Attività del Dipartimento

Subsoil Imaging with Ambient Vibrations

Albarello Dario

The most part of information about the internal structure of the Earth is deduced from the analysis of propagation patterns of seismic waves generated by strong distant earthquakes. This knowledge, however is not detailed enough to allow the forecast of seismic ground motion expected at any site of the Earth surface when future potentially damaging earthquake will occur. In fact, experimental evidence indicates that, during earthquakes, strong differences in ground shaking may exist at the scale of few hundreds of meters that are responsible for dramatic differences in the observed damages. These are mainly the effect of small-scale (hundreds of meters) variations in the mechanical properties of geological bodies present in the shallow subsoil: detecting such features before earthquakes (seismic microzoning) is mandatory to evaluate local seismic hazard and orientate prevention activities. To this purpose, the analysis of seismic registration of distant earthquakes is ineffective due to their poor content of short wavelengths components (absorbed during the long range propagation) and the lack of densely distributed seismic recording tools. On the other hand, the use of invasive direct procedures (e.g., drilling) are hampered by high costs that prevent their general application. In this context, the analysis of ambient seismic vibrations may represent an interesting alternative. These small amplitude seismic waves are generated by both natural (sea waves, wind, etc.) and anthropic (traffic, industrial activities, etc.) sources. Being ubiquitous all over the Earth surface these vibrations may represent an important tool for subsurface imaging. These vibrations are inherently stochastic in nature and this requires the development of new interpretative models that are distant from the typical ballistic approach of classical seismology. The talk will shorty report the most recent theoretical and experimental findings in this field and the efforts to promote the application of this prospecting tool as a support for seismic risk reduction practice.