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 (Md4.0, Mw 3.5), focusing on how rupture directivity influenced thedistribution of ground shaking. By analyzing peak groundacceleration (PGA) records from 21 local stations, we modeledthe event using a rupture directivity approach based on theBoatwright model.The earthquake reactivated the La Pietra fault, a known NEtrendingstructure at the eastern border of the resurgent CampiFlegrei caldera. A dip-slip fault mechanism was confirmedthrough seismic relocation and waveform analysis. The rupturewas 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 northwesternquadrant relative to the epicenter. This matches the modeledrupture scenario, emphasizing that even moderate-magnitudeearthquakes can show strong rupture directivity effects.