**1. Introduction**

It is presently well-established that earthquake ground shaking at a particular site is a function of a set of physical parameters and phenomena including the soil conditions beneath the site. Sites characterized by geological layers with a significant impedance contrast are prone to increasing the amplitude of ground motion, changing the frequency content and also lengthening the duration of the earthquake.

Various studies have investigated the effect of outcropping soft geological layers on earthquake ground motion both experimentally and theoretically [1–4]. However, whilst it is usually assumed that the shear-wave velocity (*V*S) increases with depth, soft geological layers with low-velocity "sandwiched" between higher velocity layers can also be present. Such situations create what is known as a velocity inversion.

The presence of a buried thick low-velocity layer is not uncommon, especially in sedimentary environments encompassing clay deposits. However, the effects of such stratigraphies on site effects are not often studied or documented. Moreover, seismic building codes such as the Eurocode 8 (EC8), suggest the use of the average shear-wave velocity in the upper 30 m (*V*S30) to classify sites in different categories. Hypothetically this approach is not viable for sites characterised by buried lowvelocity layers because the first 30 m could be characterised by high velocity geological material.

the islands characterised by buried clay and secondly, these profiles were used as an input to the equivalent-linear site response analysis programme SHAKE2000 to

*Assessing Seismic Site Response at Areas Characterized by a Thick Buried Low-Velocity Layer*

The Maltese archipelago, which consists of three main islands (Malta, Gozo and

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

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

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

**3. Derivation of shear-wave velocities in Maltese rocks using joint**

Twenty sites have been chosen for this investigation (14 in Malta, 5 in Gozo and 1 in Comino, shown in **Figure 1a**), all of which are characterised by the full sedimentary sequence i.e. the Blue Clay is embedded between the Upper Coralline Limestone above and the Globigerina Limestone below. Since the sites all have similar stratigraphy, any spatial geophysical variations within a particular stratum

At each site, single-station ambient noise measurements were conducted jointly with geophone array measurements. The sites were chosen not to have any major topographical slopes or irregularities so as to fulfil the 1-D assumption of the array methods. For reasons of clarity, the more detailed results in the next sub-sections are presented only for eight representative sites with a range of stratigraphical

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-

, was formed as marine sediments

determine the site response.

200 m [5].

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

*DOI: http://dx.doi.org/10.5772/intechopen.95277*

Comino) and covers an area of around 316 km<sup>2</sup>

Africa plate boundary that runs through Sicily.

Upper Coralline Limestone (UCL) (**Figure 1c**).

up of bioclastic limestones rich in the mineral glauconite.

**inversion of H/V and ESAC curves**

can also be investigated.

characteristics.

**143**

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

The research presented in this chapter was motivated by the following problems related to the Maltese islands:


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

#### **Figure 1.**

*(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.*

the islands characterised by buried clay and secondly, these profiles were used as an input to the equivalent-linear site response analysis programme SHAKE2000 to determine the site response.
