a local seismic velocity model. The amplitude and phase of the seasonal variation of the vertical and horizontal geodetic positions can be jointly adjusted only with the layered Earth model. The study emphasizes the importance of using a realistic Earth elastic structure to model surface displacements induced by surface loading. The study also shows that the modeling of geodetic seasonal variations provides a way to probe the Elastic structure of the Earth, even in the absence of direct measurements of surface load variations. Finally, it enhances the possibility of evaluating, with smaller uncertainties, the subtle stress changes given to be responsible for the seasonal variations of seismicity.