As stated earlier, the earthquakes are caused by the vibrations set up in the earth’s crust which spread outwards in all directions from the source of disturbance.
On the basis of such disturbances produced in the solid body of the earth, many causes have been assigned for causing earthquakes, minor as well as major.
Some of the earthquakes are artificial, while others are natural. But it is undoubtedly true that all the earthquakes are caused due to the disequilibrium in the earth’s crust.
Causes of the earthquakes fall into the following broad categories:
1. Volcanic activities
2. Folding and faulting
3. Plate tectonics
4. Human interference with nature (anthropogenic factors)
Volcanic explosions are certainly the most common cause of earthquakes in the neighbourhood of active volcanoes. Such earthquakes are, therefore, known as volcanic earthquakes.
This type of earthquake is caused either under the influence of the increasing pressure of volcanic gases or the subterranean movement of molten lava trying to come up on the earth’s surface.
Such earthquakes are common in the area near the volcanoes. They may occur before the volcanoes actually erupt which are, in fact, due to the intrusion of dikes and other movement of lava.
Moreover, a great and violent earthquake may be caused in the region about the volcanoes when the final eruption occurs. However, such earthquakes of volcanic origin are generally less violent and more limited in extent than those called tectonic earthquakes.
But there are exceptions too, for example, the violent earthquake associated with the great eruption of Krakatoa in the Straits of Sunda in 1883, which caused 35 meters high waves that destroyed 163 villages and killed more than 36000 people. The explosion was so violent that its noise was heard almost all round for a distance of about 3200 km.
Folding and Faulting:
A fault is defined as a fracture plane along which the rocks have been displaced. There are vertical as well as horizontal displacements. Earthquakes are caused due to sudden movements of rocks along faults. Such earthquakes are called tectonic earthquakes.
Remember that the horizontal as well as vertical movements of rocks result from the operation of endogenetic forces beneath the earth’s surface. It is due to such movements that folds and faults are created.
The fracture of the rock causing a tectonic earthquake is due to elastic strains, which are greater than the strength of the rock can withstand produced by the relative displacement of nearby portions of the earth’s crust.
However, the displacements of rocks are not sudden at the time of fracture, but attain their maximum amounts slowly during a long period of time.
In fact, the mass movements of blocks of rocks that take place at the time of the earthquake are the sudden elastic rebounds of the sides of the fracture towards positions of non-elastic strain.
The energy liberated at the time of an earthquake was present there in the form of energy of elastic strain of the rock. This is what is known as the elastic rebound theory of tectonic earthquakes.
Faults occur in rocks of all ages and of all types. Moreover, if a fault is located in a zone where one plate moves against another, the possibility of damage and destruction is great.
The most destructive Californian earthquake of 1906 was caused by the movement of rocks along the great San Andreas Fault. It is interesting to note that the visible displacements were traceable along the fault line for about 480 km, and its shock was felt over a distance of more than 11200 km in the direction of the fault.
Typical examples are present in various parts of the globe where most destructive earthquakes were caused due to the movement of rocks along the fault planes. One of the most destructive earthquake occurred in 1755 at Lisbon, Portugal.
The Calabrian earthquakes, south of Naples, were far more widespread and destructive where great tectonic shocks have been recorded on many occasions.
A great earthquake occurred in Assam in India in June, 1897, which laid in ruins a large area. The ground was fissured, and rock movements occurred along fault lines. Similarly, the Kangra earthquake devastated a very large area in northern India in 1905.
Japan is the most notable centre of seismic activity and this country suffers most from earthquakes. Sagami Bay earthquake of 1923 wrecked Tokyo and Yokohama and caused at least 143000 deaths. Surveys carried out after this earthquake showed that the surrounding mainland had been twisted round in clockwise directions.
The magnitude of the Sagami Bay earthquake on the Richter scale was 8.2. The magnitude of the Alaska earthquake which played havoc was 8.6. In 1975, Guatemala earthquake in Central America v/as caused due to movement of rocks along the Motagua fault located between the American and Caribbean plates.
According to the theory of plate tectonics, the surface of the earth consists of 15 plates comprising the rigid upper mantle, and the oceanic and continental crust. Out of the total number of plates, 6 are major plates and 9 are minor plates.
These plates are always moving. Now, it is an established fact that practically all the tectonic, seismic and volcanic activities take place at the plate margins. That is why must of the earthquakes and volcanoes are found in narrow and semi-continuous belts mostly confined to the plate boundaries.
It may be noted that the plate boundaries are placed into three distinct categories: constructive, destructive and conservative plate boundaries, each with different characteristics. Constructive plate boundaries represent such plates which move in opposite directions from the mid-oceanic ridges.
The destructive plate boundaries are those where two plates moving in opposite directions collide with each other. As the collision takes place, the heavier plate boundary undergoes subduction into the mantle beneath another plate which is made of lighter geometrical.
This is called Subduction Zone. This one is characterised by most widespread and disastrous earthquakes. Most of such earthquakes are confined to a narrow dipping zone known as Benioff Zone, after the scientist Hugo Benioff.
On the contrary, the conservative plate boundaries do not collide, rather the two plates slip past each other. The above named plate boundaries are characterised by different types of folds and faults. Needless to say those earthquakes of varying degrees of intensity are caused by different types of plate motions.
Because of certain characteristics of the constructive plate boundaries, only moderate earthquakes are associated with them. That is why only shallow focus earthquakes occur along the mid-oceanic ridges, the depth of their focus varying from 25 to 35 km.
On the other hand, at the destructive plate boundaries the most disastrous and deep focus earthquakes are caused. The Ring of Fire surrounding the Pacific basin represents the subduction edge of the Pacific plate thrusting deep into the crust and upper mantle.
Due to plate collision and subduction of one plate beneath another a lot of molten lava that comes up towards the earth’s surface is produced. This is the main cause of the presence of active volcanoes along the Pacific Rim. Volcanic eruptions also cause earthquakes.
Such processes produce volcanoes and volcanic earthquakes wherever there is convergence and collision of plate boundaries throughout the world.
Despite the fact that most of the earthquakes occur along the moving plate boundaries, the continental platforms, contrary to general expectations, are also shaken by a few shallow focus earthquakes.
Koyna earthquake in India offers a typical example of such an earthquake. Similarly East Africa, Western U.S.A. and other parts of Peninsular India experience infrequent and shallow-focus earthquakes.
Human interference with nature (Anthropogenic factor):
Sometimes human interference with nature causes artificial earthquakes. The underground testing of H-bombs produces shock waves through overlying rocks which results in an artificial earthquake. Such earthquakes can be compared with a shallow volcanic earthquake.
On certain occasions a munitions factory explodes causing an earthquake of small magnitude. Blasting of rocks by dynamites for the construction of roads in mountainous regions, deep underground mining for the extraction of minerals, blasting for the construction of dams and reservoirs and similar other human activities may also cause mild tremors.
Construction of high dams and big reservoirs storing huge volume of water disturb the equilibrium of underlying strata of rocks which cause small earthquakes in the surrounding area.
There are many other minor causes like landslides in mountainous areas, submarine slides, collapse of cavern roofs, and avalanches which cause perceptible tremors in the earth’s crust. However, such earthquakes are of very small magnitude and do not cause any damage to life and property.
However, there are certain exceptions. Due to the construction of Marathon dam in 1929, there was a severe earthquake in Greece in 1931. Koyna reservoir was constructed in 1962 in Maharashtra.
In 1967 there was a destructive earthquake in Satara district of the state which is attributed to the construction of a dam and reservoir at Koyna. The seismologists were taken by surprise as to why there could be an earthquake in this stable block of the country.
Investigations and researches that followed discovered the presence of two rift faults in Maharashtra beneath the Deccan Plateau. Similar examples of earthquakes caused due to the construction of dams and reservoirs may be found in various parts of the world.
Hoover Dam in the United States of America, Mangla dam in Pakistan, Monteynard and Grandvale in France, Kariba (Zambia), Manic dam (Canada) and Kurobe dam in Japan etc. have caused minor earthquakes of small magnitude in the past.
However, all the earthquakes which are caused by human activities are not as terrifying and disastrous as the tectonic earthquakes. All of these earthquakes can be placed in the category of shallow ones.