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International audience; This chapter provides an overview of the last two decades’ European experiences in educational seismology and describes the different contexts in which they have been developed. The basic idea of these educational projects is that seismology may represent an efficient communication vehicle for teaching a wide range of basic earth science topics through laboratory practices and educational activities. Moreover, it is also an effective tool to raise in the young citizens the awareness on the earthquake risk and possible mitigation actions. In this frame, several seismic stations with different technologies were installed in schools across Europe. The scientific support of researchers and the need to establish strong links between teachers and researchers attribute to the school an active role in the knowledge process using the scientific laboratory practice by adopting the ‘learning by doing’ modern approach of science communication (R. Schank C. Cleary, 1995). Some educational activities correlated with seismological projects are presented, following different strategies depending on the country, but all aimed at building a new way to communicate science in the schools. The new vogue is the opening towards social media and blogs. This generalises the concept of an educational Geoscience website making it an e-platform for science communication and multimedia data sharing, where researchers, teachers, students and education operators can interact and constantly be kept informed of ongoing activities and relevant events.

International audience; In this paper, we obtain an accurate estimation of the rupture velocity of the 6 April 2009 Mw 6.3 L'Aquila earthquake (Central Apennines) by analyzing its apparent source time functions (ASTFs). These ASTFs have been extracted by deconvolving an empirical Green's function (EGF) at seismic stations located 200-800 km from the earthquake. From the study of duration, shape, and directivity of ASTFs at numerous and well‐distributed stations, we show that the area between the two main slip patches of the earthquake ruptured at a relatively slow velocity (1.6 km/s). We next validate simultaneously the ASTFs and the slip model independently obtained through the joint inversion of strong‐motion, broadband teleseismic, Global Positioning System, and Interferometric Synthetic Aperture Radar data. The good agreement in shape and duration between the observed and theoretical ASTFs is an indication for the reliability of the inferred source model and for the robustness of the rupture velocity estimations of this study.

During the 1982–1984 bradyseismic crises in the Campi Flegrei area (Italy), the University of Wisconsin deployed a network of seismological stations to record local earthquakes. In order to analyse the potential of the recorded data in terms of tomographic imaging, a blind test was recently set up and carried out in the framework of a research project. A model representing a hypothetical 3D structure of the area containing the Campi Flegrei caldera was also set up, and a synthetic dataset of time arrivals was in turn computed. The synthetic dataset consists of several thousand P- and S-time arrivals, computed at about fourteen stations. The tomographic inversion was performed by four independent teams using different methods. The teams had no knowledge of either the input velocity model or the earthquake hypocenters used to create the synthetic dataset. The results obtained by the different groups were compared and analysed in light of the true model. This work provides a thorough analysis of the earthquake tomography potential of the dataset recording the seismic activity at Campi Flegrei in the 1982–1984 period. It shows that all the tested earthquake tomography methods provide reliable low-resolution images of the background velocity field of the Campi Flegrei area, but with some differences. However, none of them succeeds in detecting the hypothetical structure details (i.e. with a size smaller than about 1.5–2 km), such as a magmatic chamber 4 km deep and especially the smaller, isolated bodies, which represent possible magmatic chimneys and intrusions.

The precise relocation of micro-earthquakes using waveform cross-correlation is an efficient tool for defining the spatial characteristics of seismogenic structures in volcanic areas. To be applied and to provide meaningful results, the technique requires, however, the presence of sufficient amounts of similar events. We examined about 870 earthquakes recorded at the Campi Flegrei (CF) caldera by a temporary digital network during the end of the 1982-1984 bradiseismic crisis. The classification of these earthquakes indicates a surprising low amount of similar events, especially according to the clustered aspect of the seismicity. The relocation technique, allows only to identify 3 elongated patterns that may be interpreted as small faults which are dipping inward toward the center of the caldera. To determine up to which point this low similarity rate is unusual, we examined the rates for several data sets coming from various volcanic areas: Hawaii, Mount Pinatubo (Philippines) and Piton de la Fournaise (La Reunion Island). The results show for all areas, percentages of similar events significantly higher than for CF and indicate that the low rate observed there is not just a matter of low detection threshold or network characteristics. At CF, most of the seismicity appears to be diffused and generated along small fractures rather than along well defined fault structures. The low similarity rate is in good agreement with a seismicity generated in a fractured medium by overpressure in the hydrothermal system and low deviatoric local stress. (C) 2010 Elsevier B.V. All rights reserved.

International audience; The 6 April 2009 Mw 6.3 L'Aquila earthquake, central Italy, has been recorded by the Irpinia Seismic Network (ISNet) about 250 km southeast of the epicenter. Up to 19 three-component accelerometer stations could be used to infer the main source parameters with different seismological methods. We obtained an approximate location of the event from arrival times and array-based back-azimuth measurements and estimated the local magnitude (6.1) from an attenuation relation for southern Italy. Assuming an omega-square spectral model, we inverted S-wave displacement spectra for moment magnitude (6.3), corner frequency (0.33 Hz), stress drop (2.5 MPa), and apparent stress (1.6 MPa). Waveform modeling using a point source and an extended-source model provided consistent moment tensors with a centroid depth around 6 km and a prevalently normal fault plane solution with a dominant directivity toward the southeast. The relatively high corner frequency and an overestimated moment magnitude of 6.4 from moment tensor inversions are attributed to the rupture directivity effect. To image the rupture geometry, we implemented a beamforming technique that back-projects the recorded direct P-wave amplitudes into the earthquake source region. A northwest-southeast striking rupture of 17 km length is imaged, propagating with an average velocity up to 3 km/s. This value is significantly higher than our estimate of 2.2 km/s from S-wave spectra. Our case study demonstrates that the use of array techniques and a dense accelerometer network can provide quick and robust estimates of source parameters of moderate-sized earthquakes located outside the network.

International audience; We propose a strategy for merging both active and passive data sets in linearized tomographic inversion. We illustrate this in the reconstruction of 3D images of a complex volcanic structure, the Campi Flegrei caldera, located in the vicinity of the city of Naples, southern Italy. The caldera is occasionally the site of significant unrests characterized by large ground uplifts and seismicity. The P and S velocity models of the caldera structure are obtained by a tomographic inversion based on travel times recorded during two distinct experiments. The first data set is composed of 606 earthquakes recorded in 1984 and the second set is composed of recordings for 1528 shots produced during the SERAPIS experiment in 2001. The tomographic inversion is performed using an improved method based on an accurate finite-difference traveltime computation and a simultaneous inversion of both velocity models and earthquake locations. In order to determine the adequate inversion parameters and relative data weighting factors, we perform massive synthetic simulations allowing one to merge the two types of data optimally. The proper merging provides high resolution velocity models, which allow one to reliably retrieve velocity anomalies over a large part of the tomography area. The obtained images confirm the presence of a high P velocity ring in the southern part of the bay of Pozzuoli and extends its trace inland as compared to previous results. This annular anomaly represents the buried trace of the rim of the Campi Flegrei caldera. Its shape at 1.5 km depth is in good agreement with the location of hydrothermalized lava inferred by gravimetric data modelling. The Vp/Vs model confirms the presence of two characteristic features. At about 1 km depth a very high Vp/Vs anomaly is observed below the town of Pozzuoli and is interpreted as due to the presence of rocks that contain fluids in the liquid phase. A low Vp/Vs body extending at about 3–4 km depth below a large part of the caldera is interpreted as the top of formations that are enriched in gas under supercritical conditions.

International audience; A new technique is proposed here for the retrieval of slip images from the backprojection of high-frequency displacement records. When direct S waves are seen to be dominant in the near-source data, Green functions can be approximated by the far-field terms, as described by ray theory. Assuming that the slip rapidly reaches the final value (i.e. short slip duration), the measured displacement can be ascribed to the slip contributions lying on the corresponding isochrone on the fault plane. Here we use the far-field representation theorem to backproject on the fault plane the displacement amplitudes measured along the seismogram. Through the weighted stack of amplitude maps obtained from different stations we recover high slip zones on the fault.The resolution analysis of the backprojected images is realized with spike tests (that we refer to as 'image Green functions'), which revealed to be an useful tool for detecting and locating artificial distortions of high slip patches, due to a poor data coverage. However, when the slip is uniformly spread along the isochrones, energy is scattered everywhere on the fault, leading to defocusing effects on the final images. A partial deconvolution technique is proposed by reiterating the backprojection. An important implication of this study is that slip maps can be obtained as functions of the rupture time on the fault, that is, the method can be used to retrieve variable rupture velocity kinematic models. Since the latter parameter is not known a priori, we suggest that a data set of coupled rupture velocity and slip maps is built up and the optimal model is chosen according to a waveform fitness criterion. This procedure allows the slip inversion to be separated from the rupture velocity inversion, significantly reducing the number of parameters to be estimated. Additionally, the parametrization of the rupture velocity is done on a less dense grid than the slip.By way of example, the technique is applied to estimation of the kinematic rupture model of the 2000 Tottori earthquake (M= 6.8) , based on the inversion of near-source strong-motion data.

Geophysical Research Letters, v. 30, p. 19, 2003. http://dx.doi.org/10.1029/2003GL018173; International audience; An extended marine, active seismic survey has been performed on September, 2001 in the gulfs of Naples and Pozzuoli by recording about 5000 shots at a network of 62 sea bottom and 72 on shore seismographs. 3-D images of the shallow caldera structure are obtained from the tomographic inversion of about 77000 first P arrival times using the Benz et al. [1996] tomographic technique. The buried rim of the Campi Flegrei caldera is clearly detected at about 800-2000 m depth, as an anular high P-velocity and high density body. It has a diameter of about 8-12 km and a height of 1-2 km. According to stratigraphic and sonic log data from deep boreholes and tomographic P velocities, the rim is likely formed by solidified lavas and/or tuffs with interbedded lava. This study confirms the existence for a depressed limestone basement beneath the caldera at less than 4 km depth, while no evidence are found for shallower magmatic bodies.

Geophysical Journal International, v. 151, n. 1, p. 264-278 [452], 2002; International audience; We have used a dense wide-angle data set to test a two-step procedure for the separate inversion of first-arrival and reflection traveltimes. Data were collected in a complex thrust belt environment (southern Italy) along a 14-km line, with closely spaced sources (60 m) and receivers (90 m). We have applied a fully non-linear tomographic technique, specially designed to image complex structures, to over 6400 first-arrival traveltimes in order to determine a detailed velocity model. A bi-cubic spline velocity model parametrization is used. The inversion strategy follows a multiscale approach, and employs a non-linear velocity optimization scheme. The tomographic velocity model is adopted as the background reference medium for a subsequent interface inversion aimed at imaging a target upper-crust reflector. The interface inversion method is also based on a multiscale approach and uses a non-linear technique for model parameters (interface position nodes) optimization. We have applied the interface inversion method to over 1600 reflection traveltimes of a target event picked both in the near- and in the wide-angle offset range. The retrieved interface is well resolved in the central part of the model, where ray coverage mainly includes clear post-critical reflections and the background velocity model is accurate in depth thanks to large offset deep turning rays. The velocity and interface models thus determined are consistent with Vertical Seismic Profiling data and correlate well with the geometry of known geological structures. This study shows that the used inversion approach is efficient for target-orientated investigations in complex geological environments.

International audience; A multi-2-D non linear traveltime tomography of the shallow (3-4 km deep) structure of Mt Vesuvius volcano was performed. Data have been collected during two recent active seismic experiments using a total of 17 on-land shots and about 140 three-component digital seismographs. A newly developed technique for imaging the volcano velocity structure has been applied, based on an adaptive model space investigation where the number of grid nodes is progressively increased (multi-scale approach). The optimal model parametrization is chosen according to the minimum of the Akaike Information Criteria (AIC) parameter. This corresponds to finding the best compromise between the data misfit and simplicity of the model. The model parameter estimate is performed through the computation of an a posteriori probability density function (pdf), defined following the Bayesian approach. The maximum likelihood model is searched by an optimization technique which combines the genetic and simplex algorithms. The evaluation of the a posteriori pdf is based on traveltime computations using ray tracing techniques. Constraints on the model parameters are inserted in the form of prior pdf and error maps are inferred from cross-sections of the posterior probability around the found best fit solution. The retrieved images of Mt Vesuvius volcano show variable P-velocities in the range 1700-5800 m s −1. A fairly detailed image of the top of the Mesozoic carbonate rocks forming the basement of the volcanic area is obtained. A 9 km long, 1 km deep depression was detected at the N side of the volcano. The presence of a shallow high velocity body is evidenced underneath the Mt Somma caldera and it can be interpreted as a sub-or palaeovolcanic structure.

International audience; In this paper we evalllate the seismic amplification effects in the town of Benevento llsing records of an explo- sion of 500 kg fired at abollt 7 km. Seismic records were obtained at 43 selected sites in the city by digital three-component stations. A data selection performed on the signal-to-noise ratio reduced the available data to 26 stations. We used the spectral ratio techniques to evaluate the amplification effects of each recording site compared both to the average spectrum calculated over 26 stations and to a reference seismic station. The shapes of the spectral ratios were classified according to the geological characteristics of the site. A characteristic shape of the spectral ratio was observed to be related to the specific structure of the subsoil. In particular, the sites on basin sediments (Lagonegro Unit) and Middle Pleistocene conglomerates failed to show amplification effects; the sites on thick alluvial deposits showed amplification in the 5-9 Hz range; finally, sites on alluvial-lacustrine deposits amplified the seismic signal at frequencies depending on the characteristics and the thickness of the deposit. In addition, damage distribution caused by the 1688 earthquake in Benevento was related to the thickness of the surface layers in the ancient built-up area of the town. The study of the spectral ra- tios showed that these deposits amplify ground motion at frequencies between 9 and 12 Hz, i.e. frequencies close to the natural period of the most widespread buildings at that time in Benevento. Geological and seismic data were jointly used to carry out a zonation of the urban area of Benevento on the basis of homogeneous seismic responses. The validity of this analysis is limited to the main frequency band and amplitude of ground motion produced by the explosions.

Reflected waves carry information about the position, shape and physical properties of the geological discontinuities that originate them. The Ray+Born preserved amplitude migration gives, in a single step, an in-depth image of the discontinuities, and an estimate of the associated contrast in seismic properties. From the theoretical point of view, we show that a slight assumption on the S velocity field allows the simultaneous inversion of pp reflected and PS converted waves, and the optimization of the summations in the frequency space involved when anelastic attenuation is accounted for. The strict application of the Ray+Born migration on synthetics associated with a real experiment on Vesuvius leads to imperfect results: The amplitude of reflection events cannot be described by diffraction laws at wide angle when the expected velocity contrasts are large with respect to the available background velocity mode!. A way around this obstacle is an alternative analysis of the wave amplitudes, based on the reflective point, and no more on the scatterer, that also interprets relative changes in amplitude to avoid the estimation of the source amplitude. This technique is applied to investigate a mid-crust reflector under Mt. Vesuvius, with the data of an active seismic experiment performed in the Bay of Naples (Mareves97). With a global velocity model built through the analysis of first arrivaI times, we apply an empirical migration based on diffraction summation to draw a 2D image of the reflector. It appears flat, horizontal, at the depth of 8 Km b.s.l, and extends at least 35 Km laterally. The migration of synthetics confirm that the waves that mainly contribute to the interface image are PS converted at depth, and that their migration is strikingly stable with respect to the imperfections of the velocity model, thereby constraining the minimum lateral extension of the reflector, and its low mean dip. In a second time, with this geometrical model of the interface, we compare the PS and first arrivai amplitudes to estimate the conversion coefficient variations with angle, independently of the source amplitude and local effects in the vicinity of the receivers. These variations are found very slow, and imply that the S-wave velocity is very low in the underlying medium (Vs < 1 Km/s). The migration of synthetics support this conclusion, and further constrain the P wave velocity, bound to be very low as weil (Vp ~ 2.5 Km/s).; La migration quantitative par Rais+Born interprète la forme d'onde des phases réfléchies pour imager et quantifier les contrastes sismiques qui en sont à l'origine. Mais la non linéarité des phénomènes de réflexion dans les conditions expérimentales atypiques de la tomographie du Vésuve battent en breche la modélisation par points diffractants. Nous proposons donc une inversion basée sur le concept de surface réfléchissante, qui tolère les grands angles d'incidence, les forts contrastes de vitesse, et tient compte implicitement du signal source inconnu. Le principe est d'extraire des données les variations relatives du coefficient de réflexion avec l'angle d'incidence, puis de les interpréter en termes de contraste de vitesse. Appliquer cette technique à un réflecteur à moyenne profondeur sous le Vésuve implique d'en déterminer d'abord la géométrie 2D, par migration cinématique des données à disposition (campagne de sismique active Mareves) : il apparait horizontal, à 8 Km de profondeur, s'étend sur plus de 35 Km, et génère de fortes phases converties PS, dont la nature est confirmée par des tests synthétiques. Leur migration s'avère peu influencée par les imperfections du modèle de vitesse, et échantillone densément une zone très restreinte de l'interface. Aussi, l'analyse quantitative est-elle basée sur ces phases: La comparaison de leur amplitude à celle de la première arrivée permet, en ne modélisant ni le signal source, ni les effets de propagation locaux aux récepteurs, de contraindre les propriétés du milieu sous-jacent: Les vitesses sismiques y semblent très basses (Vs < 1 Km/s).