Different Couplings Between Precipitation and Deformation at the Same Site: A Case Study at Central Pyrenees (Spain)

Impact of hydrology-associated deformation spans from the study of the rock hydrological properties and aquifer dynamics to the identification of geological disaster precursors. We analyze a comprehensive set of hydrology-associated deformation signals, recorded by two underground 70 m long strainmeters (GAL16 and LAB780) operating under the Central Pyrenees (Spain) from the end of 2011 to the end of 2018. The two strainmeters measure extensional strain at the picostrain (10−12) level along two nearly orthogonal horizontal directions. Sign (compression or extension along any of the two directions), size, and time evolution of hydrology-associated deformation vary considerably from season to season. We identify four classes of signals, related to summer rainfalls (producing compression along both GAL16 and LAB780), late-spring and autumn rainfalls (producing extension along GAL16 and compression along LAB780), snow melting (producing opposite diurnal cycles along GAL16 and LAB780), and winter snowfalls (producing extension along both GAL16 and LAB780). Similar precipitations of the same class can give rise to differently sized signals; differences may be due to infiltration and/or surface load inhomogeneities as well as to complexities of the underground hydrogeological set up, which can lead to nonlinear, intrinsically time-dependent, responses. Our modeling sheds light on the diverse effects associated with different environmental conditions. The variety of hydrology-associated deformation we report is warning for the interpretation of all kinds of deformation data, as for their difficult modeling, the causative physical mechanisms, and the extreme care that is needed when removing hydrologically induced effects. Our insights are particularly useful in mountain areas.