Crustal Deformation and Seismic Velocity Perturbations in the Alto Tiberina Fault Zone (Northern Apennines, Italy)

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 Global Navigation Satellite System 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.