The role of alternating outcrops of sediments and basaltic lavas on seismic urban scenario: the study case of Catania, Italy

Abstract

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.

Seismic site effects observed on sediments and basaltic lavas outcropping in a test site of Catania, Italy

“…Such effects could be explained in terms of the complexity of the near-surface morphology and the existence of pronounced stratigraphic heterogeneities that, as postulated by many authors (Chavez-Garcia et al1996;Raptakis et al 1998;Bindi et al 2009), may affect the vertical component of motion. The presence of a complex morphologic and lithologic setting in the Catania area was indeed identified in several studies (Lombardo et al 2001;Panzera et al 2011bPanzera et al , 2013. Present results show, however, that major differences in the shape of HSSR and HVSR at both stations occur in the higher frequency portion of the spectral ratios.…”

Seismic site response of unstable steep slope using noise measurements: the case study of Xemxija Bay area, Malta

“…This causes the HVSR values to drop below 1 over a wide frequency range (e.g. Castellaro and Mulargia 2009;Panzera et al, 2011b). The origin of the resonance peak was confirmed by carrying out 1-D modelling, computing synthetic HVSR curves (Fig.…”

Ambient Vibrations Measurements and 1D Site Response Modelling as a Tool for Soil and Building Properties Investigation

“…Catania is characterized by alternating outcrops of sediments and basaltic lavas, with the presence of possible lateral and vertical heterogeneities, but several authors [43,44,60] used one-dimensional (1D) modelling to reproduce the seismic site effects on the outcropping lithology, with sound results. For this reason, having no earthquake recordings for the investigated site, a numerical modelling was carried out using the code STRATA (https://nees.org/resources/strata).…”

“…As a reference earthquake, we choose the M5.7 earthquake that struck southeastern Sicily in 1990, producing slight damage to the structural elements of the Church of San Nicolò l'Arena and in the eastern cloister. This strike slip event, located on Malta-Hyblean escarpment about 25-30 km from the city of Catania (see the epicentre location in the inset map of Figure 1b), can be considered as a moderate scenario for Catania e.g., [60]. The magnitude (M W ), the hypocentre distance, and the focal mechanism stand for the input of the Cauzzi et al [67] attenuation law in order to define the target spectrum.…”