Earthquake rupture velocity and stress drop interaction in the Campi Flegrei volcanic caldera

During an earthquake rupture, both dynamic and static stress drop play a key role in controlling how much energy is radiated as seismic waves, how large is the fault slip, and how quickly the rupture spreads out of the nucleation zone. Using a time-domain analysis of P- and S-wave log-displacement records, we estimate seismic moment, rupture velocity, static stress drop, and source radius of 56 Md 3+ earthquakes detected during the 2020-2025 seismic crisis at Campi Flegrei caldera, Italy. Fractures propagated at sub-shear velocities (0.4-0.9 of the shear wave velocity) along 100-1000 m in radius fault surfaces. Independent stress release estimates show a statistically significant inverse relation with the rupture velocity. The measured low seismic radiation efficiency, with a median value of 0.1, suggests that only a small portion of the stress drop is radiated as seismic waves, implying that a significant amount of energy is likely dissipated through frictional and inelastic processes, including off-fault damage. The findings suggest that in this volcanic caldera, earthquakes with higher stress drop may enhance fault-surrounding damage, which acts as a natural barrier to rupture propagation. Consequently, this mechanism could limit rupture extent and constrain the maximum magnitude of earthquakes in the area. Earthquakes at Italy’s Campi Flegrei caldera show unusually low seismic radiation efficiency, with most energy dissipated through off-fault damage and frictional processes, according to a time-domain-based analysis of seismic catalogue data.