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

Abstract

Global Positioning System observations in the Alps have now sufficient precision to reliably observe verticalsurface movement rates at the orogen scale. The geodynamic modeling of converging plate margins requiresconstraints on the origin of orogenic uplift, of which the two end members are pure crustal uplift and crustalthickening, respectively. Gravity change rates joint with uplift measurements allows to distinguish the twomechanisms. We use vertical uplift rates over the Alpine range and the southern foreland basin, to predict thegravity change for different geodynamic hypotheses of pure uplift and mantle inflow, or crustal thickening andisostatic 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 gravitychange rates, as those obtained from GRACE, are interpreted exclusively in terms of hydrologic changes tied toclimatic variation. It has been already shown that in some areas, as the Tibetan plateau and the Himalayas, thetectonic signal is not negligible. Here we estimate the effect of the tectonic signal for the uplift of smallermountain ranges, as is the Alpine arc. Our results indicate that tectonic and hydrological signals superpose andwe cannot ignore the tectonic signal when using GRACE to invert for the equivalent water height (EWH)