We study vertical ground displacement time-series from GNSS stations in the Po river basin to measure deformation signals associated with the severe drought occurring since 2021 and estimate the spatial distribution of water loss. The detection of vertical ground displacement trend changes allows us to extract a spatially correlated signal that follows the Po level trend changes and SPEI-12 drought index. GNSS stations in the basin mostly underwent uplift, up to 7 mm, since 2021, which corresponds to ~80 Gtons of water loss. Compared to GLDAS and GRACE, the GNSS results show a similar temporal evolution of water content, but a different spatial distribution. Our study indicates that using dense GNSS networks is an effective way to monitor long-term changes in water storage even in small water basins and serve as a reliable indicator of drought severity.

Crustal perturbations related to seismic activity can generally be observed with the occurrence of a large magnitude event. For less energetic seismic sequences though, the associated transient crustal variations are questionably measurable, and their observation gets easily obscured by relatively stronger perturbations such as the ones related to hydrological processes. In this study we reveal the significant role that terrestrial water-storage variations play in governing temporal crustal changes in the tectonically active Northern Apennines of Italy, and discuss the potential of accounting for its correction in order to monitor the relatively weaker transient perturbations caused by local seismic swarms. This area is characterized by an extensive level of low-energetic seismic activity, typically clustered in time and space, of which three main seismic swarms outstand during the 12-year period of study (2010-2021). Our analysis compares independent observations and processing methods of GNSS measurements and ambient seismic noise recordings. We adopt a multivariate statistical approach to discriminate between independent sources of ground deformation, and seismic noise cross-correlation analysis to monitor relative seismic-velocity variations. The result shows how the perturbation effects produced by variations in total water content are dominant in both time series of ground deformations and seismic-velocity variations. After correcting for the water-related variation effects, our monitoring results reveal perturbations in the crustal properties whose activation time and depth range correlate with the occurrences of the seismic swarms.

The 2016-2017 Central Italy earthquake sequence struck the central Apennines between August 2016 and October 2016 with Mw ∈ [5.9; 6.5], plus four earthquakes occurring in January 2017 with Mw ∈ [5.0; 5.5]. Here we study Global Positioning System (GPS) stations active during the post-seismic phase including near and far-field domains. We separate the post-seismic deformation from other, mainly seasonal, hydrological deformation signals present in ground displacement time-series via a variational Bayesian Independent Component Analysis technique. For each component, realistic uncertainties are provided to the related ICA-reconstructed displacement field. We study the distribution of afterslip on the main structures surrounding the mainshock, and we highlight the role played by structures that were not activated during the co-seismic phase in accommodating the post-seismic deformation. In particular, we report aseismic deformation occurring on the Paganica fault, which hosted the Mw 6.1 2009 L’Aquila earthquake, and is located further south of the 2016-2017 epicenters; and on a 〜2-3 km thick subhorizontal shear-zone, clearly illuminated by seismicity, which bounds at depth the west-dipping normal faults where the mainshocks nucleated. Since afterslip alone underestimates the displacement in the far-field domain, we consider the possibility that the shear zone marks the brittle-ductile transition, assuming the viscoelastic relaxation of the lower crust as a mechanism contributing to the post-seismic displacement. Our results suggest that multiple deformation processes are active in the first two years after the mainshocks.

Fluctuations in groundwater content may produce surface deformation and affect the elastic properties of the Earth’s crust. In this study we evaluate the temporal variations of the Earth’s crust elastic properties (in the form of relative seismic-velocity variations) in a tectonically active region in Northern Italy characterized by the presence of karst systems. In this area, GPS measurements already revealed hydrologically-induced deformation, modulated by changes in groundwater storage. We study the relation of our seismological observations with the geodetic and hydrological results and identify the effects of groundwater-content variations in the seismic-velocity perturbations. Our results show that hydrologically-induced changes in karstic media produce significant seismic-velocity perturbations, therefore its role in tectonic-stress adjustment studies must not be ignored. Depth sensitivity analysis of our results constrain the crustal perturbations to range between 1 and 4 km depth. Results from scattering imaging locate the crustal perturbations along the main karst systems.