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

Abstract

Ambient noise measurements and a set of 44 moderate magnitude earthquakes were used to study the role of local geology and morphology on the site response of a small hill in the northern part of Catania, on top of which the University Astronomical Observatory is located. The study area has a gentle topography with a flat surface at the top, and it is characterized by a complex sedimentary sequence lying between a clayey basement and an upper volcanic formation. The recorded data were processed through standard spectral ratio and horizontal-to-vertical spectral ratio techniques. Directional effects were also investigated by computing the spectral ratios after rotating the horizontal components of motion and performing polarization analysis. Results of noise and earthquakes analysis, although showing significant differences in amplitude, are comparable in frequency, especially in the sedimentary terrains. On the lava flows, spectral ratios show significant amplification of the vertical component, which appear related to a higher P velocity contrast with underlying soft sediments. Directional effects were identified in two frequency bands (0.2-0.4 and 1.0-10.0 Hz). The effects observed at the lower frequency interval are rather stable, and it spreads out in all the studied area. At higher frequencies, directional effects are variable and mostly observed on the slopes rather than at the hill top. Our findings appear linked to the complex wave field generated by the lithologic heterogeneities existing in the area which seem to be related to the alternation of sediments and basaltic lavas.

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

“…The lava flows are the most extended lithotype outcropping in the urban area, covering almost the entire city substratum and deeply changing the original morphology. Borehole data have highlighted the heterogeneous nature of this formation, which is characterized by alternating compact and scoriaceous levels that are extremely variable in thickness [43,44]. Moreover, the presence of pyroclastic levels is observed in the sedimentary sequence of sand and sandy clays.…”

“…It has to be remembered that the presence of lava flows at the surface could imply the existence of possible velocity inversions with the underlying soft sediments, which could give rise to a HVSR amplitude lower than one unit [51][52][53]. Panzera et al [44] tested the influence of lava thickness on the HVSR amplitude, in the frequency range 1.0-10.0 Hz. The authors observed that the amplitude of the spectral ratio peaks decayed, reaching values lower than one, when massive lava with a thickness higher than 20 m overlaid sedimentary terrains.…”

“…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).…”

An international program on Silk Road Disaster Risk Reduction–a Belt and Road initiative (2016–2020)

“…Among these hazards, the most frequent and devastating are landslides which severely affected the only route connecting the province of Gilgit-Baltistan with China (Derbyshire et al 1996;). The frequent land-sliding in the region can be mainly related to the abundance of unconsolidated, highly weathered and fractured geological formations; well-developed rock discontinuities because of the active tectonics, frequent earthquakes, uncontrolled blasting for construction and repair of the roads, presence of the largest glaciers out of the polar regions, precarious locations of alluvial, lacustrine and moraine deposits; severe climatic conditions leading to rapid weathering, poor drainage conditions, scarcity of vegetation, high rates of erosion and anthropogenic activities on unstable slopes (Derbyshire et al 2001;Kamp et al 2008;Panzera et al 2015;Shafique et al 2016).…”

When probabilistic seismic hazard climbs volcanoes: the Mt. Etna case, Italy – Part 2: Computational implementation and first results

“…Etna (stations locations are shown in the inset map of Fig. 7a), the stratigraphic sequence and local velocity model were made by integrating literature information and geological observations (Azzaro et al, 2010;Branca et al, 2011;Branca and Ferrara, 2013;Panzera et al, 2011Panzera et al, , 2015Priolo, 1999). The soil models consist of a number of viscoelastic layers, stacked over a half-space, each of them being defined by the thickness (h), the velocity of the body waves (V p and V s ), the density (ρ) and the Q factor, which controls the inelastic properties.…”