Microzonation is widely used in seismic risk evaluations to define the predominant period values, which are usually associated with extended areas of a few hundred meters. However, the representative values corresponding to these areas are obtained from few measurements in each area. Thereby, results are accurate only in the case of depth-dependent soils. However, not detected narrow and sharp lateral changes in soil are potentially the cause of imprecision and could be a source of specific errors. This paper aims to present several tests conducted in order to emphasise the importance of accurate selection of points, to underscore the necessity of more precise and detailed evaluations, and to suggest a possible methodology to select the most appropriate data acquisition points. Results highlight the need to divide microzonation areas into smaller zones for a precise evaluation in locations where sudden changes in soil characteristics exist. Therefore, in such sites the requirement of nanozonation appears; defining zones with the same soil response. Distance between vibration measurements could be the main problem for nanozonation; data acquisition in areas with irregular geology can be time-consuming when a precise analysis is required. In the most complicated environments or in dense cities, it could even be unfeasible.Consequently, it is necessary to establish a functional methodology to adequately distribute the measurement points throughout the area. On this occasion, three sites in Barcelona city were studied. This city is surrounded by mountains at NW, W and S, and by the Mediterranean Sea at N and E. As a consequence, the shallow geology is characterized by many paleochannels and streams that are currently buried. These geological structures most likely affect the soil response. Several tests were carried out to determine this dependence. The tests were based on Ground Penetrating Radar (GPR) surveys to define the paleochannels position and on vibration measurements in order to define properly the soil response.The results from both methods were compared to the known geology to accurately define the effect of the shallow geological structures in the predominant period and in the GPR images. Areas with the same geological unit but different materials were identified in the GPR images, allowing the selection of the most appropriate distance between vibration measurements in each place. As final result, predominant 2 periods that were measured over the same geological unit but over different material showed changes higher than the 40% in short distances. This procedure could improve the soil response maps, including nanozonation.
S U M M A R YAttenuation coefficients of Rayleigh waves propagating across the Iberian Peninsula, and quality factor models were obtained for different two-station great-circle paths. The data used were long-period wave trains contained in the seismograms provided by the ILIHA array installed in the peninsula in the 1980s. More than 300 seismograms were analysed to obtain 14 sets of similar paths, consisting of either paths totally included in one of the four pure tectonic regions or mixed paths crossing several regions. We used the two-station method lo determine attenuation parameters (yR) and then an inversion method to derive Qg' models. The attenuation coefficients of seismic energy in the 10-120 s range are in the range 2.8 x 10p3-0.1 x km-l, and correlate well with both previous results obtained from analogue WWSSN and most seismogeotectonic characteristics of Iberia. The Qa models obtained also agree with a few models for Iberia determined earlier, and they show significant variations from one region to another. As a general feature, the y-values are higher in the active seismotectonic regions (typical maximum of 2.6 x lop3 km-') than in the more stable ones (typical maximum of 1.4 x Moreover, the maximum of Q;' in the active regions (145 x 10-3-120 x In general, both the regionalized attenuation coefficients of Rayleigh waves and the quality factor models exhibit a good agreement with previously published L, wave attenuation and coda Q distribution patterns in Iberia.
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