Ambient noise measurements to support emergency seismic microzonation: the Abruzzo 2009 earthquake experience

After the 2009 Abruzzo earthquake (Italy), a surface geophysical survey was undertaken to support emergency microzonation studies. The technique most used was the Horizontal-to-Vertical Spectral Ratio (HVSR) applied to ambient seismic noise. More that 200 single-station measurements were performed by a group that comprised also of volunteer professionals over a two-week time span. The availability of such a large database, which is homogenous both from the instrumentation and the processing point of view, allowed us to compare the results with other expeditious techniques for microzonation. The main findings of our study reveal that: 1) the HVSR results obtained from seismic noise measurements are well correlated with the ones obtained by strong motion recordings; 2) available geological maps, even in a digital form, are unable to correlate with the observed presence/absence of amplification revealed by seismic measurements; 3) the simple lithological classification of outcrops, however detailed, cannot be used as a standalone tool to identify the presence/absence of seismic amplification phenomena. Surface geological data are not sufficient. Borehole and geophysical data are also needed to identify the seismic bedrock and provide estimates of the VS average value in the sedimentary cover.