Seismic soil classification from a different perspective

The simplified seismic soil classification in many countries still basically relies on the Vs30 parameter as a proxy to the amplification factor. The pros and cons of this approach have already been extensively debated in the literature. We present an attempt to classify the subsoils in a seismic perspective which is based on expedite geophysical field procedures (passive/active single station and multichannel surveys, all based on the surface waves analysis) aimed at determining the resonance frequencies and the very shallow Vs profile. We will show how the same effort used to measure Vs30 can be used to overcome many limits of a Vs30-based classification and to get more representative classifications. In particular, this approach 1) implies accepting the concept that the site class is not a “universal” concept but is strictly linked to the type of structure which is planned to be built on that site, 2) implies a shift from the “depth-domain” concept (30 m or 20 m or 10 m suggested by different authors) to the “frequency-domain” concept and 3) does not take into account an “average” stiffness but the abrupt stiffness changes, which play a big role in seismic amplification. Last, the role of velocity inversions (i.e. negative Vs gradients) will also be considered from the “frequency-domain” point of view.