A lithology based statistical classification and spatial analysis of Vs 30 in an area of Marche (Italy)

The Italian Department of Civil Protection has published the "Standards for representation and computer archiving for seismic microzonation" with the aim of homogenizing the geological-technical and geophysical information used for the production of seismic microzonation maps in Italy. The technical and documentary data collected by the municipal and regional Administrations are provided to designated professionals for the realization of studies and maps of seismic microzonation. Seismic microzonation studies complying with the MS standards are validated and stored in a centralized database (PostgreSQL-PostGIS) at CNR-IGAG (National Research Council - Institute of Environmental Geology and Geoengineering). For the present study, data concerning lithotypes, geophysical and geotechnical parameters have been extracted from the centralized archive, for conducting a statistical analysis on the relationships between the shear wave velocities (Vs), collected from geophysical surveys (down-hole), and the different lithotypes encountered in borehole logs. Examining the geophysical and lithological data, the Vs values show a significant variability according to the lithotype and are indicative in delineating the limits of geological change, in particular to reconstruct the surface that separates the generic sedimentary cover (Vs <800 m/s) from the seismic bedrock (Vs ≥ 800 m/s). This work is focused on the analysis of the relationships between the measured Vs values and the different lithotypes found in the geological survey stratigraphy. Different types of statistical analysis (Analysis of Variance and Multiple Post-Hoc Comparisons) were performed on the extracted data to define the range of variability of the Vs according of the lithology and the depth of sampling. The results obtained allowed us to group the different lithotypes (as defined in MS standard) in specific geological-technical categories with similar behavior (range of variability) of shear wave velocities. Finally, the geostatistical analysis techniques (such as ordinary and Bayesian Kriging) were applied in order to produce estimated maps of Vs30.