Multi-annual hydrologically-related horizontal ground deformation transient measured by GPS: insights from the Apennines

Abstract:

Temporal variations of hydrological conditions exert direct influence over the deformation of the shallow crust. In the last years, geodetic measurements have shown that annual variations in total groundwater content induce time-varying deformation of the crust that are mostly reflected in the vertical component and that are frequently reproduced by time-varying loads over an elastic half-space. Here we present new GPS observations showing transient ground deformation in the Apennines (Italy), mostly evident in the horizontal components, that correlate with multi-annual hydrological signals as measured by rainfall, GRACE (Gravity Recovery and Climate Experiment) and discharge of karstic springs. The observed transient signal is superimposed on the long-term ~3mm/yr NE-SW directed active extension, correlates with the seasonal recharge/discharge cycle of groundwater flow and its multi-annual variations and appears to modulate the intensity of tectonic extensional opening in those regions characterized by large carbonatic aquifers. In particular, periods of increasing discharge from karstic spring correlate with an enhanced extensional signal, whereas exhaustion periods (decreasing discharge) correlate with reduced extensional signal. Neither surface loading on an elastic medium nor a volumetric inflation source could explain both vertical and horizontal observed deformation. We suggest that the observed deformation is related to the presence of highly fractured shallow crust (upper 1-1.5 km) that deforms according to variable conditions of the hydraulic head within the large carbonate aquifers. Following these observations we model the observed horizontal and vertical time-dependent signal using simple tensile dislocations at shallow depth (< 1.5 km), whose opening is scaled to the discharge of the largest karstic spring, taken as a proxy for the regional elevation of the groundwater table within the carbonate aquifers. These new findings contribute to the understanding of the role of the karst systems in conveying large amount of water in the shallow crust and to an improved management of these important natural resources.