Earthquakes typically result from the movement of faults(fallas),
quasi-planar(casi
planas) zones of deformation within(dentro
de) its uppermost layers(capas
más superiores).
The word earthquake is also widely used to indicate the source region
itself. The solid earth is in slow but constant motion(movimiento)
(see plate tectonics(tectónica
de placas)) and earthquakes occur where the resulting stress(tensión)
exceeds the capacity of Earth materials to support it. This condition is
most often found(se
encuentra sobre todo) at (and the resulting frequent occurrence
of earthquakes is used to define) the boundaries(límites)
of the tectonic plates into which(dentro
de las cuales) the Earth's lithosphere can be divided. Events
that occur at plate boundaries are called interplate earthquakes(terremotos
entre placas); the less frequent events that occur in the
interior of the lithospheric plates are called intraplate earthquakes.
Earthquakes occur every day on Earth, but the vast majority of them are
minor(pequeñas,
insignifcantes) and cause no damage(daños
físicos). Large earthquakes can cause serious destruction and
massive loss(pérdida)
of life via a variety of agents of damage including fault rupture(ruptura
de falla), vibratory ground motion(movimiento
vibratorio de la tierra) (i.e., shaking), inundation (e.g.,
tsunami, seiche, dam failure(fallos
en presas o diques)), various kinds of permanent ground failure
(e.g. liquefaction(licuefacción),
landslide(desprendimiento)),
and fire or hazardous materials release(liberación
de materiales de riesgo). In a particular earthquake, any of
these agents of damage can dominate, and historically each(cada,
cada uno) has caused major damage and great loss of life, but
for most earthquakes shaking(sacudida)
is the dominant and most widespread(extendido)
cause of damage.
Most large earthquakes are accompanied(seguidos,
acompañados)
by other, smaller ones, known as
foreshocks(temblores
iniciales) when they occur before the principal or mainshock(temblors
principales) and aftershocks(temblores
finales)
when they occur following it. The source of an earthquake is distributed
over a significant area -- in the case of the very largest(los
más grandes)
earthquakes, in
excess of a(más
allá de)
thousand kilometres -- but it is usually possible to identify a
point from which the earthquake waves appear to emanate. That point is
called its "focus" and usually proves to be the point at which fault rupture
was initiated. The position of the focus is known as the "hypocentre" and
the location on the surface directly above it is the "epicenter."
Earthquakes, especially those that occur beneath(debajo
de)
sea- or ocean-covered
areas, can give rise(dar
origen)
to tsunamis(olas
gigantescas), either(o...)
as a direct result of the
deformation of the sea bed(fondo
del mar) due to(debido
a)
the earthquake, or(...o) as a result of
submarine landslips(deslizamiento
de tierra)
or "slides" indirectly triggered by it.
A class of earthquakes known silent earthquakes(terremotos
silenciosos)
are thought to be caused by
very slow slippage(deslizamiento). They are of extremely low intensity but can last(durar)
for
days or weeks releasing(liberando)
as much energy as large earthquakes.
In the 1930s, a California seismologist named Charles F. Richter devised(ideó,
diseñó) a
simple numerical scale (which he called the magnitude) to describe the
relative sizes(tamaños
relativos) of earthquakes, which has come to be called(ha
venido a llamarse)
the Richter
scale. Since Richter, seismologists have developed a number of magnitude
scales. Most of the scales in use in the Western world are mutually
consistent to a sufficient extent(hasta
cierto punto)
that the term "Richter scale" is routinely
used in reporting these numbers to the public. Other scales (and other ways
of describing the size of earthquakes) are used in some non-Western
countries, and by earthquake specialists. The press sometimes mistakenly reports(informa
por error)
such values as "Richter magnitude", and this has given rise to
public confusion.
Earthquake effects are described in terms of Intensity, a scale which
attempts(trata de,
intenta)
to quantify the severity of shaking at a given location. A number
of intensity scales are in use, and there is a significant degree of
consistency amongst(entre) them. The best known is the Mercalli (or Modified
Mercalli, MM) scale, but the more consistent and analytical European
Macroseismic Scale (EMS) is now increasingly widely used. In Japan the Japan
Meterological Agency seismic intensity scale (JMA) is used.
Some earthquakes are caused by the movement of magma in volcanoes, and such quakes(tales
seísmos) can be an early warning(primer,
temprano aviso) of volcanic eruptions. A rare few earthquakes
have been associated with the build-up(aumento,
desarrollo) of large masses of water behind dams(diques),
such as the Kariba Dam in Zambia, Africa, and with the injection or
extraction of fluids from the Earth's crust(corteza) (Rocky Mountain Arsenal). Such
earthquakes occur because the strength(resistencia,
fuerza) of the Earth's crust can be modified
by fluid pressure. Finally, earthquakes (in a broad sense) can also result
from the detonation of explosives.
Thus Western scientists have been able to monitor, using the tools of
seismology, nuclear weapons tests performed by governments that were not
disclosing information on these tests along normal channels.
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