The Umbertide 2023 Seismic Sequence: relative velocity variations, ground deformation and role of fluids

On March 9th 2023, a low-energy seismic sequence struck the town of Umbertide in the Umbria-Marche region (Central Italy), with three events with 3.8 ≤ Mw ≤ 4.5 occurring in a time span of about 4 hours. We analyze data collected by the 6 seismic stations within a radius of 20 km from the largest Mw 4.5 earthquake, computing cross-correlations of seismic ambient noise in the oceanic microseism frequency band (0.1 − 1 Hz), and we find a significant seismic velocity drop associated with the time of the main coseismic events. We carry out a dynamic strain sensitivity analysis which suggests that the crustal damaging is mainly related to the shaking induced by the passage of seismic waves. In order to interpret the deformation processes associated with these earthquakes, we exploit the high temporal resolution (1 Hz) of the geodetic observations recorded by the recently installed STrainmeter ARray (STAR), together with the pore pressure measurements. We discuss the role of elastic strain and fluids in driving the deformation time evolution the study area went through during this seismic sequence. Overall, this study sets an example of how a combined seismo- geodetic analysis may help us to resolve low-intensity tectonic signals enhancing our capability of properly investigating the seismogenic mechanisms.