بحث حول الزلازل باللغة الانجليزية Earthquakes




An earthquake (also known
as a tremor or temblor) is the result of a sudden release of energy in
the Earth's crust that creates seismic waves. Earthquakes
are recorded with a seismometer, also known as a seismograph. The
moment magnitude of an earthquake is conventionally reported, or the
related and mostly obsolete Richter magnitude, with magnitude 3 or lower
Earthquakes
being mostly imperceptible and magnitude 7 causing serious damage over
large areas. Intensity of shaking is measured on the modified Mercalli
scale.
At the Earth's surface, Earthquakes
manifest themselves by shaking and sometimes displacing the ground.
When a large earthquake epicenter is located offshore, the seabed
sometimes suffers sufficient displacement to cause a tsunami. The
shaking in Earthquakes can also trigger landslides and occasionally volcanic activity.
In
its most generic sense, the word earthquake is used to describe any
seismic event—whether a natural phenomenon or an event caused by
humans—that generates seismic waves. Earthquakes
are caused mostly by rupture of geological faults, but also by volcanic
activity, landslides, mine blasts, and nuclear experiments. An
earthquake's point of initial rupture is called its focus or hypocenter.
The term epicenter refers to the point at ground level directly above
this.

Fault typesTectonic Earthquakes
will occur anywhere within the earth where there is sufficient stored
elastic strain energy to drive fracture propagation along a fault plane.
In the case of transform or convergent type plate boundaries, which
form the largest fault surfaces on earth, they will move past each other
smoothly and aseismically only if there are no irregularities or
asperities along the boundary that increase the frictional resistance.
Most boundaries do have such asperities and this leads to a form of
stick-slip behaviour. Once the boundary has locked, continued relative
motion between the plates leads to increasing stress and therefore,
stored strain energy in the volume around the fault surface. This
continues until the stress has risen sufficiently to break through the
asperity, suddenly allowing sliding over the locked portion of the
fault, releasing the stored energy. This energy is released as a
combination of radiated elastic strain seismic waves, frictional heating
of the fault surface, and cracking of the rock, thus causing an
earthquake. This process of gradual build-up of strain and stress
punctuated by occasional sudden earthquake failure is referred to as the
Elastic-rebound theory. It is estimated that only 10 percent or less of
an earthquake's total energy is radiated as seismic energy. Most of the
earthquake's energy is used to power the earthquake fracture growth or
is converted into heat generated by friction. Therefore, Earthquakes
lower the Earth's available elastic potential energy and raise its
temperature, though these changes are negligible compared to the
conductive and convective flow of heat out from the Earth's deep
interior.[1]

There
are three main types of fault that may cause an earthquake: normal,
reverse (thrust) and strike-slip. Normal and reverse faulting are
examples of dip-slip, where the displacement along the fault is in the
direction of dip and movement on them involves a vertical component.
Normal faults occur mainly in areas where the crust is being extended
such as a divergent boundary. Reverse faults occur in areas where the
crust is being shortened such as at a convergent boundary. Strike-slip
faults are steep structures where the two sides of the fault slip
horizontally past each other ; transform boundaries are a particular
type of strike-slip fault. Many Earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip.

A
volcano is an opening, or rupture, in a planet's surface or crust,
which allows hot, molten rock, ash, and gases to escape from below the
surface. Volcanic activity involving the extrusion of rock tends to form
mountains or features like mountains over a period of time.
Volcanoes
are generally found where tectonic plates are diverging or converging. A
mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of
volcanoes caused by "divergent tectonic plates" pulling apart; the
Pacific Ring of Fire has examples of volcanoes caused by "convergent
tectonic plates" coming together. By contrast, volcanoes are usually not
created where two tectonic plates slide past one another. Volcanoes can
also form where there is stretching and thinning of the Earth's crust
(called "non-hotspot intraplate volcanism"), such as in the African Rift
Valley, the Wells Gray-Clearwater volcanic field and the Rio Grande
Rift in North America and the European Rhine Graben with its Eifel
volcanoes.
Volcanoes can be
caused by "mantle plumes". These so-called "hotspots" , for example at
Hawaii, can occur far from plate boundaries. Hotspot volcanoes are also
found elsewhere in the solar system, especially on rocky planets and
moons.

At
the mid-oceanic ridges, two tectonic plates diverge from one another.
New oceanic crust is being formed by hot molten rock slowly cooling and
solidifying. The crust is very thin at mid-oceanic ridges due to the
pull of the tectonic plates. The release of pressure due to the thinning
of the crust leads to adiabatic expansion, and the partial melting of
the mantle causing volcanism and creating new oceanic crust. Most
divergent plate boundaries are at the bottom of the oceans, therefore
most volcanic activity is submarine, forming new seafloor. Black smokers
or deep sea vents are an example of this kind of volcanic activity.
Where the mid-oceanic ridge is above sea-level, volcanic islands are
formed, for example, Iceland.
Lava enters the Pacific at the Big Island of Hawaii

Subduction
zones are places where two plates, usually an oceanic plate and a
continental plate, collide. In this case, the oceanic plate subducts, or
submerges under the continental plate forming a deep ocean trench just
offshore. Water released from the subducting plate lowers the melting
temperature of the overlying mantle wedge, creating magma. This magma
tends to be very viscous due to its high silica content, so often does
not reach the surface and cools at depth. When it does reach the
surface, a volcano is formed. Typical examples for this kind of volcano
are Mount Etna and the volcanoes in the Pacific Ring of Fire.
Hotspots
Hotspots
are not usually located on the ridges of tectonic plates, but above
mantle plumes, where the convection of the Earth's mantle creates a
column of hot material that rises until it reaches the crust, which
tends to be thinner than in other areas of the Earth. The temperature of
the plume causes the crust to melt and form pipes, which can vent
magma. Because the tectonic plates move whereas the mantle plume remains
in the same place, each volcano becomes dormant after a while and a new
volcano is then formed as the plate shifts over the hotspot. The
Hawaiian Islands are thought to be formed in such a manner, as well as
the Snake River Plain, with the Yellowstone Caldera being the part of
the North American plate currently above the hotspot.
Indonesia - Lombok: Mount Rinjani - outbreak in 1994
Volcanic features
The
most common perception of a volcano is of a conical mountain, spewing
lava and poisonous gases from a crater at its summit. This describes
just one of many types of volcano, and the features of volcanoes are
much more complicated. The structure and behavior of volcanoes depends
on a number of factors. Some volcanoes have rugged peaks formed by lava
domes rather than a summit crater, whereas others present landscape
features such as massive plateaus. Vents that issue volcanic material
(lava, which is what magma is called once it has escaped to the surface,
and ash) and gases (mainly steam and magmatic gases) can be located
anywhere on the landform. Many of these vents give rise to smaller cones
such as Puʻu ʻŌʻō on a flank of Hawaii's Kīlauea.
Other
types of volcano include cryovolcanoes (or ice volcanoes), particularly
on some moons of Jupiter, Saturn and Neptune; and mud volcanoes, which
are formations often not associated with known magmatic activity. Active
mud volcanoes tend to involve temperatures much lower than those of
igneous volcanoes, except when a mud volcano is actually a vent of an
igneous volcano.

الموضوع الأصلي : بحث حول الزلازل باللغة الانجليزية Earthquakes  المصدر : منتدى تكنولوجيا العين الذهبية