Geophysical investigations for modelling of earthquake ground shaking in the Maltese islands

Abstract

The Maltese islands have suffered damage from earthquakes in the historical past. Over the last few decades, the building density has increased dramatically, with building heights of more than five storeys becoming increasingly common. The urban development has even spilled out on to geologically more vulnerable areas. To date, no building codes are officially in force in the Maltese islands, and the National Annex for the EC8 guidelines is still in the process of being drafted. This study investigates the ground response of the islands to potential earthquake shaking, in particular on those areas characterised by a layer of clays and marls (up to 75m thick in places) embedded in the stratigraphy. The velocity inversion introduced by this layer makes the average shear-wave velocity in the uppermost 30 m (VS30), not always suitable for seismic microzonation purposes. Such a layer may still produce amplification effects, however would not contribute to the numerical mean of shearwave velocity (VS) in the upper 30 m. For this purpose, and for investigating possible local site effects over the islands, the first investigation was the establishment of shear-wave velocity profiles with a focus on areas characterised by a buried layer of clay. This was carried out through the inversion of surface wave dispersion data, using multi-station Extended Spatial Auto-Correlation (ESAC) together with single-station Horizontal-to-Vertical Spectral Ratio (H/V). Geophone arrays were set up in an L-shaped configuration at 27 sites and the effective Rayleigh wave dispersion curves extracted. At all sites characterised by the buried clay layer, the dispersion curve exhibited a normal dispersive trend at low frequencies, followed by an inverse dispersive trend at high frequencies. This lithological sequence also gave rise to a clear H/V peak that was observed consistently at all such sites in a narrow frequency range between 1 and 2 Hz. From the joint inversion procedure, the VS in the Blue Clay was found to vary between 350 and 580 m s−1, generally depending on the overburden thickness. A considerable variation in the VS within the Upper Coralline Limestone was also observed; related to the geological variety of regional facies within this formation, ranging from highly fractured (VS = 550 m s−1) to very competent (VS = 1100 m s−1). The spectral response to earthquake shaking of a surface sedimentary layer package with a measured VS profile was modelled through the equivalent linear programme SHAKE2000. The input earthquake time series was obtained from an appropriate selection of real seismograms, conformant with the local seismic hazard. 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. As expected, sites where the clay outcrops, or lies at shallow depth below the surface, are particularly vulnerable. The amplification potential of the clay layer was also demonstrated directly through the Standard Spectral Ratio (SSR) method using a bedrock reference site.