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The purpose of the work presented in the paper, is to compute the response of transversely isotropic plane layered media excited by a buried or surface source, in order to obtain a method which can help interpreting real seismograms. The computation in frequency f and wavenumber k domain based on the Kennett's reflectivity method is exposed. Displacements in space and time are calculated by numerical integration (Fourier-Hankel transform) of the response in frequency and wavenumber domain. The numerical surface response of a transverse isotropic two-layered half space excited by a surface source is presented. The mechanical properties of the proposed two-layered halfspace falls within the range of marine sediments. The frequency and offset proposed domain correspond with geotechnical surveys. Effects of anisotropy are put forward by comparing the responses in anisotropic case to the responses in isotropic case. Le but des présents travaux est de calculer la réponse des milieux stratifiés présentant une isotropie plane à l'excitation d'une source enterrée ou de surface, de façon à obtenir une méthode qui puisse aider à l'interprétation des sismogrammes réels. Le calcul exposé ici, dans un domaine de fréquence f et de nombre d'onde k, est basé sur la méthode de réflexion de Kennett. Les déplacements dans l'espace et dans le temps sont calculés par une intégration numérique (transformée de Fourier-Hankel) de la réponse en fréquence et en nombre d'onde. La réponse numérique de surface d'un demi-espace à isotropie plane et à deux marqueurs excités par une source de surface est ici présentée. Les propriétés mécaniques du demi-espace proposé correspondent à celles de la gamme des sédiments marins. La fréquence et le décalage du domaine proposé correspondent à ceux employés dans la prospection géotechnique. Les effets de l'anisotropie sont mis en avant en comparant les réponses de l'exemple anisotrope à celles de l'exemple isotrope.

This paper discusses the nature of the low-frequency seismo-acoustic waves generated by submarine earthquakes in the ocean. In a finite-depth homogeneous ocean over a semi-infinite solid crust, the derivation of the acoustic equations shows that waves propagate as modes. The waves propagating with the speed of sound in water (T waves) are preceded by waves with frequencies below the Airy phase. Furthermore, the group speeds of these modes are sensitive to the environmental setting. As a test, we applied the spectral finite-element code SPECFEM2D in a simplified configuration with an ocean layer overlaying a solid crust, and a seismic source below a Gaussian seamount surrounded by a flat seafloor. The simulations confirm that the generated T waves and their precursors follow the theoretical dispersion curves. A more realistic environment with a seismically-layered crust and a sound-speed profile in the ocean is then used to predict the expected acoustic modes. Although noisy, recordings by ocean bottom seismometers from the southwest Indian Ocean show T waves preceded by ultra-low frequency waves, which display two modes comparable to the theoretical ones. They are in good agreement for mode 1, whereas, for mode 0, a slight offset in frequency has yet to be explained.

The low-frequency (4-40 Hz) acoustic waves generated by undersea earthquakes are of great importance to monitor the low-level seismic activity associated with seafloor spreading ridges. To better understand the near-source interaction of seismic waves with the seafloor and the resulting generation of low-frequency acoustic waves, the wave propagation in a solid medium (the Earth's crust) and in the overlaying fluid medium (the ocean) were jointly simulated using a three-dimensional (3D) spectral finite-element code (SPECFEM3D). Due to numerical limitations of 3D simulations, the focus was on simple model configurations with a 1 Hz source located below a Gaussian seamount or ridge. The simulated acoustic waves (0-2.5 Hz) propagate as Rayleigh modes and are affected by modal dispersion; their horizontal speed increases away from the source and reaches the sound speed about 140 km away. The amplitude of the generated acoustic waves is affected by the shape of the seafloor topography above the seismic source, as well as their travel times to hydrophones. Consequently, localization of the acoustic sources by trilateration from arrival times may be biased by 3D-effects, and thus the seismic/acoustic conversion zone may not match the epicenter.

T-waves are underwater acoustic waves generated by earthquakes. Modeling of their generation and propagation is a challenging problem. Using a spectral element code-SPECFEM2D, this paper presents the first realistic simulations of T-waves taking into account major aspects of this phenomenon: The radiation pattern of the source, the propagation of seismic waves in the crust, the seismic to acoustic conversion on a non-planar seafloor, and the propagation of acoustic waves in the water column. The simulated signals are compared with data from the mid-Atlantic Ridge recorded by an array of hydrophones. The crust/water interface is defined by the seafloor bathymetry. Different combinations of water sound-speed profiles and sub-seafloor seismic velocities, and frequency content of the source are tested. The relative amplitudes, main arrival-times, and durations of simulated T-phases are in good agreement with the observed data; differences in the spectrograms and early arrivals are likely due to too simplistic source signals and environmental model. These examples demonstrate the abilities of the SPECFEM2D code for modeling earthquake generated T-waves.

The roles of the different retinoid receptors on the differentiation of rabbit tracheal epithelial (RbTE) cells in primary culture were analysed using selective agonists for the retinoid acid receptor subtypes RARalpha (CD336), RARbeta (CD2019), RARgamma (CD437), an RAR panagonist (CD367), a retinoid X receptor RXR panagonist (CD2624) and an antagonist for RARbeta/gamma (CD2665). Squamous differentiation was assessed via expression of cytokeratins CK13/CK4 and transglutaminase I (TGI), specific markers of metaplasia. Treatment with RARalpha and beta agonists or RAR panagonist, but not the RARgamma agonist or RXR agonist, is required for the inhibition of squamous metaplasia, evidenced by inhibition of CK13/CK4 and TGI expression. The expression of CK10 cytokeratin of keratinizing epithelia, CK14/CK5 basal cell cytokeratins, and CK6 marker of cell proliferation decreases upon exposure of the RARaalpha/beta and RXR agonists. The RARgamma agonist CD437, inactive in the decrease in CK13/CK4, CK10 and CK14, reduces CK5/CK6 amounts. CD437 is responsible for a dose-dependent apoptotic response. Nuclear labelling with propidium iodide (PI) and electron microscopy revealed chromatin condensation and nuclear fragmentation. DNA cleavage and cell fragmentation were confirmed by enzyme-linked immunosorbent assay (ELISA) and flow cytometry. The RARbetagamma antagonist was also slightly active. The results indicate that CD437 causes growth arrest in the early S-phase of the cell cycle and prevents the transition G1-S-phase. CD437 was demonstrated to induce apoptosis in the S-phase cells identified by bromodeoxyuridine (BrdU) incorporation. In conclusion, RARalpha/beta ligands are effective inhibitors of squamous differentiation. On the contrary, RARgamma ligand appears to be inefficient in metaplasia inhibition, but the selective RARgamma agonist CD437 induces growth arrest and apoptosis of basal proliferative cells.

Primary cultures of rabbit tracheal epithelial (RbTE) cells have been performed in two different ways. Quantitative analysis of both proliferative capacities and ciliated differentiation process were carried out using epithelial cell cultures from tracheal explants and from dissociated tracheal epithelial cells in air-liquid interface conditions. We show that both alpha- and beta-tubulins from RbTE cells are polyglutamylated and that this posttranslational modification is restricted to cilia axonemes and centrioles of non-ciliated cells. A monoclonal antibody raised against polyglutamylated tubulins was used to quantify the proportion of ciliated cells. Even though epithelial cells from outgrowths obtained by the explant technique highly proliferated during the first days of culture, no ciliated differentiation occurred. On the other hand, using air-liquid interface conditions after proliferation of dissociated cells, we could observe and quantify a ciliated cell differentiation in vitro by both Western blot and flow cytometric analysis. The specific detection and quantification of ciliated cells open the way for the biochemical and molecular characterization of centriolar components during ciliated differentiation.

The differentiation of tracheal epithelial cells in primary culture was investigated according to the nature of the extracellular matrix used. Cultures obtained by the explant technique were realized on a type I collagen substratum either as a thin, dried coating or as a thick, hydrated gel supplemented with culture medium and serum. These two types of substratum induced distinct cell morphology and cytokeratin expression in the explant derived cells. Where cells are less proliferating (from Day 7 to 10 of culture), differentiation was evaluated by morphologic ultrastructural observations, immunocytochemical detection of cytokeratins, and determination of cytokeratin pattern by biochemical analysis. The epithelium obtained on gel was multilayered, with small, round basal cells under large, flattened upper cells. The determination of the keratin pattern expressed by cells grown on gel revealed an expression of keratin 13, already considered as a specific marker of squamous metaplasia, that diminished with retinoic acid treatment. Present results demonstrated by confocal microscopy that K13-positive cells were large upper cells with a dense keratin network, whereas lower cells were positively stained with a specific monoclonal antibody to basal cells (KB37). Moreover, keratin neosynthesis analysis pointed out a higher expression of K6, a marker of hyperproliferation, on gel than on coating. All these data suggest a differentiation of rabbit tracheal epithelial cells grown on gel toward squamous metaplasia. By contrast, the epithelium observed on coating is nearly a monolayer of very large and spread out cells. No K13-positive cells were observed, but an increase in the synthesis of simple epithelium marker (K18) was detected. These two substrata, similar in composition and different in structure, induce separate differentiation and appear as good tools to explore the mechanisms of differentiation of epithelial tracheal cells.