In they framework of the Umbria Marche microzonation project, we prepare a first generation site effect zonation of the urban area of Fabriano by means of the analyses of seismometric recordings of aftershocks and seismic noise as well as of geological and geotechnical data collected in the area by different institutions. Velocimetric data have been employed to perform the basic elaboration aimed to the construction of the site effects map. In particular, we examine the incidence angles of P waves to obtain a qualitative appraisal of the rigidity of the deposits under the stations and Nakamura’s ratios to prepare a map of the fundamental period of the deposits in the investigated area. Both analyses show a fairly good congruity with geological and lithological information available. Then spectral ratios, which represent our leading estimate of site effects, are analysed in detail. We find relevant amplifications, up to around 7, inzones “Borgo” and “Serraloggia — Spina”, the latter is also characterised by considerable variability of expected ground motion as evidenced by the difference in the spectral ratios between stations located at distances less than 200 m. Consequently, considering these elements, we propose a subdivision of the area under examination into five zones distinguished by an overall homogeneous seismic behaviour. For each zone we also estimate the expected amplification in terms of acceleration response spectrum and Housner Intensity. The obtained results, being strongly based on weak motion recordings, should be considered valid only for small deformations. However, by means of geotechnical data available for a borehole, we obtain a preliminary appraisal of the expected entity of soil non linearity potential in one zone. The results show that when PGA in the zone with no amplilication is equal to the expected value for a return period of 475 years, amplification of response spectrum for short periods is reduced by a factor of 1.5 with respect to the linear case. For greater periods reduction is negligible.
In they framewor …
AUTORI: Tento A., De Franco R., Franceschina G., Pagani M. RIG ANNO: 2000 NUMERO: 2 Numero di pagina: 131
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