TYPES OF EARTHQUAKES

Earthquakes today and most of their observed properties are explained in terms of physical theory. This modern view holds that earthquakes are to be expected because of the constant geological reshaping of our planet.

Most earthquakes occur near plate margins, geologists conclude that global geologic, or tectonic forces that produce mountains, rift valleys, midoceanic ridges, and ocean trenches are also the underlying cause of great earthquakes. These global forces, which at present are still not fully understood are consequences of different temperatures within the earth. Different losses of heat by radiation into space and gain of heat from decay of radioactive elements within rock formations.

By far the most common of earthquakes is tectonic earthquakes. These are produced when rocks break suddenly in response to the various global forces. Tectonic earthquakes are scientifically important to the study of the earths interior an of greatest significance as they pose the greatest hazard.

The second well known type of earthquake is the one which accompanies volcanic eruptions. A volcanic earthquake is defined as one that occurs in conjunction with volcanic activity, but it is believed that eruptions and earthquakes both result from tectonic forces in the rocks and need not occur together.

Collapse earthquakes are small earthquakes occuring in regions of underground caverns and mines. The immediate cause of ground shaking is the collapse of the roof of the mine or cavern. An often- observed variation of this phenomenon is the so called "mine burst". This happens when the induced stress around the mine working cause large masses of rock to fly off the mine face explosively, producing seismic waves. Collapse earthquakes are also produced by massive landsliding.

Explosion earthquakes are produced by the detonation of chemicals or nuclear devices. Some underground nuclear explosions fired since the 1950s have produced substantial earthquakes. When a nuclear device is detonated in a borehole underground, enormous nuclear energy is released. In millionths of a second, the pressure jumps thousands of times the pressure of the Earth's atmosphere and the temperature increases by millions of degrees. The surrounding rock is vaporised, creating a sherical cavity many metres in diameter.

Some underground nuclear explosions have been large enough to send seismic waves throughout the Earth's interior; waves with amplitudes equivalent to moderate sized earthquakes that have been recorded at distant seismographic stations. Some explosons have produced waves that have shaken buildings in distant locations.

People and animals sometimes produce earthquakes, though mostly small and to note the most recorded activity in Australia is that of kangaroos jumping in the vicinity of seismograph stations.

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