As stated earlier, the earthquakes fall into two broad categories: (i) Natural earthquakes, caused by natural forces, and (ii) Artificial earthquakes, which occur due to such human activities which cause disturbance in the earth’s crust producing vibrations of various magnitudes.
Here only natural earthquakes have been classified on the basis of varying natural processes. In other words, the different types of earthquakes have been classified on the basis of the cause and the position of the stresses. Natural earthquakes are classified as under:
1. Tectonic earthquakes;
2. Plutonic earthquakes;
3. Volcanic earthquakes.
1. Tectonic earthquakes are caused by the sudden earth movements, generally along faults, usually at depths varying from about 4.5 km to 24 km below the earth’s surface. Majority of natural earthquakes including the most powerful and damaging ones belong to this category.
The term ‘tectonic’ refers to any structural change in rocks caused due to their deformation or displacement. The word ‘tectonic’ has been derived from the Greek word ‘tekton’ meaning a builder.
“The main cause of tectonic earthquakes is the building up of stresses in rocks until they are strained to breaking point, when they suddenly rupture and move”. Arthur Holmes. In tectonic earthquakes the principal shock lasts only a few seconds or in rare cases, a few minutes.
It is preceded by fore-shocks and followed by a series of after-shocks. This category of earthquakes varies greatly in intensity and magnitude, but some are highly disastrous. Generally deep seated earthquakes are less destructive than are shallower ones.
2.Plutonic earthquakes are deep focus earthquakes, the depth of disturbances being between 250 km and about 700 km. Very deep scatted earthquakes are not so well understood as other types are. Gutenberg and Richter made a critical analysis of plutonic earthquakes and concluded that the mechanism of origin of shocks seems to be the same at all depths.
3.Volcanic earthquakes are associated with explosive eruption of volcanoes. Naturally they occur in volcanic zones of the world. The Circum Pacific Belt is a typical example where volcanic earthquakes are common hazards. Their intensity and magnitude depend on the explosive power of volcanic eruption.
The most disastrous earthquakes were caused due to the violent explosions of Krakatoa volcano the impact of which was experienced at Cape Horn which is about 12,800 km away from the place of occurrence of the said earthquake.
Moreover, the earthquakes of volcanic origin are less severe and more limited in extent than those caused by the fracturing of the earth’s crust.
Volcanic earthquakes are caused by the sudden subterranean yielding of the crust under the influence of either increasing pressure of volcanic gases or the attempted escape of molten rocks (magma).