The July,18,2025 earthquake at Campi Flegrei (Md 4.0, Mw 3.5): Location, mechanism and rupture kinematics

We investigated the July 18, 2025 Campi Flegrei earthquake (Md
4.0, Mw 3.5), focusing on how rupture directivity influenced the
distribution of ground shaking. By analyzing peak ground
acceleration (PGA) records from 21 local stations, we modeled
the event using a rupture directivity approach based on the
Boatwright model.
The earthquake reactivated the La Pietra fault, a known NEtrending
structure at the eastern border of the resurgent Campi
Flegrei caldera. A dip-slip fault mechanism was confirmed
through seismic relocation and waveform analysis. The rupture
was found to propagate unilaterally down-dip at ~1.3 km/s
(about 80% of the shear wave velocity), with a fault size of ~1 km
× 0.8 km2 and a stress drop of 0.34 MPa.
The spatial variation in directivity-corrected PGA coefficients
(Cd_PGA) revealed enhanced shaking in the northwestern
quadrant relative to the epicenter. This matches the modeled
rupture scenario, emphasizing that even moderate-magnitude
earthquakes can show strong rupture directivity effects.