Modelling of ambient noise HVSR in a complex geological area: case study of the Xemxija bay area, Malta

This study will address the component of seismic risk arising from local site amplification, which is the result of the particular local sedimentary geology. Ambient noise measurements are used to infer shallow shear wave velocity structure, for which no systematic data exists on the islands. A dense microtremor measurement survey was carried out at Xemxija, using a Tromino® tromograph (Micromed SpA), with about 100 microtremor recordings performed at random positions, as well as along predetermined profiles. The time series obtained were processed using GEOPSY and Grilla® in order to compute the HVSNR (Nakamura, 1989). Whereas the overlying soil layer in the valley is clearly responsible for the amplification, the expected response is not so obvious on the hill sides and top, where the hard Upper Coralline is outcropping, at first sight implying that there should be no significant spectral ratio peaks. In fact, HVSNR curves obtained from these areas all display a clear resonance peak of amplitude greater than 2 in most cases. These resultant fundamental peaks can be generally associated with the Blue Clay layer underlying the UCL. HVSNR data using ambient noise has been shown to be a useful tool in microzonation studies. Preliminary results shows the complexity and variability of the site’s geology. The soil thickness within the valley, increasing towards the centre of the basin, is clearly reflected in the variation of resonance frequency across the valley, while the depth of the buried clay layer appears to have an effect on the amplitude value.