International audienceMicrotremors are produced by multiple random sources close to the surface of the Earth. They may include the effects of multiple scattering, which suggests that their intensities could be well described by diffusion-like equations. Within this theoretical framework, the average autocorrelation of the motions at a given receiver, in the frequency domain, measures average energy density and is proportional to the imaginary part of the Green's function (GF) when both source and receiver are the same. Assuming the seismic field is diffuse we compute the H/V ratio for a surface receiver on a horizontally layered medium in terms of the imaginary part of the GF at the source. This theory links average energy densities with the GF in 3-D and considers the H/V ratio as an intrinsic property of the medium. Therefore, our approach naturally allows for the inversion of H/V, the well-known Nakamura's ratio including the contributions of Rayleigh, Love and body waves. Broad-band noise records at Texcoco, a soft soil site near Mexico City, are studied and interpreted using this theory

During a quarter of a century, the main characteristics of the horizontal-to-vertical spectral ratio of ambient noise HVSRN have been extensively used for site effect assessment. In spite of the uncertainties about the optimum theoretical model to describe these observations, over the last decade several schemes for inversion of the full HVSRN curve for near surface surveying have been developed.In this work, a computer code for forward calculation of H/V spectra based on the diffuse field assumption (DFA) is presented and tested. It takes advantage of the recently stated connection between the HVSRN and the elastodynamic Green's function which arises from the ambient noise interferometry theory.The algorithm allows for (1) a natural calculation of the Green's functions imaginary parts by using suitable contour integrals in the complex wavenumber plane, and (2) separate calculation of the contributions of Rayleigh, Love, P-SV and SH waves as well. The stability of the algorithm at high frequencies is preserved by means of an adaptation of the Wang's orthonormalization method to the calculation of dispersion curves, surface-waves medium responses and contributions of body waves.This code has been combined with a variety of inversion methods to make up a powerful tool for passive seismic surveying.

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The damage distribution in Adra town (south-eastern Spain) during the 1993 and 1994 Adra earthquakes (5.0 magnitude), that reached a maximum intensity degree of VII (European Macroseismic Scale (EMS scale)), was concentrated mainly in the south-east zone of the town and the most relevant damage occurred in reinforced concrete (RC) buildings with four or five storeys. In order to evaluate the influence of ground condition on RC building behaviour, geological, geomorphological and geophysical surveys were carried out, and a detailed map of ground surface structure was obtained. Short-period microtremor observations were performed in 160 sites on a 100 m × 100 m dimension grid and Nakamura's method was applied in order to determine a distribution map of soil predominant periods. Shorter predominant periods (0.1-0.3 s) were found in mountainous and neighbouring zones and larger periods (greater than 0.5 s) in thicker Holocene alluvial fans. A relationship T = (0.049 ± 0.001)N , where T is the natural period of swaying motion and N is the number of storeys, has been empirically obtained by using microtremor measurements at the top of 38 RC buildings (ranging from 2 to 9 storeys). 1-D simulation of strong motion on different soil conditions and for several typical RC buildings were computed, using the acceleration record in Adra town of the 1993 earthquake. It is noteworthy that all the aforementioned results show the influence of site effects in the degree and distribution of observed building damage. Figure 4. Continuous microtremor measurements for stationarity analysis: (a) tunnel site (hard rock) and (b) beach site (soft soil).Figure 12. An example of the methodology followed to simulate ground and building responses during the 1993 earthquake in Adra: (a) E-W component of strong motion record in Adra town; (b) simulated strong motion on sediment soil site with a thickness of 30 m (Case 2); (c)-(e) simulated strong motions at the top of buildings with four, six and eight storeys, respectively; and (f) amplitude spectra of the previous strong motions.

S U M M A R YWe investigated the shallow velocity structure of Deception Island volcano, Antarctica, using correlations of ambient seismic noise. We selected long records of noise obtained by eight seismic arrays deployed along the inner coast of Deception during the period 2003-2005. Using these data, we calculated average dispersion curves and estimated local 1-D velocity models for the array sites. The combination of these profiles allowed us to obtain a comprehensive model of the shallow velocity structure of the island. The volcano is composed of relatively soft layers of pyroclastic deposits and sediments extending to a depth of about 400 m, with different degrees of compaction. Two layers with thicknesses of about 100 and 300 m and S-wave velocities of around 0.2-0.8 and 0.7-1.1 km s −1 , respectively, can be differentiated. The deeper structure is highly variable in terms of wave velocities and layer depths. Although the resolving capabilities are reduced for these layers, the larger S-wave velocities in the range 1.3-2.8 km s −1 indicate that they can be associated with pre-caldera structures and products. There are substantial differences between the different models, which can be spatially related to heterogeneities in the volcano structure. The lowest S-wave velocities may be related to the alterations produced by hydrothermal activity near the surface. On the contrary, the largest velocities occur near the caldera border, revealing the presence of compact materials at shallow depths. Sharp lateral variations can also be observed in the northwest of the bay, which points to the presence of NW-SE faults and/or strong velocity gradients.

The sedimentary structure in the Zafarraya basin, located in the south of Spain, is studied by using the horizontal-to-vertical spectral ratio (HVSR) for ambient noise. To improve the reliability of the results, the stability of the HVSR measurement was checked by using a time-dependent analysis of the noise records taking into account only clear peaks. The resonant frequency varies through the basin and does not ever coincide with the maximum of the horizontal power spectrum. Several fits between the frequency of the main peak in the HVSR for microtremors and the depth to the bedrock are obtained for data from 17 sites, where geotechnical information was available. The relation derived using a scheme based on surface waves was considered to be the most reliable one. Thus, a function describing the average velocity versus depth for the sedimentary cover of the basin is also derived under assumptions of lateral homogeneity and smooth vertical variations of velocity due to age and/or confining pressure. The measurement of microtremors at 86 uniformly distributed points provides a map of the sedimentary thickness based on the previously calculated relationships, showing values greater than 200 m at the deeper zones.

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