Earth's outer layer is broken into
pieces called tectonic plates which are about 100km thick and are constantly
moving towards, away from or past each other. . Plates are thick slabs of rock
that make up the outermost 100 kilometers or so of the Earth. Geologists use
the term "tectonics" to describe deformation of the Earth's crust,
the forces producing such deformation, and the geologic and structural features
that result.
Earthquakes occur only in the outer,
brittle portions of these plates, where temperatures in the rock are relatively
low. Deep in the Earth's interior, convection of the rocks, caused by
temperature variations in the Earth, induces stresses that result in movement
of the overlying plates. The rates of plate movements range from about 2 to 12
centimeters per year and can now be measured by precise surveying techniques. For
example, the plate containing Australia and India is moving north at the rate
of 7cm a year, causing an intracontinental collision with the Eurasian Plate in
the Himalayas. That is why these mountains are so high. Because continents are
part of these plates, they also move.
Three major types of plate
boundaries are recognized. These are called spreading, convergent, or
transform, depending on whether the plates move away from, toward, or laterally
past one another, respectively. Subduction occurs where one plate converges
toward another plate, moves beneath it, and plunges as much as several hundred
kilometers into the Earth's interior. The Juan de Fuca plate off the coasts of Washington and Oregon is subducting
beneath North America. Most earthquakes occur on the boundaries between plates,
where one plate is forced under another such as happens off island chains such
as Japan, Indonesia or the Solomon Islands, or past another as occurs in
California and New Zealand. Some regions have more earthquakes than others with
80 per cent of all recorded earthquakes taking place around the edge of the
Pacific Plate, including New Zealand, Papua New Guinea, the Solomon Islands,
Vanuatu, Japan, Canada, USA and South America.
An earthquake is the shaking
of the ground caused by an abrupt shift of rock along a fracture in the Earth,
called a fault. An earthquake occurs when the rocks break
and move as a result of stresses caused by plate movements.The stresses from
convection can also deform the brittle portions of overlying plates, thereby
storing tremendous energy within the plates. If the accumulating stress exceeds
the strength of the rocks comprising these brittle zones, the rocks can break
suddenly, releasing the stored elastic energy as an earthquake.This sudden release of energy causes the seismic
waves that make the ground shake. Within seconds, an earthquake releases stress
that has slowly accumulated within the rock, sometimes over hundreds of years
When two blocks of rock or two plates are rubbing against each other, they
stick a little. They don't just slide smoothly; the rocks catch on each other.
The rocks are still pushing against each other, but not moving. After a while,
the rocks break because of all the pressure that's built up. When the rocks
break, the earthquake occurs. During the earthquake and afterward, the plates
or blocks of rock start moving, and they continue to move until they get stuck
again. The spot underground where the rock breaks is called the focus of
the earthquake. The place right above the focus (on top of the ground) is
called the epicenter of the earthquake.
Earthquake-like seismic waves can
also be caused by explosions underground. These explosions may be set off to
break rock while making tunnels for roads, railroads, subways, or mines. These
explosions, however, don't cause very strong seismic waves. You may not even
feel them. Sometimes seismic waves occur when the roof or walls of a mine
collapse. These can sometimes be felt by people near the mine. The largest
underground explosions, from tests of nuclear warheads (bombs), can create
seismic waves very much like large earthquakes. This fact has been exploited as
a means to enforce the global nuclear test ban, because no nuclear warhead can
be detonated on earth without producing such seismic waves.
The size of an earthquake is
indicated by a number called its magnitude. Magnitude is calculated from
a measurement of either the amplitude or the duration of specific types
of recorded seismic waves. Magnitude is determined from measurements
made from seismograms and not on reports of shaking or interpretations
of building damage. ... The intensity of and earthquake is a measure of
the amount of ground shaking at a particular site, and it is determined from
reports of human reaction to shaking, damage done to structures, and other
effects.
Intraplate
Earthquakes
Earthquakes that do not occur on
plate margins are called intraplate earthquakes. All earthquakes on mainland
Australia and Tasmania are intraplate. On studying these intraplate earthquakes
in various continents, seismologists have found that most of them are caused by
thrust faulting due to the rocks being squeezed or compressed. It seems that
the movement of the tectonic plates causes the rocks away from their margins to
be compressed. Intraplate earthquakes are not as common as those on plate
margins, but major earthquakes with magnitudes of 7.0 or more do happen
occasionally.
Volcanic Earthquakes
Molten rock, called magma, is stored
in reservoirs under volcanoes. As this magma moves upwards, it can fracture the
rock it squeezes through, causing earthquakes, usually with magnitudes not much
greater than 5.0. Sometimes the magma collects in a high level reservoir prior
to a volcanic eruption and as it moves around it causes bursts of continuous
vibration, called volcanic tremor. Because of these precursors, seismographs
(earthquake recorders) are very useful for monitoring volcanoes to give warning
of an impending eruption.
Foreshocks and Aftershocks
Foreshocks are smaller earthquakes
that may occur in the same area as a larger earthquake that follows. They are
caused by minor fracturing of rocks under stress prior to the main break that
happens during the largest earthquake of the series, called the mainshock.
Foreshocks can start up to a year before the mainshock, as was the case before
the three large (magnitudes between 6.3 and 6.7) earthquakes near Tennant Creek
in January 1988. Not all earthquakes have foreshocks, and sometimes a series of
similar sized earthquakes, called an earthquake swarm, happens over months
without being followed by a significantly larger mainshock. This limits the
usefulness, at this stage, of foreshocks in earthquake prediction.
Aftershocks are smaller earthquakes
that may occur after the mainshock, in the same area. They are caused by the
mainshock area readjusting to the fault movement, and some may be the result of
continuing movement along the same fault. The largest aftershocks are usually
at least half a magnitude unit smaller than the mainshock and the aftershock
sequence may continue for months or years after the mainshock. Not all
earthquakes have aftershocks – the magnitude 5.6 Newcastle earthquake in 1989
only had one aftershock, which was very small with a magnitude of 2.1.
Occasionally, small earthquakes with magnitudes between 3.0 and 3.5 have
aftershocks. This has been observed in the Dalton-Gunning area, north of
Canberra.
No comments