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.