C. Nunziata

Interpretation of gravity and magnetic data in the Phlegraean fields geothermal area, Naples, Italy

Abstract Available gravity and magnetic data of the Phlegraean Fields geothermal area, Naples, Italy, have been interpreted and the obtained structural models discussed in the light of the other available geological, volcanological and geophysical data. On the basis of the results of a previous seismic reflection survey in the Gulf of Naples and in the Pozzuoli Bay, which delineated a basement characterized by a seismic velocity of 4–6 km/s, it has been possible to evaluate the gravity anomaly connected with the morphology of this horizon (ϱ = 2.7 g/cm3). The residual anomaly map, obtained after subtraction of the regional long-wavelength components relative to mantle and deep crustal structures and the computed components relative to the above-mentioned seismic basement, shows up as a circular low with an amplitude of 10 mgal centred in the Pozzuoli Bay. This gravity low has been interpreted as due to the occurrence, in the centre of Pozzuoli Bay, of light (Δϱ = −0.2 g/cm3) material with a maximum thickness of about 2 km. However, a contribution to the anomaly due to a narrow magmatic body intruded in the basement, as suggested by volcanological and ground deformation data, cannot be excluded. The aeromagnetic map of the Phlegraean Fields is characterized by three main anomalies which have been fitted by superficial tridimensional parallelepipedic bodies, schematically representing lava flows and domes. Their anomalies have been subsequently subtracted from the observed field, obtaining as a residual a large anomaly centred in the southwestern area of the Pozzuoli Bay. It has been interpreted as being due to a lowmagnetized body which, taking into account the thermal state of the area, should represent that part of the pyroclastic sequence which has lost part of its magnetization by thermo-chemical alteration.

The Campania-Campi Flegrei area: a contribution to discern the best structural model from gravity interpretation

Abstract Structural interpretations of negative gravity anomalies at Campi Flegrei and Campania are reviewed. The interpretations are not uniquely determined, as different models correspond to equally good anomaly fits. These models are examined using a new method based on the interpretation of the apparent density map computed from the gravity anomaly map. As regards the Campi Flegrei anomaly, a funnel-shaped caldera 3D model is readily obtained. This is more appropriate than previous interpretations, as not only anomaly but also apparent densities are fitted. As about the regional structures, the used approach suggests that the more pertinent model is a large magmatic intrusion, 9 to 14 km deep. A final scheme is proposed linking this intrusion to the funnel-shaped caldera as the main regional reservoir.

A gravity and magnetic study of the volcanic island of Ischia, Naples (Italy)

Abstract Gravity and magnetic data for the volcanic island of Ischia, Naples, Italy, have been analyzed and interpreted in light of recent geological and volcanological data to define a model of the islands shallow and deep structures. From the interpretation of the gravity data it appears that the shallow structures consist of pyroclastic material ( p =2.0 g/cm 3 ). Within these pyroclastics there are domes and lava flows of higher density and eruptive centres filled with lower density material. The basement is a “horst” with the shallowest depth at about 1.0 km, south of the centre of the island, if we assign a density contrast of 0.5 g/cm 3 relative to the above pyroclastics. Interpretation of magnetic data measured at 725 stations showed that the basement derived from the gravity interpretation is magnetized. Moreover, this basement is less magnetized on the western side of Ischia which may be caused by the anomalous thermal state of the area, as indicated by surface fumaroles, hot springs etc. and temperature measurements in deep drillings.

Seismic Ground Motion in Napoli for the 1980 Irpinia Earthquake

Abstract — The seismic ground motion in the urban area of Napoli has been computed for the 1980 earthquake (Ms = 6.9) with a hybrid technique based on the mode summation and the finite difference methods. The detailed geological setting of each quarter has been reconstructed from several stratigraphies and six geological zones have been recognized. Shear-wave velocity profiles have been assigned, based on hole tests and inversion of Rayleigh group velocities artificially generated. Realistic SH and P-SV wave seismograms have been computed along the representative cross sections of each zone, by assuming selected velocity profiles. Spectral amplifications of 2–4 have been computed at frequencies roughly corresponding to the eigenfrequencies of the most damaged buildings. Moreover, following the intensity-PGA correlations found for the Italian territory, the predicted peak ground accelerations, 0.04–0.10 g correspond to the intensity range VII-VIII on the MCS scale, in agreement with the observed data.

Spectral Amplification Effects at Sellano, Central Italy, for the 1997–98 Umbria Seismic Sequence

The seismic events recorded at two accelerographs installed at Sellano (central Italy) during the 1997–1998 Umbria seismic sequence, one on detritic material, at the historical centre, and the other one on rock, about 200 m distant, were analysed in terms of spectral amplification of the historical centre site. Epicentres were mainly concentrated in the north and south-east directions of Sellano area. The SH wave component average amplifications were evaluated from the smoothed Fourier spectral ratios of the recordings on soil and rock sites, along the two main epicentral lines. Similar amplifications resulted, with two main peaks in the frequency range of 3–5 Hz, corresponding to the eigenfrequencies of the damaged buildings. Shear velocities of the shallowest 30 m of soils were obtained by FTAN measurements along refraction seismic spreadings, and utilized to compute spectral amplification of soil station to rock station along the geological cross sections. A good agreement was found between observed Fourier spectral ratios and the computed 2D amplification modelling, which explains the damage level of the historical buildings beside the degraded conditions of brick masonry.

A gravity study of the Island of vulcano, tyrrhenian sea, Italy

A detailed gravity survey was carried out on the island of Vulcano, Aeolian Islands, Italy. Gravity was measured on 107 stations and the Bouguer anomalies were computed by assuming geological densities.Aim of this survey was to complete the island structural pattern relatively to the shallower structures.Separation of the gravity anomaly field was carried out by means of data filtering, and two main components were discerned. The λ>2.2 km wavelength component, filtered out of the longer wavelength components, was interpreted quantitatively along a NW profile. The best fitting model consists of an upper layer of recent pyroclastic products (p=2.1 g/cm3) lying upon a highly compacted pyroclastic series or lavas (p=2.4 g/cm3).The shorter wavelength residual gravity field (λ<2.2 km) is characterized by two anomalies, located on Vulcanello and the «Fossa di Vulcano» crater. Vulcanello anomaly could be interpreted, given the geothermal state of the area, as due to an increase of the rock density consequent to propylization processes by high temperature fluids (T>200°C). «Fossa di Vulcano» anomaly is instead attributable to the local volcanic chimney.A schematic comprehensive model of Vulcano is also presented, which accounts for the available main geological and geophysical data.

Seismic characterization of the shore sand at Catania

In the framework of the Catania Project supported by GNDT (Gruppo Nazionale per la Difesa dai Terremoti) seismic measurements have been carried out in the shore sands of Catania to define their detailed shear wave velocity (V) profiles and attenuation properties (Q). The method used, FTAN, is based on the analysis of the dispersion curve extracted from signals generated by controlled sources. The computation of complete synthetic seismograms enables the estimation of the attenuation properties of the surface layers. Hereinafter we analyse the shear seismic characteristics of the shore sands as indicators of potential liquefaction phenomena.

Active and Passive Experiments for S-Wave Velocity Measurements in Urban Areas

One of the key parameters for the study of the effects of local site conditions is the S-wave velocity structure of unconsolidated sediments and the S-wave velocity contrast between bedrock and overlying sediments. Detailed VS profiles with depth can be measured with standard borehole logging and hole measurements, like down-hole and cross-hole. Such measurements are expensive, very local (point measurements) and may be not representative of large areas. Powerful methods for VS measurements, that do not need drillings, are all based on the dispersion properties of Rayleigh wave phase and group velocities. Methods for phase velocity measurement of surface waves need recordings along dense arrays, with small geophone spacing, to avoid spatial aliasing, or, in case of 2 receivers, there is the problem of getting the right number of cycles and, hence, the analysis may lead to wrong values (Nunziata, 2005). Instead, the group velocity dispersion curve of the fundamental mode of Rayleigh waves can be extracted from the recorded signal at a single station by using the FTAN (Frequency Time Analysis) method (Nunziata, 2010 and references therein). FTAN is appropriate to process surface wave data both for the identification and the separation of signals and for the measurement of signal characteristics other than phase and group velocities, like attenuation, polarization, amplitude and phase spectra. When not only the fundamental mode but also the higher modes are excited, FTAN method lets to estimate the gross Q values too. In fact, the comparison between synthetic seismograms computed with extreme Q values and experimental data is based on the relative amplitude of fundamental and higher modes (Nunziata et al., 1999). FTAN method is successfully employed both in seismological and engineering field (e.g. Nunziata et al., 2009; Nunziata, 2010). At urban sites, the impossible use of explosive sources or heavy masses blows, limits the penetration depth to the uppermost 20-30 m, depending upon the rock velocities. Recently, cross correlations of long time series of ambient seismic noise have been demonstrated to recover surface wave dispersion (Green function) over a broad range of distances, from a few hundred metres to several hundred kilometres (e.g. Weaver & Lobkis, 2001; Bensen et al., 2007; Nunziata et al., 2009). Detailed VS profiles with depth are then obtained from the non linear inversion (Hedgehog method) of the average dispersion curve of the fundamental mode of Rayleigh group velocities.

Shear wave velocities from noise correlation at local scale

Cross correlations of ambient seismic noise recordings have been studied to infer shear seismic velocities with depth. Experiments have been done in the crowded and noisy historical centre of Napoli over inter‐station distances from 50 m to about 400 m, whereas active seismic spreadings are prohibitive, even for just one receiver. Group velocity dispersion curves have been extracted with FTAN method from the noise cross correlations and then the non linear inversion of them has resulted in Vs profiles with depth. The information of near by stratigraphies and the range of Vs variability for samples of Neapolitan soils and rocks confirms the validity of results obtained with our expeditious procedure. Moreover, the good comparison of noise H/V frequency of the first main peak with 1D and 2D spectral amplifications encourages to continue experiments of noise cross‐correlation. If confirmed in other geological settings, the proposed approach could reveal a low cost methodology to obtain reliable and detailed ...

Seismic ground motion in large urban areas

Seismicity and Tectonic Structures in the Site of Algiers and its Surroundings: A Step Towards Microzonation.- Seismicity of Morocco for the Period 1987-1994.- The Nile Valley of Egypt: A Major Active Graben that Magnifies Seismic Waves.- Seismicity and Seismic Hazard in Alexandria (Egypt) and its Surroundings.- Linear Amplification of Horizontal Strong Ground Motion in Zagreb (Croatia) for a Realistic Range of Scaled Point Sources.- Ground Motion Zoning of Santiago de Cuba: An Approach by SH Waves Modelling.- Duration Magnitude Scale and Site Residuals for Nothern Morocco.- Using Active Faults and Seismicity for the Strong Motion Modeling in the Eastern Rif (Northern Morocco).- Realistic Modeling of Seismic Wave Ground Motion in Beijing City.- Estimation of Site Effects in Beijing City.- Microzonation of Bucharest: State-of-the-Art.- Deterministic Approach for the Seismic Microzonation of Bucharest.- Site-specific Microzonation Study in Delhi Metropolitan City by 2-D Modelling of SH and P-SV Waves.- I: Theoretical Site Response Estimation for Microzoning Purposes.- II: Comparison of Theoretical and Experimental Estimations of Site Effects.- Deterministic Earthquake Scenarios for the City of Sofia.- Seismic Ground Motion in Napoli for the 1980 Irpinia Earthquake.- Expert Assessment of the Displacements Provoked by Seismic Events: Case Study for the Sofia Metropolitan Area.- Seismic Characterization of Neapolitan Soils.