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The Solfatara is one of the major volcanoes of the Phlegrean Fields (Campi Flegrei) volcanic complex, and it is located in a densely populated area a few kilometres west of the city of Naples. It is an active resurgent caldera that has been characterized by a rich history of surfaceground deformation and soil diffuse degassing and fumarolic emissions, which are indications of the top of a hydrothermal plume. A seismic survey was completed in May 2009 for the characterization of the main subsurface features of the Solfatara. Using the complete data set, we have carried out surface wave inversion with high spatial resolution. A classical minimization of a least-squares objective function was first computed to retrieve the dispersion curves of the surface waves. Then, the fitting procedure between the data and a three-sediment-layer forward model was carried out (to a depth of 7 m), using an improved version of the neighbourhood algorithm. The inversion results indicate a NE-SW fault, which is not visible at the surface. This was confirmed by a temperature survey conducted in 2010. A passive seismic experiment localized the ambient noise sources that correlate well with the areas of high CO2 flux and high soil temperatures. Finally, considering that the intrinsic attenuation is proportional to the frequency, a centroid analysis provides an overview of the attenuation of the seismic waves, which is closely linked to the petrophysical properties of the rock. These different approaches that merge complete active and passive seismic data with soil temperature and CO2 flux maps confirm the presence of the hydrothermal system plume. Some properties of the top of the plume are indicated and localized.

International audience; Density tomography of rock with muons of cosmic origin measures the attenuation of the flux of particles crossing the object of interest to derive its opacity, i.e. the quantity of matter encountered by the particles along their trajectories. Recent progress in micro-electronics and particle detectors make field measurement possible and muon density tomography is gaining a growing interest (e.g. Tanaka et al., 2010; Gibert et al., 2010). We have constructed field telescopes based on the detectors of the OPERA experiment devoted to study neutrino oscillation (Lesparre et al., 2011a). Each telescope may be equipped with a variable number of detection matrices with 256 pixels. The spatial resolution is adaptable and is typically of about 20 meters (Lesparre et al., 2010). The telescopes are portable autonomous devices able to operate in harsh field conditions encountered on tropical volcanoes. The total power consumption is less than 40W, and an Ethernet link allows data downloading and remote control of the electronic devices and on-board computers. Larger high-resolution telescopes are under construction. The instruments have been successfully tested on the Etna and Soufrière of Guadeloupe volcanoes were a telescope is operating continuously since Summer 2010. Muon radiographies of the Soufrière lava dome reveal its very heterogeneous density structure produced by an intense hydrothermal circulation of acid fluids which alters its mechanical integrity leading to a high risk level of destabilisation. Small-size features are visible on the images and provide precious informations on the structure of the upper hydrothermal systems. Joined interpretation with other geophysical data available on the Soufrière - seismic tomography, electrical resistivity tomography, gravity data - is presented and discussed. Density muon tomography of the internal structure of volcanoes like the Soufrière brings important informations for the hazard evaluation an is particularly adapted to brought constraints on flank destabilization and hydrothermal circulation models. Tanaka et al., Three dimensional computational axial tomography scan of a volcano with cosmic ray muon radiography, J. Geophys. Res., 115, B12332, doi:10.1029/2010JB007677, 2010. Gibert et al., Muon Tomography: Plans for Observations in the Lesser Antilles, Earth Planets and Space, Vol. 52, 153-165, doi: 10.5047/eps.2009.07.003, 2010. Lesparre et al., Geophysical muon imaging: feasibility and limits, Geophysical Journal International, Vol. 183, 1348-1361, doi: 10.1111/j.1365-246X.2010.04790.x, 2010. Lesparre et al., Design and Operation of a Field Telescope for Cosmic Ray Geophysical Tomography, Nuclear Instruments and Methods in Physics Research A, to appear, 2011a. Lesparre et al., Bayesian Dual Inversion of Experimental Telescope Acceptance and Integrated Flux for Geophysical Muon Tomography, Geophysical Journal International, to appear, 2011b.

During a 7-week microearthquake experiment conducted in Epirus, Akarnania, and the Ionian islands of western Greece, we located approximately 600 earthquakes with magnitudes between 2 and 4.2. No event was deeper than 40 km. The seismicity cannot be clearly associated with any single fault except the Lixourion right-lateral fault located west of the Ionian islands. Focal mechanisms of about 100 earthquakes show, for a narrow band of earthquakes located along the coast, ENE-WSW shortening consistent with the surface tectonics. Farther east, focal mechanisms show NNW-SSE extension beneath the foothills of the Pindus mountains, which is unrelated to surface faulting but is consistent with the presently subsiding basins. This strain pattern is seen far north and south of the Lixourion fault and is similar to the one observed in the Peloponnese. It suggests that a large-scale mechanism is responsible for the recent geodynamics of both the northwestern and southwestern Aegean

We describe how to implement seismic sources in a 2D staggered grid of a finite difference scheme of P-SV wave propagation. By comparison with analytical solutions in a homogeneous medium, we show that a very simple numerical description of sources in a second-order scheme provides good accuracy for radiated waves if one takes the elementary length where 2D stresses are applied as the grid step of the numerical scheme. The accuracy will depend on the grid step and the time step. The error is less than 10% and depends on the azimuth to the station.