What Causes Earthquakes?

By Carly Dodd on November 28 2020 in Geography

A scene from the 2015 Nepal Earthquake that was one of the most catastrophic earthquakes ever to hit the country. Image credit: Manjitkhadka/Shutterstock.com
  • Earthquakes are caused when seismic energy which was trapped within the Earth is released
  • There are four types of seismic waves: P waves, S waves, Q waves and Rayleigh waves
  • 80% of the world's earthquakes take place within the Circum-Pacific Belt in the Pacific Ocean

Earthquakes can be described as the shaking, shifting or cracking of the Earth’s surface due to movements within its crust. These “quakes” are caused by any passage of seismic wave through the Earth, and it is this energy which causes the earth to shift, buckle or wave. Seismic waves occur when energy which is trapped within the Earth’s crust escapes. This can happen in a few different ways, but it is usually caused by the slipping of one piece of the Earth’s crust against another. For this reason, earthquakes, and seismic activity more generally, tends to occur more frequently along plate borders and edges.

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What Causes Earthquakes?

Major tectonic plates of the Earth and earthquake belts around the plate boundaries. Image credit: Bogadeva1983/Shutterstock.com

The Earth’s crust itself is made up of a number of floating rock plates, known as tectonic plates, which are essentially free moving in a sea of molten rock. Because they are free and not fixed, these plates do shift, or bump into each other over time. The movements of these plates are so slow, that they normally cannot be felt by humans, but rather move incrementally over many years, decades over centuries. The plates have theoretically been in motion since the birth of the earth, and have shifted very gradually over time. Despite their slow movement, these shifts in the plates do sometimes cause major geological events which can be felt on the surface of the Earth. Great amounts of energy can become trapped within the Earth, and as plates shift or collide, this energy can be released. This energy, known as seismic energy, is what causes the waves of energy to move through the earth, or along the earth’s surface, resulting in a tremor or earthquake.

Types Of Seismic Activity

Earthquake is caused due to tectonic plate shifts as shown in the above diagram. Image credit: VectorMine/Shutterstock.com

Seismic activity, that is the energy that is released from the earth’s core in bursts of energy waves, can be classified into four types. Seismic waves are vibrations caused by earthquakes, explosions, or extreme bursts of energy. These waves either originate inside the Earth, or occur along its surface. Seismic waves can be classified into one of four different types, depending on the location of the waves, as well as the direction and pattern of the wave’s movement. The four categories are: Primary, or P waves; Secondary, or S waves; Love, or Q waves; or Rayleigh waves.

Primary Waves

Primary waves, also known as P waves, are the fastest moving waves generated by seismic activity. Because of this, they are the first type of wave registered on a seismographic, which can be used to predict and/or record earthquakes and other such activity. P waves are elastic in nature, meaning they create a compression and expansion within whatever media is transmitting the energy. This means the liquid, solid or gas through which the seismic waves are travelling will experience a back-and-forth motion. The speed of these waves is approximately 6 km/s through surface rock, and roughly 10km/s nearer the core. The speed differs due to the composition of the rock through which it is passing, as well as changes in pressure which occur deeper within the Earth.

Secondary Waves

Secondary waves are more commonly referred to as S waves, or transverse waves. These types of seismic waves move through media in a curved or rolling movement, meaning the medium will move perpendicular to the wave’s direction and rock particles will vibrate at a right angle to that of the wave’s energy. This is known as shear stress. Generally, this means the earth, or medium, will flex up and down in a waving motion, causing crests and troughs, as the seismic energy passes through it. Speeds of S waves range from 3.4km/s on the surface, to 7.2km/s near the core boundary. S waves do not transmit through liquid, and as such do not occur within the earth’s core, directly.

Love waves

Love waves are one of two surface-type waves. Love waves occur when varying degrees of vertical elastic properties experience seismic activity. In this case, the movement of the Earth or medium through which the wave passes is perpendicular to that of the direction of the energy wave. The movement spreads from the energy source in two directions. One side of the s wave is elastic in nature, while the other side borders a vacuum. This means that the earth shifts horizontally, as the ground sinks and rises along bordering areas. Due to the nature of these waves, as well as their rapid speed (thy have a lower velocity but higher speed than other types of seismic waves) Love waves can cause a much higher reading on the seismic scale. For the same reason, these are generally the most damaging waves, since the earth can shift in both directions (both up and down) at the same time. Additionally, because this is a surface type wave, the waves remain on the surface of the Earth for prolonged periods, causing sustained damage.

Rayleigh Waves

Rayleigh is the second type of surface wave, also caused by seismic activity. Rayleigh waves cause longitudinal compression as well as transvers vibration meaning they create an elliptical motion along the surface. Rayleigh waves move more slowly, and undulate, producing a long duration of seismic activity. Energy is therefore transmitted through the surface of the Earth in both perpendicular and parallel motions. Because Rayleigh waves move in multiple directions at the same time, they are very destructive to the medium through which they pass, ie the surface of the Earth and can have devastating effects.

Measurement Of Earthquakes

A seismogram is used to measure earthquakes. Image credit: BlueRingMedia/Shutterstock.com

A seismogram is used to measure earthquakes. It records a seismogram that yields data about the earthquake. A seismogram has two main parts - a base that firmly sets on ground and a heavy, hanging weight. While the former shakes when the ground trembles during an earthquake, the latter does not. The string or spring holding the weight absorbs the movement. The different in position between the moving and stationary parts of the seismograph is recorded.

A seismograph recording. Image credit: Inked Pixels/Shutterstock.com

The seismogram helps measure the size of an earthquake which depends on the extent of tectonic activity. Small earthquakes have short wiggly lines on the seismograph with less wiggling while the powerful ones are longer and tend to wiggle a lot.

Siesmograms also help determine the location of earthquakes throught the detection of the epicenter of the earthquake.

Prediction Of Earthquakes

For centuries, scientists humans have attempted to devise ways and means to predict earthquakes. However, given that the source of such natural phenomenon lie deep down within the Earth, it is highly unlikely that a foolproof method of earthquake prediction will be discovered anytime in the near future. However, precautions in areas prone to earthquakes can be adopted to prevent heavy loss of life and property when earthquakes do occur.

Frequency And Intensity Of Earthquakes

Earthquake strength is expressed in magnitude. Image credit: VectorMine/Shutterstock.com

Though devastating, natural-disaster-size earthquakes are, thankfully, not all that common, there are roughly 50,000 earthquakes every year which can be felt by human life (this figure does not include the number of seismic activity deemed too small to be felt by the average person). An estimated 100 of these noticeable 50,000 earthquakes cause damage to the earth and its surroundings, and it is only one or two of these human-detected earthquakes that is classified as a natural disaster.

Effects Of Earthquakes

Earthquakes have long affected the Earth, and formed a key part in the formation and alteration of the surface of the Earth. While minor earthquakes, and seismic activity is occurring constantly, it is only on rare occasions that humans can feel these energy bursts, and rarer still that they cause any destruction. As technology advances, humans have been able to mark, track, record and analyze more and more earthquakes with greater accuracy. This allows for a greater understanding of earthquakes, and also aids in predicting on forecasting potentially destructive seismic activity, in order to hopefully avoid any foreshadowed great disasters.

Earthquake-prone Regions Of The World

Circum-Pacific Belt

The Pacific Ring of Fire or Circum-Pacific Belt is the region of the Earth where most earthquakes occur. Image credit: Jones M/Shutterstock.com

The Circum-Pacific Belt rings the tectonic plate known as the Pacific Plate, one of the largest of the Earth’s tectonic plates. This area has increased earthquake activity and is located in the Pacific Ocean, bordering various pacific nations such as near New Zealand, Japan, and New Guinea. When looking at a plate map, it is clear how such volcanic islands were formed, many years ago, as these islands exist right along the lines of the fault, indicating that they were formed due to volcanic activity caused by plate interaction and fissures in the Earth’s crust. The Circum-Pacific belt also includes the area along the fault line near Alaska and the North and South American Western coast, which includes both the Pacific Plate and the Nazca Plate, which hugs the edge of South America. The Circum-Pacific Belt holds the most earthquakes on Earth, and is home to roughly 80% of the seismic activity on the planet. Though not all seismic activity is felt by humans, the Circum-Pacific Belt has resulted in a large number of catastrophic or damaging earthquakes and volcanic activity throughout history. Due to its highly volcanic and seismically active border rim, the Circum-Pacific Belt which forms a ring-like border, is known more commonly as the Pacific Ring of Fire.

Alpide Belt 

The Alpide Belt. Image credit: Jo weber/Wikimedia.org

The Alpide Belt is similar to the Circum-Pacific, and is another hot spot for seismic activity and volcanic vents and fissures. The Alpide is actually connected to the Circum-Pacific belt where the two converge in the East Indies. From there, the belt runs through the Mediterranean and onward through much of Asia. This area does experience increased seismic activity in comparison to other parts of the world not near plate edges, but not as much as the Circum-pacific belt. The Alpide Belt is an area where a number of plates, namely the Eurasian, Arabian, Indian, African and Indo-Australian plates all converge. It is the abundance of plate edges and cracks in the Earth’s surface that makes this region a hotspot for earthquakes and seismic activity. In comparison to the Circum-Pacific Belt, however, the Alpide belt is responsible for roughly 15% of the Earth’s seismic activity. Only 5% of the Earth’s seismic activity exists outside of the Alpide or Circum-Pacific belts.

Other Earthquake-prone Regions

Other areas of increased seismic activity include other plate borders, such as that along the oceanic ridges of the Arctic Ocean and Atlantic ocean, and the fault found along the Great Rift Valley in Africa (AfroAsian Belt).

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