Interference of tectonic signals in subsurface hydrologic monitoring through gravity and GPS due to mountain building

Global Positioning System observations in the Alps have now sufficient precision to reliably observe vertical surface movement rates at the orogen scale. The geodynamic modeling of converging plate margins requires constraints on the origin of orogenic uplift, of which the two end members are pure crustal uplift and crustal thickening, respectively. Gravity change rates joint with uplift measurements allows to distinguish the two mechanisms. We use vertical uplift rates over the Alpine range and the southern foreland basin, to predict the gravity change for different geodynamic hypotheses of pure uplift and mantle inflow, or crustal thickening and isostatic Moho lowering. The sensitivity of gravity as a tool to distinguish the two mechanisms is investigated. This model differs from the predicted isostatic movements, based on the glacial history and the mantle viscosity, since the uplift is measured and not predicted. The estimate of this tectonic signal is important, when gravity change rates, as those obtained from GRACE, are interpreted exclusively in terms of hydrologic changes tied to climatic variation. It has been already shown that in some areas, as the Tibetan plateau and the Himalayas, the tectonic signal is not negligible. Here we estimate the effect of the tectonic signal for the uplift of smaller mountain ranges, as is the Alpine arc. Our results indicate that tectonic and hydrological signals superpose and we cannot ignore the tectonic signal when using GRACE to invert for the equivalent water height (EWH).