S wave velocity and attenuation estimation by using seismic noise data and strong/weak motion recordings in boreholes

A critical element of hazard assessment is the local seismic response, defined as the variation in seismic motion occurring in the upper layers of the earth’s crust, determined by the soil properties that affect the propagation of seismic waves. The implications of this aspect are substantial in practice, as it can lead to varying levels of destruction in urban environments and to loss of human life. Past big earthquakes, such as the great California (USA) earthquake of 1906, the Niigata (Japan) earthquake occurred in 1964, and the 1985 Michoacán (Mexico) earthquake, have highlighted that targeted research is required to enhance the current knowledge base regarding site effects and related parameters. Whereas the shear-wave velocity (Vs) as well as fundamental resonance frequency (f0) have been widely investigated, less studies have been devoted to attenuation and its associated shear-wave quality factor (Qs), perhaps because their estimation from empirical data is more complicated. Arrays of ambient seismic noise have emerged as a cheap, practical, and non-invasive tool for local seismic response analysis, offering fairly precise values of Vs and, recently, also of Qs. In an attempt to improve current research on attenuation calculation, this dissertation introduces a novel linear inversion methodology to estimate Qs at the local scale based on seismic noise data obtained by arranging temporary seismological stations according to a surface array setup. This approach innovatively leverages the Simultaneous Algebraic Reconstruction Technique (SART) for an easy, rapid, and efficient computation of Qs in studies of local seismic response, particularly within urban settings. After performing reliability tests, the proposed approach is applied in a borehole testing site in northeast Italy, where Vs and Qs values from independent geophysical borehole measurements exist in the literature. Subsequently, at a second borehole site, the consistency between Vs and the factor Qs estimated using the novel approach based on SART, and the same parameters obtained from the analysis of weak motions recorded by seismometers installed within the borehole is evaluated. SART seems to hold great promise and lays the foundation for future advancement in robust estimations of Qs.