Seasonal and long-term vertical deformation in the Nepal Himalaya constrained by GPS and GRACE measurements

We analyze continuous GPS measurements in Nepal, southern side of the Himalaya, and compare GPS results with GRACE observations in this area. We find both GPS and GRACE show significant seasonal variations. Further comparison indicates that the observed seasonal GPS height variation and GRACE-derived seasonal vertical displacement due to the changing hydrologic load exhibit very consistent results, for both amplitude and phase. For continuous GPS stations whose observation time span are longer than 3 years, the average WRMS reduction is similar to 45% when we subtract GRACE-derived vertical displacements from GPS observed time series. The comparison for annual amplitudes between GPS observed and GRACE-derived seasonal displacements also shows consistent correlation. The good seasonal correlation between GPS and GRACE is due to the improved GPS processing strategies and also because of the strong seasonal hydrological variations in Nepal. Besides the seasonal signal, GRACE also indicates a long-term mass loss in the Himalaya region, assuming no GIA effect. This mass loss therefore will lead to crustal uplift since the earth behaves as an elastic body. We model this effect and remove it from GPS observed vertical rates. With a 2D dislocation model, most GPS vertical rates, especially in the central part of Nepal, can be interpreted by interseismic strain from the Main Himalayan Thrust, and several exceptions may indicate the complexity of vertical motion in this region and some potential local effects.