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summing-up
1What is an earthquake?
An earthquake is a more or less
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sometimes the fault line already
exists and is reactivated.
The point within the Earth at which
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the propagation of the seismic waves
begins is called the hypocentre (or
focus) of the earthquake. The
projection of this point on the
Earth’s surface is called the epicentre
of the earthquake.
Earthquakes occur almost
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exclusively within bands on the
Earth’s surface called seismic zones,
whilst areas where they do not occur
are called aseismic zones.
2Seismic waves
Seismic waves are classified
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the same direction as the wave
propagation;
–
S-waves (S for secundae, Latin for
second [to arrive]), which propagate
also at depth but cause the particles
of rock to oscillate in a direction that
is perpendicular to the wave
propagation;
–surface waves, which propagate
along the Earth’s surface from the
epicentre.
Seismic waves are recorded by very
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precise and sensitive tools; called
seismographs.
The wave trace produced by the
•
seismograph is recorded on paper or
in electronic form and is called a
seismogram.
A great deal of information can be
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obtained from the reading of a
seismogram (strength and duration
of an earthquake, distance from the
epicentre, the depth of the
hypocentre, etc.).
3Studying the Earth through
seismic waves
The study of the behaviour of seismic
•
waves that propagate in the interior
of the planet has revealed that:
–the speed of propagation of seismic
waves depends on the type of rocks
in which they propagate;
–consequently, seismic waves are
deflected when they travel between
diverse media;
–there is an area on the Earth’s surface
relative to an earthquake’s focus into
which P-waves do not travel directly,
this is called the P-wave shadow zone;
–S-waves cannot travel through the
entire planet, there is a portion
within which they cannot propagate.
Bringing together all this information
•
seismologists have hypothesised a
concentric shell structure for the
Earth:
–the crust, the outermost shell;
–then the mantle;
–and further inside the nucleus or
core (which in turn is divided into
the outer core and the inner core).
4Earthquake “strength”
The magnitude of an earthquake is
•
usually estimated by measuring the
maximum amplitude of vibration of
the ground as recorded on a
seismogram.
The value of the wave amplitude
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obtained provides a direct measure
of the “strength” of an earthquake,
called the magnitude, which is
measured against the Richter scale.
Before the introduction of the
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magnitude, the strength of an
earthquake was indicated only by its
intensity, which is determined
according to the effects of the
earthquake on people, buildings and
on the surrounding territory.
Reference scales have been devised
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to compare the effects of the same
earthquake in different places,
among which the most widely used is
the MCS scale, which is divided into
12 degrees (designed, together with
others, by the Italian scientist
Mercalli).
5The geographical
distribution of earthquakes
The epicentres of earthquakes that
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Seismic belts are associated with:
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–the large, deep oceanic trenches,
both the stretches where these run
close to a continent, and where they
are flanked by an arc of volcanic
islands;
–oceanic ridges;
–mountain ranges of more recent
formation, from the western
Mediterranean Sea to the Himalayas.
In Italy, shallow earthquakes occur
•
in a band that crosses the peninsula
more or less longitudinally. The
presence of hypocentres deep in the
south-eastern Tyrrhenian Sea have
also been identified.
intense vibration of the ground.
Earthquakes are caused by rocks
•
along a fault suddenly breaking,
releasing the elastic energy in the
form of violent vibrations that
propagate as seismic waves;
according to the type of deformation
they cause in the medium in which
they propagate, and according to
their speed.
• On the basis of these two criteria the
following are identified:
–
P-waves (P for primae, Latin for first
[to arrive]), which propagate at
depth and cause the “back and
forth” oscillation of rock particles in
occur every year on Earth are
regularly recorded and it is noted
that they are distributed along welldefined belts from a geographical
point of view.
Lupia Palmieri, Parotto Osservare e capire la Terra - edizione azzurra © Zanichelli 2012
unità 4•I fenomeni sismici
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summing-up
6Defence against
earthquakes
• Each year, the damage caused by
earthquakes is enormous, both in
human terms – the number of
victims they cause –and from an
economical point of view.
• Consequently, all interventions in
defence from earthquakes are of
great importance.
They include:
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–the prediction of location and
moment in which an earthquake will
occur, though not feasible for now;
–the prevention of seismic risk, which
includes the detailed analysis of the
seismic characteristics of a territory,
the construction of anti-seismic
buildings, and the preparation of the
population for seismic events; this
Lupia Palmieri, Parotto Osservare e capire la Terra - edizione azzurra © Zanichelli 2012
being the only defence currently
applicable.
Prevention is particularly important
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in Italy, a country with high seismic
risk.
unità 4•I fenomeni sismici
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