What Causes Earthquakes?

Earthquakes can kill thousands of people in seconds and destroy cities. The transfer of seismic waves from the Earth's crust and mantle to its surface, and the subsequent vibrations produced by these waves, lead to earthquakes. These vibrations have the potential to wreak havoc and cause destruction beyond imagination as their occurrence cannot be predicted.

Earthquakes are followed by aftershocks, which can be of a low magnitude, and sometimes equally destructive. Of late, there have been many natural disasters that can be associated with earthquakes, such as the Tsunami that hit the shores of India and Southeast Asia in 2004, and caused total annihilation.

Intraplate Earthquakes

• The ones that take place away from the plate boundaries are known as intraplate earthquakes. The primary cause of this type is the presence of tectonic faults in the Earth's crust.

• A fault is defined as a crustal plane, which shows a relative movement of the crustal rocks with respect to their positions. They are broadly classified as normal, thrust, and strike slip faults.

• In case of faults that are in an active state, the crustal rocks are subjected to a large amount of pressure, mainly due to the continuously active plate margins.

• These faults undergo movement, and the pressure generated is released, which further leads to the occurrence of earthquakes.

• The fault movement typically consists of compression and thrusting, which causes the displacement of rigid crustal rocks.

• The frequency of these earthquakes is less than the frequency of those caused by plate tectonic movement. A majority of the earthquakes in Australia and Tasmania belong to the intraplate category.

Plate Tectonics or Interplate Earthquakes

The crustal layer of our planet is divided into sections known as tectonic plates, which are resting/floating on the molten layer of magma (asthenosphere) beneath the Earth's crust. The movement of these plates is determined by the convection currents in the magma.

• These currents cause the tectonic plates to move on the Earth's surface, either in a convergent or a divergent manner. This leads to the formation of transform faults, which show movement of the lithospheric parts in opposite directions, but along a single plane.

• As the tectonic plates shift in such a manner, an enormous amount of strain is generated in the rigid and hard crustal rocks beneath the Earth's surface. Due to the continuous motion of plates, these rocks put massive stress and pressure on the plate boundaries.

• The stress keeps building, and at a particular point, it is suddenly released in the form of seismic waves, which travel towards the earth's surface and cause earthquakes.

• Thus, the fault planes move according to the intensity of the waves, along with a change in their positions, and the released energy is in the form of shock waves.

• The tectonic plates form a base for the stability of the continents. Most earthquakes occur near the convergent or divergent plate boundaries, wherein the plates either overlap each other forming mountain ridges or continents. This movement disrupts the balance and position of all the plates, which leads to the occurrence of earthquake tremors.

Volcanic Eruptions

• Volcanic earthquakes are not very common. The molten magma under the Earth's crust is under enormous pressure, and to release it, an opening or a vent must be present in the crust. The magmatic pressure causes strain on the lower crustal portions, in the vicinity of the volcano.

• Due to buildup of various gases, along with volatile matter and molten magma, pressure is also exerted on the walls of the opening or vent. After a critical point, the volcano erupts from this section, and magma in the form of lava is ejected along with the volatile matter and gases.

Subsequently, the lava comes in contact with the outer atmosphere and solidifies, which may result in the blocking of the volcanic mouth or the crater

• Now, the continuous generation of a tremendous amount of pressure crosses a particular breaking point in the rigid crustal rock mass. The dissipation of this strain is in the form of a massive explosion, and the subsequent generation of seismic waves. This in turn results in a high-magnitude earthquake.

• If the volcanic chamber and the opening is in the vicinity of tectonic plate boundaries, then the magnitude of an earthquake might be greater than that of the one caused by eruptions that take place away from the plate boundaries.

• Earthquakes caused due to volcanic eruptions might lead to a disturbance in the position of tectonic plates. They either move further or resettle back to their original positions.

Induced Seismicity

The occurrence of earthquakes due to various activities that are undertaken by humans is termed as Induced Seismicity. Although being a controversial topic of discussion, it has gained significant attention, especially for its inclusion in the major causes of earthquakes.

Research suggests that induced seismicity only causes minor earthquakes, but doubts have already been raised on the occurrence of major events like the Sichuan earthquake in 2008. Several studies suggest that it was caused by human activities such as the building of a dam. The various types of induced seismicity are:

Reservoir-Induced Seismicity (RIS)

Thought to be one of the most important man-made causes, RIS has been observed at numerous locations around the world. This type of seismicity is observed when a dam is built on the sites which have poor rock lithology and are susceptible to faulting and fracturing. When a reservoir is built in such areas.

The stored water increases the pressure on the underlying lithology, and this might activate any fault present beneath it. If porous rock is present around the foundation and base of the reservoir, water might seep through the loose fragments of soil and rocks, enter the fault plane act as a lubricant to activate it, further generating seismic forces.

Hydraulic Fracturing

Also known as fracking or hydrofracking, this technique utilizes pumping a particular mixture of chemicals and water beneath the Earth's surface, with the help of appropriate technological means. These chemicals are pumped at high pressure in various sedimentary layers that indicate the presence of crude oil and natural gas.

According to expert studies, this has an effect on the surrounding crustal portions, leading to earthquakes, especially if structural weaknesses like faults and dikes are present in the lithology.

The famous earthquake of Oklahoma in 2011, which had a magnitude of 5.6 on the Richter scale, was caused due to the method of hydro-fracturing in that region, and the subsequent activation of fault line in the lithology.

Mining and Groundwater Extraction

These causes are the most debatable ones. Deep underground mining might lead to the formation of empty spaces which are surrounded by structural weaknesses like joints and fractures. This might lead to their collapse within the underground mined area, generating seismic waves and minor earthquakes.

The same is true in the case of extensive extraction of groundwater, especially in case of confined aquifers (surrounded by rock strata on all sides). Groundwater present in such chambers is stored at a high pressure as compared to the atmospheric pressure.

When the water is extracted, the pressure is suddenly lost, and this might lead to the caving of the aquifer walls, and subsequent generation of seismic activity.

The point of origin of an earthquake beneath the surface is known as the hypocenter. Maximum destruction is caused at the epicenter, which is the point on the ground surface that is projected exactly above the hypocenter.

• When there are sudden movements in various layers of the crust and mantle, concentric waves are produced. They are known as seismic waves, and are of three types: Primary Waves (P-Waves), Secondary Waves (S-Waves), and Surface Waves (L-Waves and R-waves).

• These waves show different patterns of movement.

• Earthquakes usually consist of foreshocks and aftershocks.

• In a series of earthquakes, one with the maximum magnitude is known as the main shock. Its intensity is calculated by the magnitude of seismic waves which is measured with the help of a seismograph. This measurement is taken on the Richter scale.

• Earthquakes can be forecasted by using technological means like continuous monitoring of seismic waves in seismic stations, observing changes in the surrounding lithology, i.e., sudden formation of a depression or upheaval on the ground surface, or change in the behavior of animals. However, the forecast can't be exact.

Though earthquakes cannot be predicted accurately, we can stay safe by using some precautionary methods like building earthquake-resistant houses, and training ourselves to take the right action when an earthquake strikes.