Coco, G.

The study of the spectral features of volcanic tremor and low frequency events (l.f.e.) recorded before and during the preliminary phases of the powerful 1991-1993 eruption of Mt. Etna is briefly described. Significant modifications were observed in the spectral signature of l.f.e. before the onset of the eruptive event, as well as in the temporal distribution of the volcanic tremor dominant frequencies. We interpret both l.f.e. and tremor changes in terms of a spatial modification of the source, as the paroxysmal eruptive activity is approaching. Such findings also appear quite interesting for the identification of markers of the modifications which some seismic events of the volcano undergo in the early stages heading the occurrence of an eruption.

More than 200 time histories of ambient noise have been recorded at 158 sites in the urban area of Catania. Among them, 144 sites are aligned along 15 profiles crossing the most representative lithologies outcropping in the study area. The standard Nakamura (1989) technique has been applied to compute the H/V spectral ratio along these profiles, where the upper-layer structure has been reconstructed in detail using surface geology surveys as well as data from available wells. The geological peculiarity of Catania is the presence of an extended, high- velocity lava cover of varying thickness that fills a large part of the urban area; lower-velocity sedimentary layers outcrop only in small windows in the northern part of the town, however, they predominate in the southern part. In such a complex geological setting, the application of the Nakamura technique provides results that do not correspond strictly to the expectation for usual hard and soft-site spectral shapes. Measurement results have indicated that, in general, the H/V amplitudes do not attain large values in the study area: only at 15 sites are the spectral peaks greater than 3 units, and this occurs predominantly on lava outcrops, where the maximum amplification occurs between 7 and 10 Hz. This frequency band is consistent with weathering processes of the lava flows. At soft sites the observation of significant amplitude (>3) spectral peaks is limited to a few cases. The recordings of six broad-band stations laying on or near the selected profiles have been used for a preliminary comparison between microtremor results and amplifications observed during individual earthquakes. The H/V spectral ratios are generally similar for microtremor and earthquake data, microtremor tending to underestimate the amplitude of horizontal ground motions of earthquakes. But amplifications at sedimentary outcrops (with reference to a massive lava site) can be significant during individual earthquakes, and in some cases include frequency bands where no tendency to amplify was inferred from the microtremor H/V spectral ratios. Even though this comparison needs more data before reaching a stable conclusion, a preliminary analysis of earthquake data confirms that caution is required in using ambient noise for engineering purposes in complex and laterally sharply varying nearsurface geological structures such as those presented by the urban area of Catania.

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.

In this study, we investigate the mechanical properties of the substrate underlying the historical Franciscan Friars Minor convent of Ispica (southern Sicily). The convent, where new fractures recently occurred, is located on top of a calcarenite cliff formed along one of the planes of the regional Pozzallo-Ispica-Rosolini normal fault system. Taking advantage of three existing mechanical drillings 30 m deep, we have carried out down-hole tests (DHT) and a seismic tomography survey in up-hole configuration. The down-hole tests provided vertical profiles of P- and S-wave velocity measured at 1-m depth intervals, from which we have derived the basic elasto-dynamic and seismic parameters, while the tomography survey imaged vertical sections of P-wave velocity across the cliff. The results highlight variable mechanical properties of the subsoil and a step-like pattern of velocity variations parallel to the cliff face that could suggest the occurrence of secondary ruptures related to the main Ispica fault.

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, ML 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.

We describe the results of a sonic tomography survey and penetration tests carried out inside the S. Agata church of Ragusa Ibla, in southeastern Sicily (Italy). The purpose of this work was to evaluate the extent of decay in some of the ancient wooden trusses of the nave's roof, in view of possible strengthening interventions. Sonic tomography is entirely non-invasive and is suitable to investigate large portions of a structure, although in a qualitative way, while penetration tests are little invasive, point measurements that enable high-resolution detection of wood decay and cracks. We combined the two techniques to investigate the internal condition of 4 trusses that looked most deteriorated at a preliminary visual inspection. Results showed that decays occur mainly next to the walls, due to rainwater infiltration, and on the side of the timbers facing the rear of the nave; in general, chords have worse mechanical properties than rafters.