The magnitude of damaging volcanic earthquakes of Mt. Etna: are the commonly used scales adequate?

Abstract

On October 2002 a seismic swarm occurred on the eastern flank of Mt. Etna. One of the strongest events caused severe damage, up to EMS intensity of VIII that contrasts with its local magnitude of 4.4. The occurrence of significant damage at such small magnitude is repeatedly observed in the Mt. Etna area and is traditionally attributed to the shallow source of volcanic earthquakes. Strong-motion accelerograms and broad-band seismograms recorded during the swarm demonstrate that there is a more cogent cause for the severe damage, i.e. an anomalously strong low-frequency (0.1 < f < 1 Hz) radiation deviating from the conventional Brune (1970) spectral scaling. Therefore, these earthquakes cause unexpectedly large ground displacements and long ( 20 sec) durations of shaking. The integration of digital accelerograms recorded on October 2002 yields a maximum peak ground displacement as large as 1.8 cm at a distances of 18 km, out of the largest damage zone. Based on the sharp local attenuation of ground motion amplitudes observed during the Mt. Etna earthquakes, we infer that displacements near the epicentres can have attained 10 cm. So large displacements are consistent with the maximum observed damage. Moreover, the frequency cutoff below 1.25 Hz in the Wood-Anderson response attenuates the peak-to-peak amplitudes used to assess local magnitudes. This instrumental deamplification at low frequency yields underestimated values of local magnitude that are not representative of the real ground shaking. Since a prompt, correct magnitude (and potential damage) assessment is crucial for efficient Civil Protection actions, a procedure is proposed which, in near-real-time, can be successful in identifying potentially damaging earthquakes of Mt. Etna through the computation of response spectra. The procedure provides a magnitude value that is derived on a statistical basis from the Housner (1952) spectral intensity computed in the low-frequency band. This parameter is a suitable near-real-time indicator of large earthquake-induced building shaking and could also be applied for a preliminary determination of the epicentral macroseismic intensity of volcanic events of Mt. Etna through consolidated relationships established for tectonic earthquakes in Italy.