1,721,014 research outputs found
Use of Refraction Microtremor (ReMi) technique for the determination of 1-D shear wave velocity in a landslide area
No full textIn the context of an ongoing study on seismic response of landslide-prone hill-slopes in Central Italy (area of Caramanico Terme), we tested the applicability of the Refraction Microtremor (ReMi) analysis technique (Louie, 2001) to obtain geometrical and physical parameters needed for numerical modelling. In particular, we used this technique to determine one-dimensional shear-wave velocity profiles (Vs) at sites located on and close to a recent landslide that mobilized 30-40 m thick Quaternary colluvium overlying Pliocene mudstones. The use of this technique in unstable slope areas presents difficulties related to rough topography and lateral lithological heterogeneities, which prevent the extension of geophone array up to the minimum lengths (100 - 200 m) commonly adopted in standard applications. Moreover, sites distant from anthropic sources of microtremors can have unfavourable noise conditions in comparison with other well established cases of application. To check the stability of the ReMi data in these operative conditions and the confidence level of the results, three ReMi campaigns were conducted at different times using different acquisition parameters (seismograph channel number, geophone frequency and spacing). We also tested simultaneous noise recording along orthogonal arrays to investigate a possible presence of directional variations of soil properties. The Rayleigh wave velocity dispersion data derived from picking carried out on p (slowness)-f (frequency) matrix showed the presence in noise recordings of different Rayleigh wave vibration modes (fundamental and first two higher modes), which prevail at different frequency intervals. This indicates that it is essential to correctly identify the different vibration modes to avoid erroneous data interpretation (e.g. fictitious identification of velocity decrease with depth). An analysis of the influence of changing environmental conditions and of different acquisition parameters was conducted through the comparison of data obtained from different campaigns with equal acquisition parameters and from simultaneous acquisition with different parameters. We show that different data acquisition can give quite stable results if spatial aliasing does not contaminate the signal in the p-f matrix near the picking area. Regarding the presence of directional variations, the differences found between velocities measured in two orthogonal directions were not very large (up to 10-20 %). These differences were more probably due to an anisotropic distribution of the noise sources rather than to lateral variations in material properties. The Rayleigh wave velocity dispersion curves, obtained from microseismic noise recording, were then inverted with the software Dinver (Wathelet, 2005) to derive shear-wave vertical distribution. This resulted in a large number of models compatible with data uncertainties estimated from measurement repetitions. The major variability characterizing the models at depth implies that this part of profiles is poorly constrained. However, if different vibration modes are recognised, the number of solutions can be considerably reduced by simultaneously inverting the relative dispersion curves and also by introducing into the models additional constraints (e.g. subsurface information from boreholes and seismic refraction data). References Louie J.N.; 2001: Shear wave velocity to 100 meters depth from refraction microtremor arrays. Bull. Seism. Soc. Am., 91, (2), 347-364. Wathelet M.; 2005: Array recordings of ambient vibrations: surface-wave inversion. PhD thesis, Université de Liège, Belgium
Liquefaction damage potential for seismic hazard evaluation in urbanized areas
No full textThe liquefaction susceptibility of granular soils under seismic actions is commonly estimated by means of the liquefaction safety factor and recently by the potential index also. Since its original formulation the potential index has been developed and modified according to both deterministic and probabilistic approaches in order to draw liquefaction microzonation maps. In this study a new approach to potential index definition is proposed in order to relate the liquefaction potential prediction to the loss of bearing capacity for shallow foundation. Such new method has been used to estimate the so called liquefaction damage potential PDL at Barletta site, located in Puglia Region, where strong seismic events may occur. © 2011 Elsevier Ltd
Panorama migratorio
Full text linkLa migrazione è uno dei fenomeni maggiormente diffusi nei nostri tempi. Nel corso degli anni ha acquisito
una portata planetaria coinvolgendo tutti i paesi del mondo in almeno uno dei suoi due vettori (emigrazione,
immigrazione). Se il fenomeno delle migrazioni forzate è tendenzialmente in crescita, l’Unione europea con le sue
politiche migratorie e le misure intraprese per gestire detto fenomeno si pone in controtendenza. Per quanto riguarda la situazione italiana, secondo i dati UNHCR, tra il 1° gennaio e il 31 dicembre 2018
sono sbarcate nel nostro Paese 23.371 persone, quasi 100mila in meno rispetto al 2017
Normativa in materia migratoria
Full text linkSecondo il report dell’Oms Europa del 2019 è
importante tutelare la salute di rifugiati e migranti
per tre motivi: perché quello alla salute è un diritto
fondamentale, perché migranti e rifugiati
contribuiscono attivamente allo sviluppo dei Paesi
ospitanti e di quelli nativi e perché l’assistenza
sanitaria è il modo migliore per salvare vite umane,
tagliare costi sanitari e proteggere la salute delle
popolazioni residenti. Tuttavia a livello europeo la
normativa in merito risulta ancora debole e
frastagliata, disseminata in ambito diversi delle
competenze e dell’azione dell’Unione europea
Integration of ambient noise and ERT data to investigate the structure of the Yang Jia Gou rock avalanche deposits (Sichuan - China)
No full textThe present study investigates the landslide dam deposits of a rock avalanche triggered in Yang Jia Gou, in Sichuan Province, using single-station three component recordings of ambient noise, with the aim of obtaining information about thickness and mechanical properties of the deposits from their resonance properties. Three noise measurement campaigns and two ERT surveys were conducted to support data interpretation. The data were analyzed using the traditional Nakamura’s technique, HVNR, and the innovative technique HVIP, both based on the calculation of ratios between horizontal and vertical amplitude of ground motion. Both methods revealed the presence of resonance peaks, a major one at lower frequency, and a minor one at higher frequencies, representative of the deposit layering. HVNR showed a considerable instability in terms of amplitude of H/V, likely because this technique analyzes the entire noise wave field recorded, so to be subject to a large variability related to a variable composition of the noise field. This problem does not affect the HVIP method, which is based on the analysis of the ellipticity of Rayleigh waves, isolated from the recording.
Rayleigh wave ellipticity curves were used as targets in the inversion phase to obtain the velocity profile of the site. The subsoil model was constrained by the data derived from the resistivity profiles. The results revealed: different velocity layers inside the deposit; lateral variations in thickness, in accordance with the higher frequency peak, and in mechanical properties, with an increase of stiffness, probably due to a major portion of rocky blocks; an increase in thickness of the entire deposit, probably because of the irregularities of the substrate.
Further investigations are in progress through other kinds of noise analysis exploiting the synchronization of simultaneous recordings. This can provide additional constraints (to be derived from the dispersion of group velocity of Rayleigh waves) and aid resolving interpretation ambiguities.
How to cite: Capone, P., Del Gaudio, V., Wasowski, J., Hu, W., Venisti, N., and Li, Y.: Integration of ambient noise and ERT data to investigate the structure of the Yang Jia Gou rock avalanche deposits (Sichuan - China), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4578, https://doi.org/10.5194/egusphere-egu21-4578, 2021
Site classification of Italian accelerometer stations from analysis of residuals of GMPE application to the national seismic database
Full text linkGround motion prediction equations (GMPE) are a basic tool for seismic hazard assessment, in that they provide an empirical method to estimate ground shaking expected for future earthquakes of given magnitude and distance. The calibration of these relationships relies on seismic databases derived from accelerometer large-scale networks. However, locally recorded shaking are strongly influenced by site effect that can cause ground motion amplification phenomena. This has led to introduce, in data processing, some type of site classifications, typically expressed in form of soil categories, or to adopt some parameter representative of site propensity to amplification, e.g. the mean velocity of shear-waves over the upper 30 meter of subsoil (Vs30). Soil categories defined from geological evaluations, possibly supported by Vs30 measurements, or even, directly, the same Vs30 values, have been thus proposed among the explanatory variables of GMPE, which implies the need of a classification of accelerometer sites whose data are used for GMPE calibration. In the last decade, however, several doubts have been raised about the effectiveness of Vs30 as a proxy of seismic amplification and, more in general, on the reliability of anticipatory site classifications, preceding the direct observation of site response to seismic shaking. Since the Italian National accelerometer network has accumulated a great deal of recordings, it would be now possible to base station classification on a statistical analysis of such data. In particular, we propose a method based on the application of GMPEs to the events recorded and on an analysis of residuals resulting from the comparison between observed and predicted values of shaking parameters. In view of an effort for GMPE optimisation, the effectiveness of the resulting classification, in comparison with the pre-existing ones, was tested subdividing the accelerometer database into a “training” and a “validation” dataset. The former was used for the calibration of GMPEs having different functional forms, whereas the latter was used to compare the predictive efficacy of the different functional forms when employed on data different from those used for the calibration. Tests were focused, in particular, on GMPE predicting peak ground acceleration (PGA) and peak ground velocity (PGV). After having calibrated GMPEs not including a term representative of site effect, a cluster analysis was applied to the residuals in order to redefine the accelerometer station classification based on the minimisation of the rms of differences of residuals relative to single stations from the average value of each class. Then, the new classification was applied to the calibration of GMPE including explanatory variables representing the site classes. The effectiveness of the introduction of such variables in the GMPE functional form was checked applying GMPE with or without such variables to the validation dataset. Test results showed that, while for PGV the introduction of site terms actually improve the GMPE predictive capacity, this does not occur for PGA, for which the site amplification factor shows a strong variability depending on event magnitude and distance, so that its influence on ground motion cannot be effectively modelled without introducing a dependence on event characteristics
Risultati dell'indagine conoscitiva
Full text linkNel corso del 2018 si sono verificate delle oscillazioni del numero dei cittadini stranieri presenti nel sistema
di accoglienza. Tra le motivazioni che le hanno determinate si possono menzionare gli eventi traumatici nei paesi
di origine (guerre, calamità naturali, etc.) e la stagionalità del fenomeno. Per quanto riguarda i CARA/CAS il
numero degli ospiti è determinato anche dalla distribuzione degli stessi nelle strutture presenti sul territorio
nazionale. La tutela della salute e le cure mediche sono garantite agli ospiti dei Centri durante tutto il percorso
dell’accoglienza. Al momento dell’arrivo, viene eseguito uno screening sanitario sui migranti al fine di verificare
il loro stato di salute. Tale prassi è prevista anche nelle Linee guida sui controlli sanitari all’arriv
Site classification of Italian accelerometric stations from cluster analysis of residuals of peak ground motion data regressions
Full text linkOne basic element for seismic hazard assessment is the empirical definition of ground motion prediction equations (GMPE) to estimate shaking expected for earthquakes of given magnitude and distance. GMPEs are calibrated from data of accelerometric stations, distinguishing among site categories of different lithological type (e.g. hard rocks, more or less stiff soils) expected to cause different levels of ground motion amplification. Such a site classification is commonly based on geological observations and/or geophysical parameters like the mean propagation velocity of seismic waves through subsoil surficial layers. However, doubts have been raised about the effectiveness of results obtained from these conventional methods. Here we propose a methodology of accelerometric site classification relying on peak ground motion observations, exploiting the large amount of such observations available in the Italian National accelerometric database. The method is based on a cluster analysis of differences between observations and predictions provided by GMPEs whose functional form does not comprise site class among the explanatory variables. The new method was applied to the ITalian ACcelerometric Archive (ITACA), extracting a "training" dataset (used to calibrate some GMPEs through regressions) and a "validation" dataset (to select the optimal GMPE form). A cluster analysis was then applied to regression residuals, grouping stations into three categories with increasing value of residual average. Checking the reclassification effectiveness through the examination of differences between independent "validation" observations and predictions of GMPEs adopting the new classification, these proved to be more consistent with site response properties than predictions provided by GMPEs using current classification
Tomographic Study of the Adriatic Plate
No full textTwo well-known methodologies have been used, for the first time, to derive a detailed and reliable lithospheric model of the Adriatic Plate, consistent with the thus far available data: non linear inversion of phase and group velocity dispersion curve obtained from surface-wave tomography, and body-waves tomography. To carry out the body-waves tomography, about 16500 P phases and 2000 S phases from 1219 seismic events, 73 seismic stations located at the border of the investigated region and a vertically heterogeneous starting model derived from the nonlinear inversion of average dispersion measurements, are used. The 3-D velocity model of the upper lithosphere, obtained from the combined analysis, shows a rather clear structural anomaly on the northeastern side, where the crust is thicker, and an uprising of the top of the lid in the northern part of the plate with a very thin, if any, transition zone from crust to mantle
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