Abstract
Detailed knowledge of the 3D basin structure underlying urban areas is of major importance for improving the assessment of seismic hazard and risk. However, mapping the major features of the shallow geological layers might become expensive where large areas must be covered. In this study, we propose an innovative tool, based mainly on single station noise recordings and the horizontal-to-vertical-spectral ratio (H/V), to identify and locate the depth of major impedance contrasts. The method is based on an identification of fingerprints of the major impedance discontinuities and their migration to depth by means of an analytical procedure. The method is applied to seismic noise recordings collected in the city of Almaty (Kazakhstan). The estimated impedance contrasts versus depth profiles are interpolated in order to derive a three-dimensional (3D) model, which after calibration with some available boreholes data allows the major tectonic features in the subsurface to be identified.
Highlights
The use of seismic noise for investigating the subsurface structure has already been done for several decades
In order to highlight the peaks in the horizontal-to-vertical spectral ratio (H/V) that are expected to indicate the presence of impedance contrasts at depth, we propose a procedure similar to that introduced by Wüster (1993) and applied by Parolai et al (2002b) for identifying particular features in sonograms
In order to interpret the identified features, a calibration of the depth migrated H/V ratios was carried out using the available data from four deep boreholes in the area (Silacheva, 2002)
Summary
The use of seismic noise for investigating the subsurface structure has already been done for several decades. Methods involving seismic noise spectra (Kanai, 1957), the spectral ratio (Field and Jacob, 1993), and the horizontal-to-vertical spectral ratio (H/V) (Nakamura, 1989; Bard, 1999) have been proposed as tools for providing information about the S-wave velocity structure below a site. The derivation of the S-wave velocity structure is mainly of importance in areas where seismic hazard is of concern, since the frequency-dependent modification of the Swave amplitude, generally described as site effects, contributes to the impact of a seismic event. The undertaking of site response studies is a crucial step toward preparing for future events, namely the mitigation of earthquake risk and the definition of the optimal engineering designs for civil structures in different geological settings.
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