How and Why Earthquakes Occur


One of the most devastating and terrible natural disasters is an earthquake. It is possible to believe that the earth is securely grounded on a solid rock foundation, but in truth, nations are situated on tectonic plates, which are rocky slabs that are readily moved by the molten rock beneath the earth's surface. Numerous lives have been lost as a result of earthquakes, which are defined as tremors or quake movements of the earth's crust that result in a sudden release of energy. A seismometer is used to measure the vibrations, also known as seismic waves. Seismograph data is recorded and utilized to calculate an earthquake's magnitude. The occurrence of an earthquake could result from natural forces like volcanic eruptions or could originate from human causes such as explosions.

Purpose of Study

This paper will study how earthquakes occur and the reasons that trigger its occurrence. The paper will provide valuable information to the undergraduate engineering students and make them understand how earthquakes happen. The students will also be also to get an insight into the reasons that cause earthquakes. It will also explain why some areas are more prone to earthquakes than others.

How Earthquakes Occur

The Dislocation of Earth's Crust
Earthquake is a vibration which could be violent on the surface of the earth and is followed by an energy release inside the earth's crust. An abrupt dislocation of the crust's segments is responsible for generating gigantic energy. The dislocation of the segments occur due to eruptions resulting from volcanic force or through explosions that are could be manmade. When a dislocation occurs, the crust starts by bending due to the intensive pressure and stress exerted on it. Once the strength becomes more that the strength of the rock, the crust breaks and snaps to a newer position. Seismic waves, vibrations resulting from the breaking of the crust, are generated (Chen, 2014). The seismic waves produced then travel outwardly from the earthquake's source to the surface of the earth. The speed of travel depends on the materials through which the waves travel causing vibrations of different intensities. The vibrations cause a "ring like bell" or quiver effects on the earth.

A dislocation of the earth's crust leads to the formation of faulty line

This is a fracture in the crust of the earth and which results from dislocation where two earth crust blocks slip against one another. The fault lines could either be normal, reverse or lateral. A normal fault line occurs in a response to tension or pulling forces. Under this, there is a movement of the block overlying down the dip of the faulty planes. Thrust or reverse fault line happens in a response to compression or squeezing forces in which the overlying block will move upwards dip of the fault plane. On the other hand, the reverse also called strike-slip fault will occur in a response to the two types of stress forces. In this case, the blocks move in a horizontal manner against each other. Essentially, normal faulting will occur along spreading zones, trust along subduction zones and strike-slip will happen along transform lines.

Geologists do report that most earthquake will tend to re-happen along the fault lines as this indicate a zone of weakness in the earth's crust (Bouhadad, 2013). Ostřihanský (2012) strengthens the point by stating that, even though a faulty zone lately did experience an earthquake, it does not guarantee that all the stress was relieved and thus another earthquake is likely to reoccur at the same place. In New Madrid for instance, a greater earthquake that happened was followed by another large aftershock after six hours in 1811, on December 6 (Chen, 2014). Besides, the stress release along a section of the fault lines could raise stress in some other places. The earthquake that happened in January and February of 1812 in the new Madrid was as a result of these phenomena.

Primary and Secondary waves in Earthquake Occurrence

The seismic waves usually travel in two types of waves; the primary (P-waves) and the secondary (S-waves). The P-waves will cause vibrations on the earth surface in the regions in which they directly move. These waves travel in alternating manner stretching and squeezing patterns of rarefactions and compressions. This is known to be longitudinal waves and according to Wang et al., these waves will travel at the absurd speed of approximately 25,000 km/h (Wang et al., 2013). The secondary wave does travel in a less speed compared to primary waves. This is normally half the speed of the primary waves. These waves, as they move forward they vibrate up and down owing to the speed they travel at (Wang et al., 2013).

Earthquake Focus and Epicenter

The earthquake's focal point is described as the depth from the surface of the earth to the area where the energy of the earthquake originates. This is well known as the focus. Wang et al. (2013) note that the focal depth of an earthquake that emanates from source to the surface is approximately 43.5 miles (70 kilometers) are termed to be shallow. Those which range between 43.5 and 186 miles (70 and 300 kilometers) are intermediate the focal depths while an earthquake that is deep could go up to about 435 miles (700 kilometers). The concentration of an earthquake's focus is on the upper mantle and the crust. The depth that goes to the core of the earth's center is approximately 3,960 miles (6,370 kilometers). Therefore, earthquakes that tend to be the deepest emanate in a relatively shallow parts of the interior section of the earth. The point that is directly above the focus is an epicenter. The epicenter is the point that is likely to have the most destructive effects.

Occurrence of Tsunami

The earthquakes that occur underneath floor of an ocean occasionally produce enormous sea waves known as tsunamis. The sea wave do travel transversely in the ocean with speed of up to 597 miles per hour (960 km) and they could be 49 feet higher when they appear on the shore. For instance, in the Alaskan's earthquake that happened in 1964, the tsunami that engulfed the coastal regions lead to greatest devastation at Cordova, Seward and Kodiak (Bouhadad, 2013). This tsunami culminated into severe damages along the North Americas west coast and especially in California at Crescent City. Some of the waves did race across the ocean coastal regions of Japan.

Liquefaction Process during Earthquakes

Liquefaction occurs when sediments containing water are packed loosely, shake and thus lose their strength. This leads to chief damages during the occurrence of the earthquakes. For example, during the Loma Prieta earthquake that occurred in 1989, the liquefaction process of the debris and soils utilized to fill the lagoon lead to major fractures, subsidence and horizontal slides of the ground surface in San Francisco, Marina district. The landslides that are triggered by earthquakes usually lead to huge destructions. In the Alaska earthquake, 1964, the landslides that were shock – induced did devastate the development of Turnagain Heights residential and many of the region's downtown of Anchorage.

Causes of Earthquakes

Tectonic Movement

Tectonic plates are always in constant moves. The grinding of two plates suddenly jolts into completely new locations. Along the faults, the jagged edges of the tectonic plates do grind against each other. Majorly, as Ostřihanský (2012) documents, many earthquakes will occur in middle parts of the ocean at the points where the plates push apart on the floor of the sea. Surely, among the major earthquakes that occur around the edges of large tectonic plates in the Pacific Ocean forms an intense regional activity called the "ring of fire" (Chen, 2014). When the grinding happens, massive energy is released from the movement which causes earthquakes. Energy is build up and the earthquake will continue until when the entire energy gets released from the focus. The structural disturbances that result into a relative displacement of the sections of the lithosphere are termed to be the major causes of earthquake occurrence (Ostřihanský, 2012).

Surface Causes

Massive explosions, slip occurrence on the steep coasts and the dashing of the sea waves could result into earthquakes. Besides, landslides, heavy trucks, and avalanches together with huge projects of engineering trigger minor tremors. These surface causes could be manmade such as the engineering projects while others like avalanches and landslides are caused by natural causes.

Volcanic Causes

Earthquakes can also result into earthquakes. The occurrence of a volcanic eruption creates void spaces in the mantle and the earth's crust causing a misbalance of the materials. Thus, movements happen in order to balance the materials in the mantle. These movements are what causes the earthquakes.

Other Causes

Other causes of earthquakes include the adjustment of the earth's inner bed rocks and buildup of pressure in the interior of the earth. When adjustments occur on the bedrocks, it is characterized by adjustments between sima. Beneath the ocean silica and magnesium are present. (Si + Ma = sima) (Ostřihanský, 2012). The adjustment normally occurs in the interior of the earth's crust and these earthquakes leading to Plutonic Earthquakes. Furthermore, the pressure of gasses formed in the earth's interior cause expansion and contraction thus instigating sudden shocks that shake the surface. Other causes include the disposition of materials and denudation of landmasses.


Earthquakes are phenomenal experiences of sudden movement of the earth crust. The earth's plates sit on the mantle which if essentially formed by a liquid rock. Shifting of the earth plates result in enormous energy that forms waves causing earthquakes. The occurrence of an earthquake is followed or accompanied by series of vibrations which causes destructions.


Bouhadad, Y. (2013). Occurrence and impact of characteristic earthquakes in northern Algeria. Natural Hazards, 72(3), 1329-1339.

Chen, H. (2014). Research status of tectonic stress and the relationship between tectonic stress and oil-gas accumulation. IOSR Journal Of Engineering, 4(1), 49-52.

Ostřihanský, L. (2012). Causes of earthquakes and lithospheric plates movement. Solid Earth Discussions, 4(2), 1411-1483.

Wang, M., Shimizu, K., & Uesu, K. (2013). An Analysis of Earthquakes Latitude, Longitude and Magnitude Data by Use of Directional Statistics. Japanese Journal Of Applied Statistics, 42(2), 29-44.

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