VDP Srl

PROJECT DESCRIPTION

The Assignment was commissioned by the Department of Civil Protection, part of the Prime Minister Office. In particular, it was authorized by the Office for Evaluation, Prevention and Mitigation of seismic risk and activities and post-emergency works - Land monitoring services and management of databases (MOT) .
The Department manages the RAN (National Accelerometer Network or Italian Strong Motion Network), a dense network of accelerometer systems located throughout the country in the most seismically active areas.

PROJECT DESCRIPTION

The accelerometer stations are placed on publicly owned lands, giving priority to sites situated on non-fractured outcropping rocks.

The definition of the dynamic parameters of the lands, which has a significant importance in local seismic response studies, can be obtained through different techniques such as passive methods based on environmental seismic noise (microtremor). These measurements provide information about the relation between the average speed of S-waves and the substrate depth below the observation point, and they allow the identification of possible seismic resonance phenomena.

The HVSR (Horizontal to Vertical Spectral Ratios) methodology, better known as H/V ratio, is based on the determination of the average ratio between the spectral amplitudes of the horizontal and vertical components of the microtremors.

The resonance frequencies correspond to the maximum values of the function that represents the average spectral ratios as a function of the oscillation frequency.
The Department of Civil Protection provided the seismic instrumentation necessary for the environmental noise measurements, which consisted of 6 digital loggers equipped with metric velocity sensors with periods of 5 seconds and GPS timers. The measurements were performed on 157 accelerometer stations in Italy according to the specifications requested by the Department of Civil Protection.

The data loggers were suitably set in order to obtain a reliable and accurate sampling of the signal with a step of 8 ms (125 Hz) and with a recording time not less than 6 hours. During the subsequent data elaboration, the signal intervals affected by obvious anthropogenic noise were discarded. Afterward the FFT (Fast Fourier Transform) was calculated for all the remaining signal components leading to the smoothing procedure with a triangular window. Eventually, the average spectral ratios and their respective variances between the horizontal and vertical components were determined. Moreover the original registration data in the acquisition format (binary) and the ASCII files regarding the elaborated information were recorded on an optical disc along with the reports concerning the measurements performed at each accelerometer station.