Estimating the magnitude of historic earthquakes from macroseismic intensity data: new relationships for the volcanic region of Mount Etna (Italy)

Abstract

In the present work, we propose new intensity-magnitude relationships specifically for the volcano seismicity in the Mt. Etna region, which provide magnitude estimations of historic earthquakes by the macroseismic intensity. Starting from the local and national earthquake catalogues, we selected a dataset of 150 events occurring from 1971 to 2010, for which both epicentral intensities and instrumental magnitudes were available. Using empiric models that fit, by least-square method, the average values of magnitude for each class of intensity (I0), we derive different I-M regressions calibrated on duration (MD), local (ML) and moment magnitudes (MW). Taking into account the values of confidence limits, the results show that the magnitudes calculated for intensity VIII EMS, the highest value of our dataset, range from 4.0 to 4.6 according to the adopted magnitude scale. These differences become even more significant in applications aimed at assessing the seismic potential of local seismicity and related hazard. In fact, considering an intensity IX-X EMS, which is the maximum value historically observed at Mt. Etna for the typical shallow earthquakes, the calculated MD-ML-MW magnitudes reach values up to 4.4, 5.1 and 5.3, respectively. This finding is coherent with the well-known problem of saturation of MD magnitude scale, that appears to be “contracted” with respect to the other ones, and provides more reliable estimations compared with the previous I-M relationship adopted so far.