The Role of Long-Period Ground Motions on Magnitude and Damage of Volcanic Earthquakes on Mt. Etna, Italy

Abstract

On October 2002, a seismic swarm occurred on Mt. Etna. One of the strongest events caused severe damage, up to a European Macroseismic Scale intensity of VIII that contrasts with its local magnitude of 4.4. The occurrence of significant damage at such a small magnitude is repeatedly observed in the area and is traditionally attributed to shallow source. Recorded strong-motion accelerograms and broadband seismograms demonstrate that there is one more cause for the severe damage, that is, an anomalously strong low-frequency (0:1 < f < 1 Hz) radiation deviating from the conventional Brune (1970) spectral scaling. Therefore, these earthquakes cause large ground displacements and long (≈20 sec) durations of shaking. The integration of digital accelerograms yields a maximum peak ground displacement as large as 1.8 cm at a distance of 18 km. Based on the sharp local attenuation of ground motion in the study area, we infer that peak ground displacements near the epicenters did exceed 10 cm. The occurrence of large displacements caused selective damage to medium-rise (≥ 3 stories) reinforced concrete buildings and elements like church façades.The frequency cutoff below 1.25 Hz in the Wood-Anderson response attenuates the peak-to-peak amplitudes used to assess local magnitudes. Therefore, M L values are not representative of the real strength of volcanic earthquakes. Because a prompt magnitude (and damage potential) assessment is crucial for 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 pseudovelocity 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 estimate of the epicentral macroseismic intensity.

When probabilistic seismic hazard climbs volcanoes: the Mt. Etna case, Italy – Part 1: Model components for sources parameterization

“…The main reasons for this behavior are (i) the extremely shallow focal depths of Etna earthquakes (0-4 km, see Sect. 4.1.1) compared with those of regional events (typically in the range 10-15 km) and (ii) an anomalously strong low-frequency (0.1 <f <1 Hz) radiation deviating from the conventional Brune (1970) spectral scaling, which causes large ground displacements and long (≈ 20 s) durations of shaking (Milana et al, 2008). The final dataset therefore covers the time span 1805-2015 and consists of nine earthquakes, the causative faults of which are clearly recognized through extensive evidence of coseismic surface faulting (Azzaro, 1999).…”

Ground-motion scenarios on Mt. Etna inferred from empirical relations and synthetic simulations

“…PGA and PGV were measured at ca. 15 gal, and 4.5 cm/s, respectively (Milana et al 2008). The extremely low ratio PGA/PGV is a strong clue to prevailing low frequencies in the records.…”

“…3 Bull Earthquake Eng is available (see, e.g. Milana et al 2008), which is used to verify the choice of basic input parameters. At the same time, the accelerometric station located near the city of CAT belonging to the national accelerometer network of Italy (Rete Accelerometrica Nazionale, RAN) recorded this event.…”

“…a local magnitude M L = 4.4 and a stress parameter s = 5 bar, we obtain a general underestimation of ground motion with respect to the values reported at the site CAT (see Table 2). However, a considerable uncertainty in moment magnitude estimation has been reported in Milana et al (2008), who referred to values obtained from MEDNET stations and revised the local magnitude on the basis of Wood-Anderson simulations of the accelerograms. For M L a value of 4.8 is reported whereas the (MedNet) moment magnitude M W is given with 4.7.…”

“…In terms of impact to the densely urbanized sectors on the eastern flank, the most relevant event was the October 29 earthquake (10:02 UT). With a magnitude of M L 4.4 (Milana et al 2008), the earthquake produced heavy damage even to reinforced concrete structures in the epicentral area (Azzaro et al 2006b), along a narrow but elongated zone (over 4 km in length) where the macroseismic intensity reached degree VIII on the European Macroseismic Scale (hereafter EMS, see Grünthal 1998). In general, the entire eastern flank has been affected in the past by similar or even more severe seismic events.…”