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Lombardo, Giuseppe
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Lombardo, Giuseppe
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- PublicationOpen AccessWhen probabilistic seismic hazard climbs volcanoes: the Mt. Etna case, Italy – Part 2: Computational implementation and first results(2017)
; ; ; ; ; ; ; ; ; ; ; ; ;; ;; ; ; ; ; ; ; ;This paper describes the model implementation and presents results of a probabilistic seismic hazard assess- ment (PSHA) for the Mt. Etna volcanic region in Sicily, Italy, considering local volcano-tectonic earthquakes. Working in a volcanic region presents new challenges not typically faced in standard PSHA, which are broadly due to the nature of the local volcano-tectonic earthquakes, the cone shape of the volcano and the attenuation properties of seismic waves in the volcanic region. These have been accounted for through the development of a seismic source model that integrates data from different disciplines (historical and instrumental earthquake datasets, tectonic data, etc.; presented in Part 1, by Azzaro et al., 2017) and through the development and software implementation of original tools for the computa- tion, such as a new ground-motion prediction equation and magnitude–scaling relationship specifically derived for this volcanic area, and the capability to account for the surfi- cial topography in the hazard calculation, which influences source-to-site distances. Hazard calculations have been car- ried out after updating the most recent releases of two widely used PSHA software packages (CRISIS, as in Ordaz et al., 2013; the OpenQuake engine, as in Pagani et al., 2014). Re- sults are computed for short- to mid-term exposure times (10% probability of exceedance in 5 and 30 years, Poisson and time dependent) and spectral amplitudes of engineer- ing interest. A preliminary exploration of the impact of site- specific response is also presented for the densely inhabited Etna’s eastern flank, and the change in expected ground mo- tion is finally commented on. These results do not account for M > 6 regional seismogenic sources which control the hazard at long return periods. However, by focusing on the impact of M < 6 local volcano-tectonic earthquakes, which dominate the hazard at the short- to mid-term exposure times considered in this study, we present a different viewpoint that, in our opinion, is relevant for retrofitting the existing buildings and for driving impending interventions of risk re- duction.601 106 - PublicationOpen AccessCharacteristics of volcanic tremor accompanying the September 24th, 1986 explosive eruption of Mt. Etna (Italy)(1996-03)
; ; ; ;Gresta, S.; Istituto di Geologia e Geofisica, Università di Catania, Italy ;Lombardo, G.; Istituto di Geologia e Geofisica, Università di Catania, Italy ;Cristofolini, R.; Istituto di Scienze della Terra, Università di Catania, Italy; ; Features of the volcanic tremor recorded before, during and after the eruptive event which occurred at Mt. Etna on September 24th 1986, are described. The whole eruption was particularly short in time (about eight hours) and characterized by an extremely violent explosive activity with lava fountains a few hundred meters high. As the complete record of the seismic signals generated during the whole eruptive episode was available, a detailed spectral analysis of the volcanic tremor recorded at four stations, located at increasing distance from the summit of the volcano, was carried out. Fourier analysis, that was performed using temporal windows of about 11 min in duration, pointed to some large fluctuations of the overall spectral amplitude, as well as some frequency variations of the dominant spectral peaks. The ratio of the overall spectral amplitude recorded at the highest station and at the peripheral ones, was calculated in the two spectral bands 1.0-2.5 and 2.6-6.0 Hz, respectively. The significant contribution of energy at low frequency values supports the hypothesis of a subvertical planar source, which was active during the paroxysmal stage of the eruption. Such results are also supported by the analysis of the attenuation function of the spectral amplitude.168 168 - PublicationRestrictedExploratory seismic site response surveys in a complex geologic area: a case study from Mt. Etna volcano (southern Italy)(2017)
; ; ; ; ; ; ; ;Panzera, F. ;Lombardo, G. ;Langer, H.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Branca, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Azzaro, R.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Cicala, V. ;Trimarchi, F. ; ;; ; ; ;A preliminary study targeting to evaluate the local seismic response was performed in the eastern flank of Mt. Etna (southern Italy) using ambient noise measurements. The obtained spectral ratios were subdivided through cluster analysis into different classes of fundamental frequency permitting to draw an iso-frequency contour map. The analysis set into evidence the extreme heterogeneity of lava sequences, which makes difficult to identify a single seismic bedrock formation. Another important outcome, concerning the local seismic effects in terms of frequency and azimuth, is the important role played by the fracture fields associated with the main structural systems of the area. The existence of two zones with strong directional effects striking WNW–ESE and NW–SE, nearly orthogonal to the orientation of the main fracture fields, corroborate such hypothesis.877 71 - PublicationRestrictedWavefield Polarization in Fault Zones of the Western Flank of Mt. Etna: Observations and Fracture Orientation ModellingAmbient noise measurements performed on the western flank of Mt. Etna are analyzed to infer the occurrence of directional amplification effects in fault zones. The data were recorded along short (\500 m) profiles crossing the Ragalna Fault System. Ambient noise records were processed to compute the horizontal-to-vertical noise spectral ratio as a function of frequency and direction of motion. Wavefield polarization was investigated in the time–frequency domain as well. Peaks of the spectral ratios generally fall in the frequency band 1.0–6.0 Hz pointing out directional amplifications that are also confirmed by the results of the time–frequency analysis, the largest amplification occurring with high angle to the fault strike. A variation of the frequency of the spectral peak is observed between the two sides of the fault, possibly related to a damage fault asymmetry. Measurements performed several kilometers away from the fault zone do not show behavior that is as systematic as in the fault zone, and this suggests that the observed directional effects can be ascribed to the fault fabric. We relate the polarization effect to compliance anisotropy in the fault zone, where the presence of predominantly oriented fractures makes the normal component of ground motion larger than the transversal one. In order to test the direction and the type of fractures that are expected in the fault zone, we modeled the brittle deformation pattern of the investigated fault. Theoretical results are in good agreement with field observations of the fracture strike.
249 3 - PublicationRestrictedThe role of alternating outcrops of sediments and basaltic lavas on seismic urban scenario: the study case of Catania, Italy(2011)
; ; ; ; ; ; ;Panzera, F.; Dipartimento di Scienze Geologiche, Università di Catania, ;Rigano, R.; Dipartimento di Scienze Geologiche, Università di Catania, ;Lombardo, G.; Dipartimento di Scienze Geologiche, Università di Catania, ;Cara, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Di Giulio, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Rovelli, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; ; ; Experimental data and numerical modelling were used to study the effect of local geology on the seismic response of the Catania area. The town extends on a marly clays bedrock and terraced deposits made up by coastal sands and alluvial conglomerates. This sedimentary substratum is deeply entrenched by paleo-valleys filled by lava flows and pyroclastics. Available borehole data and elastic parameters were used to reconstruct a geotechnical model in order to perfome 1D numerical modeling. Seismic urban scenarios were simulated considering destructive (M w = 7.0), strong (M w = 6.2) and moderate (M w = 5.7) earthquakes to assess the shaking level of the different outcropping formations. For each scenario seven real accelerograms were selected from the European Strong Motion Database to assess the expected seismic input at the bedrock. PGA and spectral acceleration at different periods were obtained in the urban area through the equivalent linear numerical code EERA, and contour maps of different levels of shaking were drawn. Standard and horizontal-to-vertical spectral ratios were achieved making use of a dataset of 172 seismic events recorded at ten sites located on the main outcropping lithotypes. Spectral ratios inferred from earthquake data were compared with theoretical transfer functions. Both experimental and numerical results confirm the role of the geological and morphologic setting of Catania. Amplification of seismic motion mainly occurs in three different stratigraphic conditions: (a) sedimentary deposits mainly diffused in the south of the study area; (b) spots of soft sediments surrounded by lava flows; (c) intensely fractured and scoriaceous basaltic lavas.112 18 - PublicationOpen AccessPossibili correlazioni tra attività sismica profonda e fenomeni vulcanici all'Etna(1995-01-11)
; ; ; ; ;Castellano, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Bianco, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Imposa, S.; Università di Catania ;Lombardo, G.; Università di Catania; ; ; ; ; ; ;Ferrucci, F.Innocenti, F.In the text154 254 - PublicationRestrictedResults of microtremor measurements in the urban areas of Catania, Italy(2001-09)
; ; ; ; ; ; ; ;Lombardo, G.; Dipartimento di Scienze Geologiche, University of Catania, Italy ;Coco, G.; Dipartimento di Scienze Geologiche, University of Catania, Italy ;Corrao, M.; Dipartimento di Scienze Geologiche, University of Catania, Italy ;Imposa, S.; Dipartimento di Scienze Geologiche, University of Catania, Italy ;Azzara, R. M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Cara, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Rovelli, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ;; ; ; ; 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.199 23 - PublicationOpen AccessEvidence for ground motion polarization on fault zones of mt. etna volcano(2008-07-22)
; ; ; ; ;Rigano, S.; Dipartimento di Scienze Geologiche, Università di Catania, Catania, Italia ;Cara, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Lombardo, G.; Dipartimento di Scienze Geologiche, Università di Catania, Catania, Italia ;Rovelli, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; During local and regional earthquakes, an evident amplification of horizontal ground motion is observed at two seismological stations near the Tremestieri fault, on the southeastern flank of Mt. Etna volcano. Rotated-component spectral ratios show a narrow spectral peak around 4-Hz along a N40°E direction. A conventional polarization analysis using the eigenvectors of the covariance matrix confirms the very stable directional effect enhancing the approximately NE-SW elongation of the horizontal ground motion in the fault zone. The effect is evident during the entire seismogram and independent of source backazimuth as well as distance and depth of earthquakes. The same polarization is observed in ambient noise as well. This consistency allowed us to use microtremors for checking ground motion polarization along and across the Tremestieri fault zone with a high spatial resolution. The result is a stable polarization of horizontal motion in the entire area, interesting a broad frequency band. To check whether this ground motion property is recurrent and understand a possible relationship with fault strike, faulting style, or orientation of fractures, ambient noise was recorded on other mapped faults of the Mt. Etna area, the Moscarello, Acicatena and Pernicana faults. The latter, in particular, is characterized by different strike and faulting style. A systematic tendency of ambient noise to be polarized is found in all of the faults. A picture emerges where normal faults of the eastern flank show a E-W to NE-SW polarization that changes on the Pernicana fault, which develops approximately E-W and is characterized by a prevailing NW-SE to NS polarization. Directions of polarization were never parallel to the fault strike. Moreover, polarization persists too far away from the fault trace, excluding an effect limited to a narrow low velocity zone hosted between harder wall rocks. Both these observations rule out an interpretation in terms of fault-trapped waves. The cause of observed polarizations will be the subject of future studies. However, the consistency with recent results of velocity anisotropy in a part of the investigated area suggests a possible role of attenuation anisotropy on horizontal amplitude variations versus azimuth.200 1110 - PublicationOpen AccessFeatures of seismic events and volcanic tremor during the preliminary stages of the 1991-1993 eruption of Mt. Etna(1996-03)
; ; ; ; ;Lombardo, G.; Istituto di Geologia e Geofisica, Università di Catania, Italy ;Coco, G,; Istituto di Geologia e Geofisica, Università di Catania, Italy ;Corrao, M.; Istituto di Geologia e Geofisica, Università di Catania, Italy ;Gresta, S.; Istituto di Geologia e Geofisica, Università di Catania, Italy; ;; 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.179 142 - PublicationOpen AccessEvidences for strong directional resonances in intensely deformed zones of the Pernicana fault, Mount Etna, Italy(2009-10-10)
; ; ; ; ; ;Di Giulio, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Cara, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Rovelli, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Lombardo, G.; University of Catania ;Rigano, R..; University of Catania; ; ; ; In this paper we investigate ground motion properties in the western part of the Pernicana fault. This is the major fault of Mount Etna and drives the dynamic evolution of the area. In a previous work, Rigano et al. (2008) showed that a significant horizontal polarization characterizes ground motion in fault zones of Mount Etna, both during earthquakes and ambient vibrations. We have performed denser microtremor measurements in the NE rift segment and in intensely deformed zones of the Pernicana fault at Piano Pernicana. This study includes mapping of azimuth-dependent horizontal-to-vertical spectral ratios along and across the fault, frequency–wave number techniques applied to array data to investigate the nature of ambient vibrations, and polarization analysis through the conventional covariance matrix method. Our results indicate that microtremors are likely composed of volcanic tremor. Spectral ratios show strong directional resonances of horizontal components around 1 Hz when measurements enter the most damaged part of the fault zone. Their polarization directions show an abrupt change, by 20° to 40°, at close measurements between the northern and southern part of the fault zone. Recordings of local earthquakes at one site in the fault zone confirm the occurrence of polarization with the same angle found using volcanic tremor. We have also found that the directional effect is not time-dependent, at least at a seasonal scale. This observation and the similar behavior of volcanic tremors and earthquake-induced ground motions suggest that horizontal polarization is the effect of local fault properties. However, the 1-Hz resonant frequency cannot be reproduced using the 1-D vertically varying model inferred from the array data analysis, suggesting a role of lateral variations of the fault zone. Although the actual cause of polarization is unknown, a role of stress-induced anisotropy and microfracture orientation in the near-surface lavas of the Pernicana fault can be hypothesized consistently with the sharp rotation of the polarization angle within the damaged fault zone.224 498