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Understand
35.
Find out if an earthquake
with a magnitude of 6
is twice as strong as one
with a magnitude of 3. If
not, how much stronger
is it?
36.
Explain how the following
factors will affect personal
and material damage
causedbyanearthquake:
a)
Focal depth of the
earthquake
b)
Day and time that it
occurs
c)
Type of substratum (hard
rock or poorly consolidated
sediment)
d)
Type of buildings
5.
SEISMIC AND VOLCANIC RISK
The risk not only depends on the intensity and the frequency of the phenomenon,
but also on the density and size of the population and the type of infrastructures
in the place affected.
So, in an uninhabited area, the risk is zero. However, in an area with medium
danger but a very high population density and poorly designed infrastructures,
the risk can be the similar to an area with high seismic danger that has buildings
designed to resist earthquakes.
Earthquakes
and explosive
volcanic eruptions
are the internal geological
phenomena that cause the most victims and material damage.
5.1.
Seismic risk
Every year there are close to a million earthquakes; about 150000 are felt by
the population and the rest only by seismographs. To evaluate the strength of
earthquakes, there are two types of measurements: magnitude and intensity.
❚❚
The
magnitude
of an earthquake measures the energy released in the hypocentre.
To express this, we use the
Richter scale
, which is exponential. For each unit that
goes up on the scale, the energy released is multiplied by 30. For example, an
earthquake with a magnitude of 6 on the Richter scale releases 30 times more
energy than one with a magnitude 5.
❚❚
The
intensity
evaluates the damage caused by the earthquake in a certain area.
Nowadays, it is measured on a European macroseismic scale EMS-98, that has
twelve levels, from1 (‘not felt’) to 12 (‘completely devastating’).
An earthquake will only have one value of magnitude, but its intensity will vary
depending on the area where it is measured: the maximum is found in the epicentre
and decreases further away from it.
The main risk of earthquakes is the movement of the ground and the collapse
of buildings, as occurred in 1976 in Tangshan, China, where more than half a
million people died. Other historic examples of the effects of an earthquake are
the following:
Geological risks
are threats originating from geological processes that cause
damage to the lives of human beings, their belongings
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and buildings.
Landslides
of mud and rocks like the
one that buried the Peruvian city of
Yungay in 1970, after the earthquake
of Ancash.
Fires
that burn out of control because
underground gas and water pipes are
destroyed, like the earthquake that
completely destroyed San Francisco in 1906.
Tsunamis
are caused by large earthquakes
that affect the ocean floor. The most
recent and devastating tsunamis occurred
in Indonesia (2004) and in Japan (2011).
Risk =
danger
×
vulnerability
Value of the damage that people, or
their belongings, can suffer as a
consequence of the phenomenon.
Probability that the phenomenon
happens in this place at a certain
time.
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belongings
:
things that a person
owns
Key structure
conjunction
not only… but
also
:
The risk not only depends on…
but also on…
