The structure of the Archaean crust of the North America has been studied based on the synthesis of geological and geophysical data, including seismic sections along LITHOPROBE Geotransects, magnetic and gravity anomaly maps, and seismic tomography data. The authors rely on the experience gained in the Russian Program of the deep geological and geophysical studies of the East European Craton. The juvenile Neoarchaean crust, containing the fragments of reworked Meso-and Paleoarchaean rocks, forms an asymmetric round-oval-shaped domain, wherein the geophysical, structural, and metamorphic parameters display a concentric zoning pattern. The Central zone occupies the Hudson Bay basin. The Internal zone (the northeastern and northern Superior Province) is mainly composed of the granulite facies of metaplutonic, metavolcanic and metasedimentary rocks. The External zone encompasses the southern Superior Province together with Hearne and Rae Provinces. This paper presents 3D crustal models of southern Superior Province. The crust development resulted from rifting and a partial disruption of the continental crust, short-term opening of the linear oceans, successive northward subduction and accretion of the ancient continental and juvenile Neoarchaean oceanic and island-arc terranes between ~2.78 and ~2.70 Ga. Subsequent events in the epicontinental environment, including formation of the metasedimentary belts, granulite facies metamorphism and intense ore formation processes, took place within the range from ~2.71 to ~2.63 Ga. The SCLM morphology within the limits of the Archaean North American Craton can be represented as a flattened overturned cone with a vertical axis (down to a depth of ~350 km). The Hudson Bay basin is located right above the lithospheric keel. A number of the main features of the structure and evolution of the Archaean crust of the North American Craton, primarily the ovalconcentric zoning, the important role of high-temperature magmatic and metamorphic processes and mainly intracontinental magmatism and sedimentation, indicates the leading role of the mantle-plume type processes. The Neoarchaean evolution of the North American craton represents the plate-tectonic processes initiated by a superplume. The Neoarchaean North American Craton is one of a series of similar phenomena that occurred ~2.75 Ga ago in a number of continental regions. The most important features, repeated to a certain degree in tectonic units of this type, are: (1) synchronous formation between 2.79 and 2.58 Ga; (2) mainly intracontinental development; (3) the prevalence of oval-shaped synformal tectonic structures of different ranks with some form of concentric zoning; (4) high-temperature magmatism (usually with the participation of enderbite-charnockites and gabbro-anorthosites) and metamorphism of the granulite facies; (5) a frequently repeated combination of high-grade (granulite and hightemperature amphibolites facies) and low-or moderate-grade (greenschist and epidote-amphibolite facies) metamorphic rocks; (6) the lower-c...

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An extended marine, active seismic survey has been performed on September, 2001 in the gulfs of Naples and\ud Pozzuoli by recording about 5000 shots at a network of 62 sea bottom and 72 on shore seismographs. 3-D images of the\ud shallow caldera structure are obtained from the tomographic inversion of about 77000 first P arrival times using the Benz\ud et al. [1996] tomographic technique. The buried rim of the Campi Flegrei caldera is clearly detected at about 800 –\ud 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\ud 1 – 2 km. According to stratigraphic and sonic log data from deep boreholes and tomographic P velocities, the rim is\ud likely formed by solidified lavas and/or tuffs with interbedded lava. This study confirms the existence for a\ud depressed limestone basement beneath the caldera at less than 4 km depth, while no evidence are found for shallower\ud magmatic bodies

[1] In 1996, LITHOPROBE acquired two $600 km long seismic refraction/wide-angle reflection profiles oriented parallel and orthogonal to the regional geologic strike of the Archean western Superior Province of the Canadian Shield. We present results from ray-based travel time inversion of these data, combined with modeling of the longwavelength gravity field and independent estimates of V p /V s (g), all of which constrain the structure and composition of the lithosphere to depths of 120 km. Large variations in lower crustal V p (6.7-7.5 km s À1 ), lower crustal g (1.72-1.86), upper mantle V p (8.0-8.8 km s À1 ), and Moho depth (32-45 km) are determined. Velocities along orthogonal profiles indicate 8% azimuthal V p anisotropy (fast propagation normal to strike) in part of the lower crust and >6% azimuthal V p anisotropy (fast propagation parallel to strike) in a 15-20 km thick layer (layer H) in the upper mantle which dips northward at $10°from a minimum depth of 48-50 km. Wide-angle reflections identify a deep boundary at $110-120 km depth along both lines. Lower crustal chemical compositions estimated from V p , g, and density are generally intermediate (57-66% SiO 2 ) requiring the presence of 25-60% granitic rocks, which is significantly higher than previous estimates (30% intermediate or felsic granulite) for average Archean crust. The anisotropic lower crustal layer characterized by anomalously high V p , high g lc , and low (À7%) relative density values is interpreted as an amphibolite (basic-intermediate composition) having a subhorizontal lineation to account for 8% azimuthal V p anisotropy. The high V p and intermediate anisotropy of upper mantle layer H requires a harzburgite peridotitic composition with the a axis of olivine aligned E-W. We suggest that both of these layers are relic oceanic lithosphere accreted at the base of the crust during the final stages of lithospheric assembly in the western Superior Province at $2.7 Ga.

We present a singular value decomposition (SVD) based algorithm for polarization filtering of triaxial seismic recordings based on the assumption that the particle motion trajectory is essentially 2-D (elliptical polarization). The filter is the sum of the first two eigenimages of the SVD on the signal matrix. Weighing functions, which are strictly dependent on the intensity (linearity and planarity) of the polarization, are applied. The efficiency of the filter is tested on synthetic traces and on real data, and found to be superior to solely covariance‐based filter algorithms. Although SVD and covariance‐based methods have similar theoretical approach to the solution of the eigenvalue problem, SVD does not require any further rotation along the polarization ellipsoid principal axes. The algorithm presented here is a robust and fast filter that properly reproduces polarization attributes, amplitude, and phase of the original signal. A major novelty is the enhancement of both elliptical and linear polarized signals. Moreover as SVD preserves the amplitude ratios across the triaxial recordings, the particle motion ellipse before and after filtering retains a correct orientation, overcoming a typical artifact of the covariance‐based methods.

Geophysical experiments next year in Romania may provide insight into a common but short‐lived seismic process that can be observed and understood at only one spot on Earth at present. About 150 stations will be set up in the Vrancea area in the southeast Carpathian Mountains to, in effect, record the terminal phase of the detachment of a subducting slab of oceanic lithosphere. This is a major regional tomographic study using a large number of broadband seismometers, which will operate for 6 months. Images will be used for hazard assessment as well as for a delineation of detachment history. Active subduction of oceanic lithosphere at convergent plate boundaries involves earthquakes, magmatism, metamorphism, and deformation—some of the most vivid manifestations of any plate tectonic process. The initiation and termination of subduction, however, remains relatively poorly understood. When convergence of lithospheric plates ceases and the suction force of the subducting plate becomes negligible, the subducting slab moves into an almost vertical position. If subduction occurs in an arcuate geometry, the slab is likely to be segmented.

In July 1998, an M w = 6.2 earthquake struck the islands of Faial, Pico and San Jorge (in the Azores Archipelago), registering VIII on the Modified Mercalli Intensity scale and causing major destruction in the northeastern part of Faial. The main shock was located offshore, 8 km North East of the island, and it triggered a seismic sequence that lasted for several weeks. The existing data for this earthquake include both the general tectonic environment of the region and the teleseismic information. This is accompanied by one strong-motion record obtained 15 km from the epicentre, the epicentre location of aftershocks, and a large collection of the damage inflicted to the building stock (as poor rubble masonry, of 2-3 storeys). The present study was carried out in two steps: first, with a finite-fault stochastic simulation method of ground motion at sites throughout the affected islands, for two possible locations of the rupturing fault and for a large number of combinations of rupture mechanisms (as a parametric analysis); secondly, the damage to buildings was modelled using a well-known macroseismic method that considers the building typologies and their associated vulnerabilities. The main intent was to integrate different data (geological, seismological and building features) to produce a scenario model to reproduce and justify the level of damage generated during the Faial earthquake. Finally, through validation of the results provided by these different approaches, we obtained a complete procedure for the parameters of a first model for the production of seismic damage scenarios for the Azores Islands region.

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