On March 9 th $9\mathrm{th}$ , 2023, three small earthquakes ( 3.8 ≤ M w ≤ 4.5 ) $(3.8\le {M}_{w}\le 4.5)$ occurred near Umbertide (Central Italy), within 4 hours. Analyzing seismic ambient noise in the frequency band 0.1–1 Hz from six seismometers within 20 km of the M _w  4.5, we detect a significant drop in seismic velocity at the time of occurrence of the seismic sequence. A strain sensitivity analysis suggests that crustal damage was primarily caused by the passage of seismic waves, likely enhanced by the presence of fluids. Analyzing geodetic data from recently installed strainmeters and pore pressure transducers, we highlight the role of elastic strain and fluids in driving deformation during this sequence, suggesting the presence of a pressure source that strongly influences the deformation pattern. We find that fluid‐induced stress may cause the reloading of a few kPa of a co‐seismic asperity within hours, unveiling the contribution of pore pressure sources to the stress budget of seismogenic faults.

On March 9, 2023, a sequence of small earthquakes M w ≤ 4.5 $\left({M}_{w}\le 4.5\right)$ occurred in the northern Apennines (Italy). Although the events were relatively small, we observed a clear and sudden reduction in the speed of seismic waves traveling through the Earth's crust. This drop is a sign of physical changes in the rocks, likely caused by the shaking of the earthquakes and the presence of underground fluids. We combined data from seismic sensors, strainmeters, and instruments that measure underground water pressure. Together, these observations showed that underground fluids may have amplified the effects of the earthquakes and contributed to how the ground moved afterward. The data also suggest that fluids may have moved more easily through the crust after the shaking, changing the pressure conditions in ways that might help trigger further seismic activity. Our findings show that even small earthquakes can cause important changes underground when fluids are involved. Monitoring these changes using a mix of seismic and geodetic tools helps scientists better understand how earthquakes work and how fluid‐filled faults behave.

We exploit a dense near‐fault observatory network of seismo‐geodetic instruments to resolve faint signals of a low energy M _w ≤ 4.5 swarm