Site characterisation and response study in Rabat, Malta

Abstract

The investigation of local ground conditions is an important part of seismic hazard assessment (Fäh et al., 2003). Local geology can greatly alter the seismic waves from earthquakes by amplifying their amplitude, changing the frequency content and increasing the shaking duration during an earthquake (Kramer, 1996). Sedimentary structures hosting dense settlements are likely to suffer from heavy damage, even though they can be situated away from the epicentre of the earthquake (Zor et al., 2010). The main parameters responsible for such effects are the shear-wave velocity (VS) structure and thickness of the sedimentary cover, the impedance contrast between the soft sediments and the underlying bedrock as well as the geometry of their interface (Parolai et al., 2006). Site response studies contribute to earthquake-hazard mitigation strategies such as seismic risk assessments, emergency response-preparedness and land use planning by considering existing and proposed buildings (Zor et al., 2010). Since 1530, an earthquake intensity of VII on the European Macroseismic Scale (EMS-98) scale was experienced on the Maltese archipelago at least four times, with the major source of seismic hazard being the northern segment of the Malta Escarpment. Earthquake activity can also be attributed to active fault zones in the Sicily Channel and the Hellenic Arc. Even though the latter is situated relatively far away from the islands, an earthquake in 1856, with an epicentral location around 1000 km away from the islands, caused significant damage to buildings, with many houses suffering serious cracks to their walls (Galea, 2007). The public perception about seismic risk remains one of negligence and complacency and up to date, no comprehensive seismic site response study has been done on the islands. This study is the first of a series of site response analyses which are to be carried out. It is divided in two parts. Firstly, a series of ambient noise measurements were done at a site in Rabat (Malta) to investigate and evaluate different techniques for estimating one-dimensional shear-wave velocity profiles. The chosen site is characterised by outcropping Blue Clay overlying the harder Globigerina Limestone. The data from the first investigations then serve as input to the equivalent-linear analysis programme SHAKE2000 (Ordonez, 2002) which is used for the site response analysis. In this research work, some advantages and limitations of chosen surface-wave techniques are also assessed. Moreover, any difference between equivalent profiles (satisfying the same experimental data) in site response results is investigated.