Earthquake Rupture Velocity and Stress-Drop Interaction in the Campi Flegrei Volcanic Caldera

During earthquake ruptures, dynamic and static stress releases 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. Here we exploit a time-domain technique using P- and S-waves log-displacement records to estimate the seismic moment, rupture velocity, and radius of the ruptures of 32 Md 3+ earthquakes detected during the 2020-2024 seismic crisis at Campi Flegrei caldera, Italy. Seismic ruptures propagated at sub-shear velocities (0.4-0.9 of shear wave speed) along 100-1000m in radius fault surfaces. Stress-drop estimates show a statistically significant inverse relation with the rupture velocity. The low seismic radiation efficiency confirms that the radiated energy is only a small fraction of the total available strain energy at the source which is mostly dissipated by friction and fracture-related processes. We argue that in volcanic calderas, the energy dissipation related to off-fault rock damage processes is the main cause for low radiation efficiency and inverse relation between the rupture velocity and stress drop. In this geologic environment, ruptures with higher stress drop favor extensive off-fault damage, which can prevent rupture propagation for long distances, limiting the maximum attainable earthquake magnitude.