Preliminary results on the seismic properties of main lithotypes outcropping on Malta

A quick characterization of the seismic properties of main lithotypes outcropping in Malta is performed as well as the evaluate of the general features of the local seismic response. The litho-stratigraphic sequence is relatively simple consisting of five major layers namely the Lower Coralline Limestone (LCL), the Globigerina Limestone (GL), the Blue Clay (BC), the Greensand Formation and the Upper Coralline Limestone (UCL). The Upper Coralline, Globigerina and Lower Coralline strata are essentially considered as being stiff rocks, whereas the Blue Clay layer and the Greensand layer are softer (Pedley, 1978; 2002). A total of ten measurements, founded on non-invasive seismic prospecting techniques that use the vertical component of surface waves (MASW and ReMi), were carried out. The site response was evaluated using ambient noise records, processing the signals through spectral ratio techniques (HVNR). The results show that lower shear wave velocities (between 150 and 450 m/s) are observed in soft sediments and detritus with respect to limestone, in which the velocity rises to about 700 m/s. The limestone formations show different HVNR features according to their position in the litho-stratigraphic sequence. The measurements performed m the GL depict a flat spectral ratio whereas, the UCL sites are characterized by spectral ratio peaks in the frequency range 1.0-2.0 Hz that could be related to the presence of a BC layer between the UCL and GL formations. We can however assert that the GL formation can be considered as the local bedrock while major amplifications occur on soft soils that cover it.