Modelling and assessment of earthquake ground response in areas characterised by a thick buried low-velocity layer

Abstract

It is known that local geology can alter the ground motion characteristics at a particular site. Whereas some sites are characterised by outcropping low-velocity layers, others might have buried ones of considerable thickness which might still be a cause of ground motion amplification. The Maltese islands (Central Mediterranean) provide a good case study for profiles with a buried low-velocity layer. Parts of the islands are characterised by Upper Coralline Limestone covering a soft Blue Clay layer of considerable thickness (up to 75 m). This gives rise to a velocity inversion and makes the travel-time average shear-wave velocity (VS) in the upper 30 m not suitable for seismic microzonation purposes. This study presents a comprehensive site response analysis for such areas. Previously obtained shear-wave velocity (VS) profiles were used as an input to the equivalent linear programme SHAKE2000. Maps of amplification factors and 5% damped elastic response spectra confirm that the clay, even when buried under a hard outcropping layer, can still produce significant amplifications at frequencies which are of engineering interest when considering the recent urbanisation patterns. Moreover, the suitability of the VS30 and the travel-time average shear-wave velocity till bedrock parameters as proxies for site response was investigated, with the results highlighting the inadequacy of these parameters for sites characterised by buried low-velocity layers and the necessity for site-specific response studies.