**2. The geology of the Maltese islands**

Hypothetically this approach is not viable for sites characterised by buried lowvelocity layers because the first 30 m could be characterised by high velocity

covering a clay layer which can be up to 75 m thick [6].

• the lack of site response studies on a national scale;

• the *V*S30 might not be the right proxy for site response;

• the lack of a national annex for the Eurocode8.

The Maltese islands (Central Mediterranean) present the perfect case for studying the effect of buried low-velocity layers. The islands are characterised by a fourlayer sequence of limestones and clays [5] (**Figure 1**). While the eastern half of the archipelago is characterised by limestone layers, the western half exhibits limestone plateaux and hillcaps, ranging in thickness from thickness from 2 m up 162 m,

The research presented in this chapter was motivated by the following problems

• the lack of information about average *V*<sup>S</sup> values of different lithotypes making

• the lack of knowledge about the effect of the buried clay on site response;

Even though this study is based and intended for the Maltese islands, results from it can still shed light on the effect of low-velocity layers in any context globally. To tackle the above-mentioned issues, a two-fold process was taken: firstly *V*<sup>S</sup> profiles were obtained using ambient-noise techniques at various sites around

*(a) A geological map of the Maltese islands with the location of the studied sites; (b) the location of the Maltese islands in the Central Mediterranean; (c) schematic showing the geological formations of the Maltese islands.*

geological material.

**Figure 1.**

**142**

related to the Maltese islands:

*Earthquakes - From Tectonics to Buildings*

up the Maltese islands;

The Maltese archipelago, which consists of three main islands (Malta, Gozo and Comino) and covers an area of around 316 km<sup>2</sup> , was formed as marine sediments during the Oligocene and Miocene epochs. The islands lie in the Sicily Channel (**Figure 1b**) on a relatively stable plateau of the African foreland, known as the Pelagian Platform, about 200 km south of the convergent segment of the Europe-Africa plate boundary that runs through Sicily.

Geologically, the islands are made up of four main strata of lime-rich sedimentary rocks, with the composition and texture of each layer depending among other things, on the grain size of the sediment and depth of deposition [7]. Starting from the oldest and the bottom-most layer, the formations are: the Lower Coralline Limestone (LCL), the Globigerina Limestone (GL), the Blue Clay (BC) and the Upper Coralline Limestone (UCL) (**Figure 1c**).

The compact LCL forms most of Malta's southern and south-western coastline along with some inland outcrops associated with faults. The base of the LCL cannot be seen above sea-level and it is exposed through a thickness of up to 140 m. It is non-homogeneous and composed of five different 'facies' according to depositional environment of the sediments [7, 8]. The GL covers large areas of central and southern Malta and Gozo [9]. It is a chalky and soft yellowish fine-grained limestone, which is further subdivided into three layers separated by two thin hardground conglomerate layers. Its thickness can vary from as little as 20 m to over 200 m [5].

The BC layer is the softest in the layer package making it easily erodible. It is mostly found beneath the UCL which is the youngest of the layers. The latter can have variable characteristics, ranging from fractured and friable to highly compact. These two formations are absent in the central and eastern parts of Malta, whereas the western half of Malta and some areas in Gozo retain the full sedimentary sequence. In limited areas, on the uppermost part of the BC layer, one can find a thin layer (between 1 m and 11 m), known as Greensand Formation which is made up of bioclastic limestones rich in the mineral glauconite.
