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Earthquakes - What Is The Underlying Causes?

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.



 

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