Abstract Fluids in the crust influence earthquake nucleation by affecting fault strength and rupture dynamics, but direct observations at seismogenic depths are rare. We study the Irpinia Fault System in Southern Italy, site of the 1980 M 6.9 earthquake, to understand how fluid overpressure, fault orientation, and regional stress control earthquake generation (up to Mw 7). Using 16 years of seismic data from national and local networks, we estimate microearthquake (2.5 < M ≤ 4.2) focal mechanisms with advanced methods. Applying focal mechanism tomography, we quantify fluid overpressure at depths beyond direct measurement and assess its impact on fault activation. Our results reveal pervasive overpressured fluids along the fault, actively involved in microseismic ruptures. This challenges the view that mainly large Apennine earthquakes are fluid‐driven and advances understanding of normal fault mechanics. Our approach, providing the selection of the microearthquake fault preferential plane, highlights fluids’ role in earthquake nucleation and low‐energy swarm‐like seismic sequences related to variable permeability within fault zones.