Inversion of surface wave data for subsurface shear wave velocity profiles characterized by a thick buried low-velocity layer

Abstract

The islands composing the Maltese archipelago (Central Mediterranean) are characterized bya four-layer sequence of limestones and clays. A common feature found in the western halfof the archipelago is Upper Coralline Limestone (UCL) plateaus and hillcaps covering a softBlue Clay (BC) layer which can be up to 75 m thick. The BC layer introduces a velocityinversion in the stratigraphy, implying that the VS30 (traveltime average sear wave velocity(VS) in the upper 30 m) parameter is not always suitable for seismic microzonation purposes.Such a layer may produce amplification effects, however might not be included in the VS30calculations. In this investigation, VS profiles at seven sites characterized by such a lithologicalsequence are obtained by a joint inversion of the single-station Horizontal-to-Vertical SpectralRatios (H/V or HVSR) and effective dispersion curves from array measurements analysedusing the Extended Spatial Auto-Correlation technique. The lithological sequence gives riseto a ubiquitous H/V peak between 1 and 2 Hz. All the effective dispersion curves obtainedexhibit a 'normal' dispersive trend at low frequencies, followed by an inverse dispersive trendat higher frequencies. This shape is tentatively explained in terms of the presence of highermode Rayleigh waves, which are commonly present in such scenarios. Comparisons madewith the results obtained at the only site in Malta where the BC is missing below the UCLsuggest that the characteristics observed at the other seven sites are due to the presence ofthe soft layer. The final profiles reveal a variation in the VS of the clay layer with respect tothe depth of burial and some regional variations in the UCL layer. This study presents a steptowards a holistic seismic risk assessment that includes the implications on the site effectsinduced by the buried clay layer. Such assessments have not yet been done for Malta.