Site frequency response characterisation of the Maltese islands based on ambient noise H/V ratios

The Maltese islands in the Central Mediterranean are composed of a simple 4-layer sedimentary sequence of Oligocene–Miocene limestones and clays, highly disturbed by synsedimentary to recent tectonic activity, so that the surface outcrops present a variety of lithologies, as well as complex geomorphology. The seismic history of the islands includes a number of damaging earthquakes, the largest intensity since 1500 being EMS-98 VII–VIII in 1693. In this study a first seismic site response survey was undertaken over the whole archipelago, by recording ambient noise and applying the H/V (Horizontal-to-Vertical Spectral Ratio) technique. The results show a clear correlation between site response and outcrop geology. In general, where the two oldest layers, the Lower Coralline Limestone and the Globigerina Limestone outcrop, the H/V is flat above 0.5 Hz. A thick layer of clays and marls (Blue Clay) deposited originally on top of the Globigerina Limestone is preserved in the western half of the archipelago, and this strongly influences the site response. Where the clay outcrops at the surface, it produces the highest amplitudes, and peak frequencies ranging between 2 and 5 Hz, whereas all sites on the hard Upper Coralline Limestone, which overlies the clay in many regions, still exhibit H/V peaks at a consistent frequency between 1 and 2 Hz, which we attribute to the presence of the underlying clay. Results were validated by a second measurement survey and numerical modelling. The site effect induced by the clay layer has implications for the assessment of seismic risk, as well as for the validity of using VS30 as a proxy for site class and site amplification.