On the Importance of Geolithological Features for the Estimate of the Site Response: The Case of Catania Metropolitan Area (Italy)

The evaluation of seismic site response in the urban area of Catania was tackled by selecting test areas having peculiar lithological and structural features, potentially favourable to large local amplifications of ground motion. The two selected areas are located in the historical downtown and in the northern part of Catania where the presence of a fault is evident. Site response was evaluated using spectral ratio technique taking the horizontal- to-vertical component ratio of ambient noise. Inferences from microtremor measurements are compared with results from synthetic accelerograms and response spectra computed at all drillings available for this area. Such method is particularly suitable in urban areas where the nature of the outcropping geological units is masked by city growth and anthropic intervention on the surface geology. The microtremor H/V spectral ratios evaluated at soft sites located within the downtown profile tend to be smaller than that usually reported in the literature for such soils. A tendency for amplifications to peaks near 2 Hz is observed only in some sites located on recent alluvial deposits. Evidences for amplifications of site effects (frequency range 4–8 Hz) were observed in the sampling sites located on the fault, with a rapid decrease of spectral amplitude just a few tenths of metres away from the discontinuity. Numerical simulations evidenced the importance of geolithological features at depth levels even greater than 20–30 m. Besides this, the results strongly confirm the importance of the subsurface geological conditions, in the estimate of seismic hazard at urban scale.     

Relationship between soil CO<Subscript>2</Subscript> flux and volcanic tremor at Mt. Etna: Implications for magma dynamics

Large variations of the CO₂ flux through the soil were observed between November 2002 and January 2006 at Mt. Etna volcano. In many cases, the CO₂ flux was strongly influenced by changes in air temperature and atmospheric pressure. A new filtering method was then developed to remove the atmospheric influences on soil CO₂ flux and, at the same time, to highlight the variations strictly related to volcanic activity. Successively, the CO₂ corrected data were quantitatively compared with the spectral amplitude of the volcanic tremor by cross correlation function, cross-wavelet spectrum and wavelet coherence. These analyses suggested that the soil CO₂ flux variations preceded those of volcanic tremor by about 50 days. Given that volcanic tremor is linked to the shallow (a few kilometer) magma dynamics and soil CO₂ flux related to the deeper (~12 km b.s.l.) magma dynamics, the “delayed similarity” between the CO₂ flux and the volcanic tremor amplitude was used to assess the average speed in the magma uprising into the crust, as about 170–260 m per day. Finally, the large amount of CO₂ released before the onset of the 2004–2005 eruption indicated a deep ingression of new magma, which might have triggered such an eruption.     

Minimum 1D Velocity Model from Local Earthquake Data in the Provence Region,South-Eastern France

We computed a one-dimensional (1D) velocity model and station corrections refering to the Provence region (South-eastern France) by inverting P-wave arrival times recorded on an eight-station local seismic network. Using this velocity model and the program HYPOELLIPSE (Lahr, 1989), we relocated a set of 108 local events. The quality comparison between previous earthquake locations and new relocated shows a good improvement.The obtained Minimum 1D velocity model can be used in a better-quality routine for earthquake location and represent a first step towards more detailed seismic studies in the Provence region.     

Volcanic tremor at Mt. Etna: State-of-the-art and perspectives

Volcanic tremor on Etna seems to have its origin within the main magma feeding system. On the basis of both spectral analyses at two permanent seismic stations and periodical measurements along the slopes of the volcano, two distinct sources are proposed. The former, characterized by low frequency contents (f<1.5 Hz), is located in a 2 km deep flat magma chamber, whereas the latter source seems to be linked to the upper part of the active vents.Turbulent motions in the magma-gas mixture, induced by escaping gases within the conduits, is one proposed cause of volcanic tremor on Etna (Seidl et al., 1981).From spectral analyses we propose approximate models of the feeding system of the main summit craters.Time variations of tremor energy were also investigated, and no regular patterns have been observed for the studied eruptions.More systematic information seems to be needed for a better knowledge of both the source model and location, and correlation between tremor features and volcanic activity.     

Seismic activity preceding the 1983 eruption of Mt. Etna

The March–August, 1983 eruption of Mt. Etna can be considered as one of the most important in the last years.The analysis of seismic activity during the three months immediately before the eruption showed interesting variations of theb coefficient, in the frequency-magnitude relationship, that have been linked to possible changes of the stress field in the Etnean region.The eruption start was also preceded by a strong seismic crisis with epicenters mostly on the southern, eastern and southwestern flanks of the volcano, and characterized by the shallowness of most of the events (h3 km).The data analysis has led to a hypothesis on the eruption occurrence based on a model of dynamic evolution of the stress field acting on Mt. Etna.     

A procedure to estimate the seismic hazard in an urban area: an application to Acireale (Eastern Sicily)

A “standard procedure” to characterize the seismic hazard of a given area was proposed. It is based on a multidisciplinary approach implying: (1) the knowledge of the seismic history of the area; (2) detailed geological surveys; (3)seismic noise measurements; (4) simulations of earthquake scenarios. The downtown of Acireale, a typical baroque town located on Eastern Sicily, was chosen as the “test area”. A catalog of the local seismogenic faults (able to generate earthquakes in historical times) has been compiled, as well as a seismic catalog for the effects of both local and regional earthquakes. The analysis of both catalogs allowed us to make the following conclusions: (1) the most important seismogenic faults affecting the Acireale municipality do not affect the downtown, while the related local earthquakes attenuate their energy (and intensity) in short (few km) distances; (2) the highest seismic intensity (degree X) experienced in Acireale downtown was caused by the 1693 regional earthquake; (3) over the last 140 years, the downtown has experienced the highest intensity value of VII only once, while six times the intensity was VI. On the whole, this implies a moderate seismic hazard. The estimation of the seismic hazard has been also approached by the experimental method of recording seismic noise. Measurements have been performed at seven different sites, where drills gave detailed information on the shallow subsurface geology to obtain HV (horizontal/vertical) spectral ratios. On the whole, the highest site amplification factor was moderate (about 7). A further investigation based on synthetic seismograms (and spectra) produced by simulating two given earthquake scenarios was also performed. The two scenarios are, respectively, representative of the largest expected earthquake in the area (the 1693 shock) and of a moderate (magnitude ca. 5.5) local earthquake (as the 1818 one). Moderate to strong locally expected accelerations were evidenced.