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directions and causing ground movement in the areas around the fault. Given the appropriate
geological conditions, earthquake motions can be felt (and even cause losses) in areas located
several hundreds of kilometres away from the initial rupture. The point on the fault where
rupture initiates is called focus or hypocentre, while its projection on the earth surface is
called epicentre; the distance between these two points is called focal depth.
In the 1960s the mechanism of strain accumulation at faults was understood and the theory
of plate tectonics was developed, whereby the lithosphere (i.e. the upper part (or shell) of the
earth) including the crust as well as part of the mantle, consists of several discrete segments,
called plates, which move with respect to each other at the rate of a few centimetres a year;
this relative movement is caused by convection currents in the mantle of the earth. The six
main tectonic plates, as well as other smaller ones, are shown in Figure 4.1; note that some
continents are on a single plate, whereas others straddle more than one plate. The plate
boundaries can be either divergent (sea floor spreading at mid-ocean ridges), or convergent;
particularly important in the latter case is the phenomenon of subduction (i.e. when a plate is
pushed below the neighbouring plate). As seen in Figure 4.1, that depicts the distribution of
epicentres of recent (1960–2000) earthquakes, the most serious tectonic activity takes place at
the boundaries of the plates (different size and colour of circles correspond to different
magnitude and focal depth). Earthquakes occurring close to the plate boundaries are called
interplate events, while earthquakes remote from the boundaries are referred to as intraplate
events; the latter are far less common and much more difficult to explain than the former
(Bolt, 1993; Reiter, 1991).
Although earthquakes can be triggered by other phenomena, such as volcanic eruptions,
sudden changes in the stress state of soil layers due to filling of reservoirs behind dams,
‘mine-burst’ (masses of rock collapsing explosively in mines), or even underground nuclear
explosions (Bolt, 1993), the vast majority of them are due to faulting. There are essentially
two types of faults, those associated with horizontal movement (strike-slip), and those
associated with vertical movement (dip-slip). Fault orientations have a strong effect on the
resulting earthquake motion; for instance, reverse dip-slip faults are usually the ones
associated with the most catastrophic ground motions.
As mentioned previously, fault rupture gives rise to seismic waves. These propagate either
by compression and dilation (like sound waves), with the ground particle motion in the same
direction as the propagation, and are called longitudinal or P-waves, or by shear (particle
motion perpendicular to the direction of the propagation), and are called transverse or S-
waves; these two types of waves are referred to as body waves. The velocity of shear waves is
given by

(4.1)

where G is the shear modulus of the ground and ρits mass density; v is a very useful
s