Low-magnitude seismic swarms in the Calabrian Arc (Italy)

Seismic swarms of low magnitude earthquakes occur frequently in the Calabrian Arc. During the last few years, several earthquakes of magnitude up to ML4.4 occurred both on land and offshore near the coast of Calabria. Some of them were followed by a sequence of tens to hundreds of smaller, well-clustered earthquakes that occurred during the following weeks or months. In other cases, swarms of low-magnitude earthquakes occur without a classical mainshock-aftershock evolution. In this work, we selected swarms that were well recorded by a high number of seismic stations to perform a detailed analysis consisting of the determination of the relative location and focal mechanism for as many earthquakes as possible. In some cases, the relative location allows to recognize the seismogenic fault and to distinguish the fault plane from the auxiliary plane of the focal mechanism solution. In other cases, the relative location unravels a small cloud of events that is not compatible with a unique fault plane, suggesting the occurrence of the swarm in highly fractured seismogenic volume. The relative hypocenter positions allow to estimate the size of the seismogenic volume, which is very small in most of the cases, often less than 1 km3. However, its extension is greater than the size computed for the mainshock rupture in many cases. The most common source mechanism is of normal type, but strike-slip and reverse kinematics are also found, in particular for swarms located offshore and near the coast. The temporal distribution of events does not show any evident migration of the sources, thus suggesting that the driving mechanism is not related with aseismic phenomena like fluid diffusion and stress waves.