Francesco Panzera

New evidence of wavefield polarization on fault zone in the lower NE slope of Mt. Etna

Fault zones are generally characterized by a damage zone, formed by highly fractured materials, that is seismically characterized by low velocity and rigidity compared to the host rock. Such geometrical setting, coupled with impedance contrast, lead to the development of local amplification of the ground motion and directional effects. In the present study, the results of measurements performed in the lower north-eastern area of Mt. Etna are shown and discussed. Ambient noise data were recorded and processed through the horizontal to vertical spectral ratio technique, setting into evidence directional effects and providing a direct estimate of the polarization angle as well. The data were recorded along and across the Piedimonte-Fiumefreddo fault system, a set of NE-SW trending extensional segmented faults, that is flanked by small conjugate segments and is accompanied by several WNW-ESE oriented minor faults and fractures of the Pernicana fault system.Results point out that in most of the investigated sites the polarization effects are nearly perpendicular with respect to the minor faults, striking WNW-ESE. This highlight that the local extensional deformation field, related to the shear zone of the Pernicana fault system, plays an important role and it appears as the predominant one in the seismic response that characterize this tectonically complicated area.

Evaluation of building fundamental periods and effects of local geology on ground motion parameters in the Siracusa area, Italy

The Siracusa area, located in the southeastern coast of Sicily (Italy), is mainly characterized by the outcropping of a limestone formation. This lithotype, which is overlain by soft sediments such as sandy clays and detritus, can be considered as the local bedrock. Records of ambient noise, processed through spectral ratio techniques, were used to assess the dynamic properties of a sample survey of both reinforced concrete and masonry buildings. The results show that experimental periods of existing buildings are always lower than those proposed by the European seismic code. This disagreement could be related to the role played by stiff masonry infills, as well as the influence of adjacent buildings, especially in downtown Siracusa. Numerical modeling was also used to study the effect of local geology on the seismic site response of the Siracusa area. Seismic urban scenarios were simulated considering a moderate magnitude earthquake (December 13th, 1990) to assess the shaking level of the different outcropping formations. Spectral acceleration at different periods, peak ground acceleration, and velocity were obtained through a stochastic approach adopting an extended source model code. Seismic ground motion scenario highlighted that amplification mainly occurs in the sedimentary deposits that are widespread to the south of the study area as well as on some spot areas where coarse detritus and sandy clay outcrop. On the other hand, the level of shaking appears moderate in all zones with outcropping limestone and volcanics.

A microtremor survey to define the subsoil structure in a mud volcanoes area: the case study of Salinelle (Mt. Etna, Italy)

Abstract A microtremor survey aiming to obtain information on subsoil structure was performed in the lower southwestern Etnean flank (Sicily, Italy). The area is characterized by emissions of muddy and salty water which create specific pseudo-volcanic edifices. Recording sites were both randomly selected in the area and along two profiles. Horizontal-to-vertical noise spectral ratio, polarization and surface wave prospecting techniques were used. The spectral ratios results set into evidence different frequency ranges that appear interesting to infer information on the subsoil structure. The depth of the hydrocarbon reservoir was inferred through 1D modeling, by using the information coming from the literature and experimental surveys. Moreover, a strongly directional effect suggests the existence of a NNW-SSE-oriented mud feeding dike.

Exploratory seismic site response surveys in a complex geologic area: a case study from Mt. Etna volcano (southern Italy)

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.

Performance evaluation of Wied Dalam (WDD) seismic station in Malta

The continual operation of a permanent seismograph, now exceeding a couple of decades in some cases, naturally involves changes of hardware and soft- ware over time. Nonetheless, the long-term, consistent performance of the seismic station, and the good quality of its data, is very important for national seismic stud- ies investigating the local seismicity, and also important for the international seismological community research- ing regional tectonics and deep Earth structures. Here we investigate the data availability and quality of the currently only seismic station on Malta (WDD) since its installation in 1995, and establish spectral patterns in the seismic data that may be inuenced by diurnal vari- ations, seasonal weather changes, and/or site-specic settings. The results are important for the future de- ployment of permanent seismic stations on the Maltese islands, and for the analysis of local seismic hazard and ground motion studies.

Evaluation of seismic site response in the Maltese archipelago

The investigation of local ground conditions is an important part of seismic hazard assessment (Fäh et al., ). It is now well–established that earthquake ground shaking is not only a function of the earthquake magnitude and epicentral distance, but also of the site conditions, including soft layers in the sub–soil stratification and topographical features. Local geology can greatly alter the seismic waves from earthquakes by amplifying their amplitude, changing the frequency content and increasing the shaking duration during an earthquake (Kramer, ). In fact, several unconsolidated soft sites have suffered significantly greater damage than rock sites. One case was the  Michoacán earthquake which showed low peak ground acceleration near the epicenter, yet caused severe damage in Mexico City, which is found more than  km away and is characterized by soft shallow sediments (Campillo et al., ). The main parameters responsible for such effects are the shear– wave velocity (VS) structure and thickness of the sedimentary cover, the impedance contrast between the soft sediments and the underlying bedrock as well as the geometry of their interface (Parolai et al.,). Knowledge of the VS structure and/or the resonance frequency of soft soil layers is of utmost importance for the prevention or mitigation of earthquake disasters (Arai and Tokimatsu, ). Such information should contribute to earthquake–hazard mitiga-

Overview of the seismic hazard in the Sicily channel archipelagos

A joint Italo–Maltese research project (Costituzione di un Sistema Integrato di Protezione Civile Transfrontaliero Italo–Maltese, SIMIT) was financially supported by the European community. One of the aims of SIMIT was to improve the geological and geophysical information in Lampedusa and in Malta and ultimately to mitigate natural hazards. Although this region lies on the Sicily Channel Rift Zone, a seismically active domain of Central Mediterranean, the knowledge about seismotectonic and seismic hazard is not satisfactory. At present, seismic hazard assessment (SHA) for Italy (MPS Working Group, ), Tunisia (Ksentini and Romdhane, ) and more generally for whole European areas (Giardini et al., ) do exist, whereas no specific SHA for the Sicily channel archipelagos are available. The Sicily Channel appears to be a region of moderate seismic activity, with the seismicity mainly located in the surrounding areas (Fig. ). For the Malta archipelago a first catalogue, listing historical and felt earthquakes, was made by Galea (), whereas the Database Macrosismico Italiano (DBMI; Locati et al., ) does not list any data as regards earthquakes felt in Lampedusa. For this reason, in the present study, a theoretical seismic history was derived (Fig. ) for Lampedusa and Malta, using the European–Mediterranean Earthquake Catalogue (EMEC) (Grünthal and Wahlström, ) and the attenuation relationship for macroseismic intensity data by Pasolini et al. (). The two study areas do not appear to have been affected by strong earthquakes occurring in the Sicily channel, but they were somehow struck by major earthquakes occurring in the surrounding area. Although the present description seems to exclude large shaking effects, SHA for the study region is

Seismic refraction tomography surveys as a method for voids detection: an application to the archaeological park of Cava Ispica, Sicily, Italy

ABSTRACT Geophysical surveys are commonly used in areas where the presence of ancient civilizations is historically documented. These investigations are able to detect through indirect methods the presence of bodies or structures in the subsoil measuring the variations of some physical parameters, obtaining information about archaeological remains without causing harm to them. In the present study, we investigated the presence of anthropogenic buried cavities partially filled with rubble material, in an area located in the northern part of Cava Ispica, one of the most important archaeological sites located in south-eastern Sicily. The results of seismic refraction prospections, processed with tomographic methods, are presented and discussed. The entire valley contains prehistoric burial sites, Christian catacombs and residential units of various kinds. Despite the numerous searches made over past years, much still remains to be discovered. The seismic refraction tomographic data, analyzed in 3D, revealed the presence of low velocity values (<400 m/s) areas that, considering the lithotypes locally present and the features of other archaeological structures discovered during previous surveys, can be ascribed to the presence of possible cavities. The obtained results show how such geophysical inspections represent an important preliminary tool for archaeological surveys.