An earthquake is a natural phenomenon, which has many causes behind it. Some of these are natural while others are man-made. Among these causes, the most important are as follow;
(1). Tectonic Movement causes Earthquakes: Earth’s crust is divided into large segments (fragments) separated along ridges. These segments are called plates (Lithospheric Plates). The term tectonic plates are used to describe the active mobile nature of these plates. There is a continuous process of convection within the lower mantel due to the extremely high temperature and its molten state. The convection causes the lower magma to move upward and the upper magma to move again downward. The type of movement is known as convection. Convection may be divergent as well as convergent. The convection within the mantle put an impact on the solid crust and made the crust crack into many pieces. Each cracked piece of the crust is known as the tectonic/ lithospheric plate. The convection of the mantle moves these plates. The plates under which the convection is divergent, move the plate apart from each other, while the plates, which are above the converging magma, move convergently (one plate overrides the other plate). There is a third type of tectonic movement, which is known as a transverse movement. The transverse movement of the two plates moves them horizontally across each other. The part of the crust where two plates transversely move across each other is known as the transverse boundary. Two transversely moving plates collide with each other and cause stress between the boundaries of the plates. When this stress is (abruptly) released, a huge amount of seismic energy is produced. The seismic energy results in an earthquake by sending strong seismic waves across the earth.
(2). Volcanic Eruption: Volcanic eruption is one of the major causes of earthquakes. These eruptions result in the expulsion of huge amounts of lava, stones, steam, and ashes from the crust of Earth. Thus the surface of Earth near the volcano is shaken. This phenomenon is called an earthquake. In 1883, an earthquake took place in Krakatoa (Indonesia) as a result of the volcanic eruption. It destroyed many villages around Java and Sumatra.
(3). Internal Movement of Earth: Internal movements of Earth comprise of the effects of all the internal forces which build up the features of the crust of our planet. The internal movements of Earth are also called the Tectonic Movements. These include diastrophic forces (faulting, folding, depression, and uplift) and vulcanicity. These forces are the major source of the internal movement of the Earth. Mostly tensional forces and compressional forces are responsible for an earthquake. The best example of an earthquake caused by the internal forces of Earth is the one, which shook Bihar (India) in 1934.
(4). Isostasy: Isostasy means the condition of vertical equilibrium between the floating landmasses and the asthenosphere beneath them. This system of equilibrium is maintained despite the many internal forces, which are constantly at work to change the landmasses. The isostasy adjustments cause earthquakes. In 1949, an isostatic Earthquake shook Hindukush mountainous region causing great destruction. Thus isostasy is maintained between the Sialic (Silicon Aluminium) crust and the Simatic (Silicon Magnesium) substratum resulting in earthquakes.
(5). Landslides: In mountainous regions, landslides also cause minor earthquakes. In this case, a large mass of rocks and soil travel downhill very rapidly. The downslope movement of the landmasses causes tremors.
(6). Ejection of Steam: Sometimes the huge volume of hot steam is ejected or moves underneath the surface of Earth. The pressure developed through this movement causes tremors in the region.
(7). Cavern Collapses: The cavern collapses are another source of minor earthquakes.
(8). Displacement of Ice Blocks: In snow-covered mountains, the fall of huge ice blocks into the valley cause minor tremors and shocks.