Tracking Space and Time Changes of Physical Properties in Complex Geological Media

An important issue in seismology concerns the characterization of the propagation medium, aiming to analyze the behavior of rocks in relation to the generation of earthquakes (both natural and human-made). The basic idea is that seismic waves can be used to image the medium’s physical properties. In this context we placed our research project, concerning the reconstruction of the spatial and temporal changes of physical properties (velocity, attenuation, rock parameters) in complex geological media. In the first part of this thesis we present a detailed description of known and new methodologies useful to track the seismicity, the propagation medium’s features and their temporal variation. In particular, a new rock modelling approach is constructed, allowing the conversion of velocity and attenuation values in rock micro-parameters; and a new equalization procedure for the 4D tomography is developed, allowing at once to optimize the choice of time-windows in the case of massive data-sets and to completely handle seismic tomography issues. In the second part, we show the results obtained by applying this methodologies to three complex areas: the Irpinia fault zones, The Geysers geothermal area and the Solfatara volcano. The relevance of these three areas lies not only in their different physical nature, but also in their different dimension. The obtained results show how the described methodologies can be used in seismogenic and volcanic areas to improve the knowledge of the medium’s properties, in order to mitigate the risk associated to destructive events, and in geothermal areas, to monitor the induced seismicity through the tracking of the medium properties’ temporal variation. Therefore, this thesis represents a useful tool for the characterization of the propagation medium, by providing a compendium of different methodologies and by showing the results of their application to three complex areas characterized by different physical nature and dimensional scale.