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National audienceAmbient vibration techniques such as the H/V method may have the potential to significantly contribute to site effect evaluation, particularly in urban areas. Previous studies interpret the so-called Nakamura's technique in relation to the ellipticity ratio of Rayleigh waves, which, for a high enough impedance contrast, exhibits a pronounced peak close to the fundamental S-wave resonance frequency. Within the European SESAME project (Site EffectS assessment using AMbient Excitations) this interpretation has been tested through noise numerical simulation under well-controlled conditions in terms of source type and distribution and propagation structure. We will present simulations for a simple realistic site (one sedimentary layer over bedrock) characterized by a rather high impedance contrast and low quality factor. Careful H/V and array analysis on these noise synthetics allow an in-depth investigation of the link between H/V ratio peaks and the noise wavefield composition for the soil model considered here: (1) when sources are near (4 to 50 times the layer thickness) and surficial, H/V curves exhibit one single peak, while the array analysis shows that the wavefield is dominated by Rayleigh waves; (2) when sources are distant (more than 50 times the layer thickness) and located inside the sedimentary layer, two peaks show up on the H/V curve, while the array analysis indicates both Rayleigh waves and strong S head waves; the first peak is due to both fundamental Rayleigh waves and resonance of head S waves, the second is only due to the resonance of head S waves; (3) when sources are deep (located inside the bedrock), whatever their distance, H/V ratio exhibit peaks at the fundamental and harmonic resonance frequencies, while array analyses indicate only non-dispersive body waves; the H/V is thus simply due to multiple reflections of S waves within the layer. Therefore, considering that experimental H/V ratio (i.e. derived from actual noise measured in the field) exhibit in most cases only one peak, we conclude that H/V ratio is (1) mainly controlled by local surface sources, (2) mainly due to the ellipticity of the fundamental Rayleigh waves. Then the amplitude of H/V peak is not able to give a good estimate of site amplification facto

or semianalytical solutions. A set of testing examples is included, together with their thorough descriptions and reference solu­ tions, allowing participants to fully replicate the model with their own code. Reference solutions can be downloaded for compari­ son purposes. Alternatively, the simulation result then can be uploaded to the server, which will calculate and display the time-frequency misfits between selected solu­ tions (Kristekova et al. [2006]; see also the SPICE code library). The source-imaging benchmark is a blind test on kinematic source inversion. A syn­ thetic data set for a hypothetical source model (based on the 2000 Tottori earthquake in Japan) is provided for a set of receivers. Participants can download this data set, try to invert for the slip distribution, and compare the result with the correct one that was used to obtain the benchmarking database. Outlook We hope the library helps researchers get started in using computational tools in seis­ mology. The next step is to develop a more sophisticated infrastructure that facilitates the implementation and execution of large simulations on a routine basis. At a joint meeting of SPICE and CIG (Com­ putational Infrastructure in Geodynamics, a U.S. project providing community codes for geodynamics; Web site: http://www .geodynamics.org) held 8-12 October 2007 in Jackson, N. H., participants discussed how to facilitate the use of computational methods in seismology, and they indicated the need to coordinate developments (e.g., benchmarking, on-demand computing, model and data exchange formats, and train­ ing) on an international scale. To meet this need, it was decided to initiate a new Inter­ net platform (http://www.computational-seismology.org) and an associated mailing list, through which information on meetings, recent developments, publications, projects, and new software in the field of computa­ tional seismology can be disseminated.