New relationships between macroseismic intensity and local magnitude for the volcanic region of Mt. Etna (Italy)

The volcanic region of Mt. Etna (Italy) has a well-documented historical seismic activity, with records of seismic and volcanic events on the volcano dating back to late 1633. This historical data, covering a time span longer than that recorded by instrumental seismological data, is a testament to the reliability of the intensity-magnitude relations, the only means to obtain macroseismic information, the sole indicator of the energy released by earthquakes. Previous studies in the literature have proposed various methods for converting epicentral intensity into macroseismic magnitude for the Etna region. Still, these methods were based on older datasets with limited instrumental data. The updated relationship proposed in the paper significantly improves the accuracy of macroseismic magnitude estimates, aligning them more closely with local magnitudes calculated for recent earthquakes. The study uses a dataset of 58 volcano-tectonic events from 1997 to 2018, with magnitudes between 2.5 and 4.8 and intensities ranging from IV to VIII on the EMS scale. The instrumental magnitudes were obtained from the Mt. Etna seismic catalogue and the Italian seismological database, while macroseismic data were sourced from the macroseismic catalogue of Etnean earthquakes. In the volcanic area of ​​Etna, macroseismic epicenters are often located very close to the sites where the maximum intensity is observed, this is due to the strong attenuation of seismic energy and the shallowness of the epicenters. For this reason, the epicentral intensity is generally assumed to be equal to the maximum intensity. The new relationship is tailored explicitly for shallow earthquakes (H ≤ 3 km), which are the most recurrent. It includes a correction factor for depth, making it applicable to deeper events and enhancing its relevance in real-world scenarios.