Task 3.2: Local Geology Effect on Seismic Hazard

Abstract

The main purpose of Task 3.2 was to introduce the role of local geology in the seismic hazard evaluation at national scale. This was achieved by grouping the geological formation of the 1:500,000 Italian geological map into three classes A, B, C according to the EuroCode8 provisions (GCM, Geological-Class Map). These classes are associated to a variation of the ground motion due to the different seismic properties of the lithologies included in each group (ANNEX 1). For a critical evaluation of the proposed approach, we have checked the occurrence of a systematic increment of macroseismic intensity as a function of the geological classes at a national scale. The study of distribution of Intensity anomalies associated to the 3 classes show a statistical significant tendency of positive anomalies to be related to classes B and C and negative anomalies to be related to class A (ANNEX 2).

We then assumed that the effect of local geology on ground motion is the one described by variation of elastic response spectra prescribed by EC8 (in terms of PGA, PSA, Housner Intensity). This variation is controlled by a set of parameters depending on the soil classes and earthquake magnitude (ANNEX 3). We compared the increment factors to those deriving from a numerical simulation of the Città di Castello (Central Italy) basin where many geological and seismological data were available. The results at local scale show a good fit (within a factor of two) with the variations of hazard parameters for classes B e C (ANNEX 4).

Moreover, we launched a national-scale enquiry over the 8101 Italian Municipalities whose main goal was the estimation of the buildings distribution on the three soil classes corresponding to those of the CGM. We collected and organized the received forms in a database (see product 3). The comparison at a Municipality-scale of the GCM and the questionnaire results shows that their information is complementary, the first referring this percentage to the housing, the latter considering all the municipality territory (ANNEX 5). The questionnaire is a good data gathering tool for pointing out area where more detailed microzonation investigation must be performed. The questionnaire can be a powerful tool to increase awareness of Municipality Administration and the lack of involvement could be overcome by coordination at the Region levels.

Products 1. National geologic map at 1:500,000 scale modified according to the EC8 soil classes (geological-class map, GCM) 2. Map of the geology-dependent correction of hazard parameters 3. Database of the Italian municipalities containing information about the distribution of buildings in relation to the surface geology as well as geographical and geotechnical data availability.

The three products are organized in an attached CD and they are included in the GIS of the project.