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The seismic ground motion of a test area in the eastern district of Naples is computed with a hybrid technique based on the mode summation and the finite difference methods. This technique allows us the realistic modelling of source and propagation effects, including local soil conditions. In the modelling, we consider the 1980 Irpinia earthquake, a good example of strong shaking for the area of Naples, which is located about 90 km from the epicenter.The detailed geological setting is reconstructed from a large number of drillings. The sub-soil is mainly formed by alluvial (ash, stratified sand and peat) and pyroclastic materials overlying a pyroclastic rock (yellow neapolitan tuff), representing the neapolitan bedrock. The detailed information available on mechanical properties of the sub-soil and its geometry warrants the application of the sophisticated hybrid technique.As expected, the sedimentary cover causes an increase of the signal's amplitudes and duration. If thin peat layers are present, the amplification effects are reduced, and the peak ground accelerations are similar to those observed for the bedrock model. This can be explained by the backscattering of wave energy at such layers, that tend to seismically decouple the upper from the lower part of the structure.For SH-waves, the influence of the variations of the S-wave velocities on the spectral amplification is studied, by considering locally measured velocities and values determined from near-by down-hole measurements. The comparison between the computed spectral amplifications confirms the key role of an accurate determination of the seismic velocities of the different layers.The comparison performed between a realistic 2-D seismic response and a standard 1-D response, based on the vertical propagation of waves in a plane layered structure, shows considerable difference, from which it is evident that serious caution must be taken in the modelling of expected ground motion at a specific site. 相似文献
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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. 相似文献
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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 ( = 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 (Δ = −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. 相似文献
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The Campi Flegrei caldera (Italy) has been the site of intense seismic activity over the past decades. This area is densely populated and includes important towns such as Napoli with historical sites and supporting many industries. For the proper use and management of the region, the evaluation of the dynamic properties of near-surface rocks is necessary.
The volcanological pattern has been reconstructed from the lithostratigraphies of several drillings. The most interesting and widespread pyroclastic products are the pozzolana deposit (soil) and the Neapolitan Yellow Tuff (rock). Both pozzolana and tuff products are covered by recent eluvial and coastal sandy deposits and younger volcanic products (<12 000 years). The characteristic ranges of the shear wave velocity (Vs) of the Campi Flegrei–Neapolitan soils and tuffs are defined and the primary influencing factors are evaluated. For the sandy deposits, the results show that eluvial and lacustral products have lower shear wave velocities than coastal products. For the volcanic products younger than 12 000 years b.p. the influence of vertical pressure is emphasized. As regards the pozzolana deposit (soil) and the Neapolitan Yellow Tuff (rock), a major influencing factor is shown by the textural characteristics and the different hardening degrees as a consequence of the diagenetic processes. The scattering of the Vs velocities for the same formation is so wide that only the variability ranges can be individuated. These results suggest a need to carry out detailed Vs measurements or, at least, to make a parametric study of the effect of the Vs ranges on seismic response analysis in order to give safe building codes. 相似文献
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Ambient noise measurements have been performed at local and small scales in the Neapolitan and surrounding areas (Campania, southern Italy) by employing two broad-band Kinemetrics Q330 stations, equipped with Episensor ES-T three component accelerometers. In both experiments frequency time analysis (FTAN method) has been performed on the vertical and radial components of noise cross correlations to retrieve the Rayleigh wave dispersion (Green??s function). At local scale, over an interstation distance of about 26?km, the group velocity dispersion values have been compared with those obtained from FTAN analysis on recordings of two earthquakes with similar path. At small scale, measurements have been carried out over an interstation distance of about 440?m in the public gardens of Scampia, the northernmost quarter of Naples. The Rayleigh wave group velocity dispersion data obtained from noise cross correlation, have been combined with those from active seismic experiment along the same alignment, but shorter (120?m offset). The non linear inversion of such a dispersion curve has allowed the definition of V S models to depths of 100?m, in agreement with nearby stratigraphy. Moreover, a good agreement has resulted for the resonance frequency among the H/V ratio, the ellipticity of the fundamental mode computed for the chosen V S model, and the average two-dimensional (2D) spectral amplification computed along a cross section representative of the Scampia quarter. 相似文献
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Shear wave velocities of the crust and upper mantle are defined beneath the Roccamonfina volcano and surrounding Apennines (southern Italy) from the simultaneous nonlinear inversion of the local group velocity dispersion data, obtained from seismic events recorded in 1988–2004 at Roccamonfina station of the INGV-RSNC network, and regional dispersion data obtained in previous studies. The main features of the representative VS models are a carbonatic basement and a low velocity zone at 6–10?km of depth. The sedimentary succession is ~5?km thick below the Roccamonfina volcano and lays above a high VS (3.8?km/s) ascribable to solidified magma body, while it is ~10?km thick below the surrounding Apennines. A low velocity layer with an average thickness of 10?km is detected below the Roccamonfina volcano which can be associated with the presence of partial melting and interpreted as magmatic reservoir. Such low velocity layer, also found below the surrounding Apennines but with a reduced thickness of 2–3?km, extends to the Campanian Plain and to the Neapolitan volcanic area, from Campi Flegrei to Somma-Vesuvius. 相似文献
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Shear wave velocities of the lithospheric structure to 73 km depth have been defined along three profiles crossing the Campanian Plain (Southern Italy) from the simultaneous non linear inversion of the local and regional dispersion data. The former consist of group velocity dispersion data obtained from some seismic events which occurred at the borders of the Campanian Plain and recorded at Napoli, and the latter of group and phase dispersion data obtained in previous studies. The main features of the representative VS models are a carbonate basement deepening to ～5 km in the central part of the Plain and a low velocity zone at a depth of ～15 km, rising to 7 km in the southern part, close to Somma-Vesuvio. The low velocity layer can be correlated with that found at ～10 km of depth below Campi Flegrei and the Neapolitan area, and at 5 km below the Somma-Vesuvio caldera area. Such regional velocity reduction can be associated to the presence of a zone with less than 5% partial melting that can be interpreted as magmatic reservoir of the Campanian volcanism. 相似文献
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The Sannio seismogenic area turns out to be responsible for the highest peak ground accelerations (PGA) and seismic response spectra (SRS) at Napoli. The 1688 earthquake is considered representative of the area, and realistic synthetic seismograms have been computed for this scenario earthquake at the historical center and the eastern sector of Napoli. The use of a hybrid technique based on mode summation and finite-difference methods is fully justified by the available detailed knowledge about the geological and geophysical properties of the Neapolitan subsoil. This modeling makes it possible to recognize that amplifications of ~2 for PGA and >3 for SRS are to be expected because of the pyroclastic soil cover. Based on the information contained in the available catalogs, different magnitudes have been considered. Taking into account the correlation, valid for the Italian territory, between synthetic PGA and observed intensities, it turns out that the most probable magnitude (M) of the 1688 earthquake is 6.7, while M?=?7.3 should be assigned to a conservative scenario earthquake. Comparison of the computed response spectra for the 1688 scenario earthquake with the Italian seismic building code shows that the code is adequate with respect to the expected effects at the historical center of Napoli, but that it underestimates the possible ground motion at the eastern sector, in particular at the newly developed area built after the 1980 earthquake. 相似文献