Characterisation of shallow sediments by processing of P, SH and SV wavefields in Kaštela (HR)

Abstract

A seismic study was conducted in a historical town in Croatia to characterize the shallow subsurface and evaluate the response to an earthquake for engineering purposes. The study involves three seismic lines acquired along the existing roads of the town. For each of the lines, we acquired vertical compressional P and two horizontally polarized, S-wavefields; SH and SV, respectively orthogonal and parallel to the direction of the seismic line. We processed the data from these three wavefields with both first-break tomography, to obtain wave-velocity profiles, and reflection seismic imaging, to obtain stacked sections, and integrated the results. From such analyses, we obtained an in-depth characterization of the shallow subsurface. Specifically, we obtained standard engineering parameters like the equivalent shear-wave velocity of the upper 30 m of the subsurface (VS30), which indicates A-class soil, and compressional to shear-wave velocity ratio (VP/VS), which gives an indication regarding the presence of fractures in the rock. In addition to this, we evaluated the presence of anisotropy thanks to the SH- and SV-wave tomography inversions, which allowed to notice that VSV > VSH on most of the area. The presence of anisotropy is consistent with the known geological features of the area, in particular the subvertical bedding of the flysch, of which we were able to estimate the bedding plane orientations (dip and strike angles). Finally, by superimposing the stacked sections obtained from reflection seismic imaging with the velocity profiles computed with the traveltime tomography, we confirmed the reliability of the tomographic velocity models.