Earthquake Types and Mechanism

In Earthquake, there is a sudden release of energy in the Earth’s crust, which leads to a series of motions because of the waves created due to this energy (called seismic waves) released. These seismic waves originate in a limited region and spread in all directions.

Types of Earthquakes

Earthquakes can be generated by a number of sources, most of which are result of  natural tectonic processes, usually caused by the interaction between two lithospheric plates. Other quakes can be generated by volcanoes as magma is injected into the Earth’s crust. For example, earthquakes in the island of Hawaii are generally volcanic earthquakes. Rest of the Earthquakes are artificially generated by nuclear test explosions. Thus, there are several types of Earthquakes such as:

• Tectonic Earthquakes: Tectonic Earthquakes are most common and generated due to folding, faulting plate movement.
• Volcanic Earthquakes: Earthquake associated with volcanic activity are called volcanic earthquake. These are confined to areas of volcanoes and pacific ring of fire is best example of these types of earthquakes.
• Collapse Earthquakes: They are evident in the areas of intense mining activity, sometimes as the roofs of underground mines collapse causing minor tremors.
• Explosion earthquakes: This is a minor shock due to the explosion of the nuclear devices.
• Reservoir Induced Earthquakes: Large reservoirs may induce the seismic activity because of large mass of the water. They are called reservoir induced earthquakes

Foreshocks, Mainshocks and Aftershocks

The Earthquakes come in three forms of clusters called foreshocks, mainshocks, and aftershocks. Foreshocks are quakes that occur before a larger one in the same location; around a quarter of all mainshocks happen within an hour of their foreshock. Mainshocks and aftershocks are better known. Mainshocks are of the highest magnitude. Aftershocks are smaller quakes that occur in the same general geographic area for days-and even years-after the larger, mainshock event.

Hypocentre and Epicentre

The point, where earthquakes are generated first, is called focus or hypocenter. A hypocenter is below the surface, where the first rock displaces and creates the fault. Epicentre is the point on the Earth’s surface that is directly above the hypocenter or focus. This is the point where the shock waves reach the surface. Earthquakes originate at depths ranging from about 5 to 700 kilometers. Nearly 9o percent of all earthquakes occur at depths of less than 100 km. Shallower is the depth, more destructive an earthquake is.

Mechanism of Tectonic Earthquakes

Theory of plate tectonics explains that earth’s crust is formed by a number of large plates that move very slowly in various directions on the earth’s surface. These plates are 60-200 km thick and float on top of a more fluid zone, much in the way that icebergs float on top of the ocean.  Most earthquakes occur near a boundary between two plates. As one plate pushes past or moves over another, great stresses build up in the rock along the edges of the plates because friction prevents them from sliding past each other. Subsequently, the stresses become great enough so that the rocks can rupture. The edges of the plates slip a short distance in different directions, causing an earthquake. Greater the stresses, greater is the resulting earthquake. The movements are of three kinds

Divergent:

In divergent movements the plates move apart from each other. This is most common type of movement in mid-oceanic zones.

Convergent

In convergent movements the plates move towards each other and the border overlap. This is most common type of movement in subduction zones where the dense oceanic plates collide and slide beneath the continental plates.

Transformational

In this type of movement the plates move in opposite side, on parallel. Some earthquakes are caused by the movement of lave beneath the surface of the earth during volcanic activity.

Earthquake Belts

There are two major belts of earthquakes in the world. They are as follows:

Circum-Pacific Belt

This belt is along a path surrounding the Pacific Ocean This zone included the regions of great seismic activity such as Japan, the Philippines, and Chile. This path coincides with the “Pacific Ring of Fire”.

Alpine-Himalayan Belt

Another major concentration of strong seismic activity runs through the mountainous regions that flank the Mediterranean Sea and extends through Iran and on past the Himalayan Mountains. This zone of frequent and destructive earthquakes is referred to as the Alpine-Himalayan belt.