What Causes Earthquakes? Discover Top 10 Earthquakes in History!

You must have heard about earthquakes, but have you ever felt one? Have you ever felt the ground shaking beneath your feet? What is the reason behind this? And what should you do during an earthquake? Let’s find out all about earthquakes in this lesson!

What Is an Earthquake?

An earthquake is a shaking, rolling or sudden shock of the surface of Earth.

What Causes Earthquakes?

The Earth is made up of four layers: inner core, outer core, mantle and crust. The mantle and crust are a thin skin compared to the other layers. The mantle is about 1,800 miles thick, while the crust is only about 8 miles thick. The surface of Earth, or the crust, is not one solid piece. It is broken into pieces of rock masses called “tectonic plates”. 

The plates are always moving and pushing into each other, but they move slowly. Sometimes the plates slide under or against each other and become stuck. When they slide apart, they create pressure force. When the force becomes too great, the rock masses suddenly shift causing a fault in the crust. A fault is a crack in the Earth’s crust. This force causes the Earth to shake and ripple, and this is how we get an earthquake. 

Sometimes earthquakes occur because of other natural or human causes. For example, the movement of magma beneath a volcano can cause an earthquake. Holding great amounts of water behind dams causes pressure which can cause quakes. Furthermore, the digging of mines and underground nuclear explosions can also cause quakes. 

Types of Faults That Cause Earthquakes

There are three main types of geological fault that may cause an earthquake: normal, reverse and strike-slip. Normal faults occur when the crust is being extended. Reverse faults occur when the crust is being shortened. Strike-slip faults occur when the two sides of the fault slip horizontally past each other.

Types of Earthquakes

1. Tectonic Earthquakes

This is the most common type of earthquake. They occur when the tectonic planets move and cause rocks in the Earth’s crust to break. 

2. Volcanic Earthquakes

Volcanic earthquakes are a form of tectonic earthquake that occur because of volcanic activity along with tectonic forces.

3. Collapse Earthquakes

Collapse earthquakes are small earthquakes in underground areas such as caverns and mines. Collapse earthquakes are usually less powerful than other earthquakes, but they can cause much damage if the collapsed cavities are underneath structures.

4. Explosive Earthquakes

Explosion earthquakes occur because of the explosion of nuclear or chemical devices or weapons. So, this type of earthquake is man-made. 

Epicentre and Focus of Earthquakes

The epicentre is the point on the Earth’s surface directly above where the rocks cracked. The focus of the earthquake is where the rocks actually cracked. The focus is often miles beneath the Earth’s surface.

What Are Seismic Waves?

Shock waves from an earthquake that travel through the ground are called seismic waves. They are most powerful at the centre of the earthquake, but they travel through much of the Earth and back to the surface. They move quickly at 20 times the speed of sound.

Types of Seismic Waves

There are two basic types of seismic waves: Body Waves and Surface Waves. 

1. Body Waves

They are the waves that travel through the Earth. They are classified into two types: 

a. Primary Waves (P-Waves)

The first waves of an earthquake. They cause the ground to move back and forth in the direction of travel. They travel faster than other waves through the Earth, and they are the first waves to arrive at seismic recording stations. These waves can travel through solids, liquids, and even gases.

b. Secondary Waves (S-Waves)

The next waves to arrive. They are also called transverse waves. They shake the ground in a crosswise motion that is perpendicular to the direction of travel. These are the shake waves that move the ground up and down or from side to side. They travel slower through the Earth, and they can travel only through solid materials. These waves cause the most damage.

2. Surface Waves

They are the waves that travel at the Earth’s surface. They are usually generated when the source of the earthquake is close to the Earth’s surface. Although the surface waves travel slower than the body waves, they are much larger and more destructive. There are two kinds of Surface Wave:

a. Rayleigh Waves

Also called ground roll. They cause the ground to shake in an elliptical pattern. This motion is similar to that in ocean waves.

b. Love Waves

They cause the surface to move horizontally from side to side perpendicular to the direction of travel. They move faster than Rayleigh waves. 

How Are Earthquakes Measured?

Earthquakes occur very frequently though you might not always feel the shaking. This is because not all earthquakes are strong. But how are they measured? Earthquakes are measured according to two aspects: their magnitude and their intensity.

1. Magnitude

The magnitude of an earthquake is a measure of how large the earthquake is. It only measures the size and does not change depending on where you are or how the shaking feels. Earthquakes spread energy in the form of seismic waves. Instruments called seismographs are used to measure how large the earthquake was.

Earlier, the instrument was using a weighted pen and a spring, and the vibrations from the earthquake made the pen draw lines onto some paper. A short wiggly line that didn’t wiggle very much meant a small earthquake, while a long wiggly line that wiggled a lot meant a large earthquake.

Then, from 1930 to 1970, scientists used the Richter scale to measure earthquakes. This scale, invented in 1934 by California scientist Charles Richter, measures the magnitude of an earthquake, and the result is a number from 0 to 10. The larger the number on the scale, the larger the earthquake.You usually won’t even notice an earthquake unless it measures at least a 3 on the scale.

However, the Richter scale only measures the largest wiggle of an earthquake. So, it was replaced with the Moment Magnitude Scale, which measures different parts of the earthquake and records all the different seismic waves to seismographs across the world. 

Earthquake Magnitude Classes

Earthquakes are classified into classes depending on their magnitude. These classes also provide earthquake measurement. The classification starts with “minor” for lower magnitudes and ends with “great” for largest magnitudes. The classes are: 

MagnitudeClassDescription and Damage
3.0 – 3.9MinorMay be felt.
4.0 – 4.9LightCould shake your house. Some people may not notice.
5.0 – 5.9ModerateStuff may fall off of shelves. There may be minor damage.
6.0 – 6.9 StrongMay cause damage to buildings and other structures. Walls in some houses may crack and windows break.
7.0 – 7.9MajorSerious damage. Weaker buildings will collapse, and cracks will occur in bridges and on the street.
8.0 – 8.9GreatThere will be significant damage. Large cracks will appear in the Earth. Sometimes whole cities could be flattened.

2. Intensity

The intensity of an earthquake is a measure of how much damage it caused. It tells the severity of the shaking and its effects on people and their environment. So, intensity measurements differ from a location to another depending on how near it is to the epicentre. This means that there can be multiple intensity measurements as opposed to one magnitude measurement. 

Earthquake Intensity Scale

Scientists use the Modified Mercalli Scale to measure the intensity of an earthquake. The scale is composed of 12 levels that range from observable quake impacts to catastrophic destruction. Intensity is reported by Roman numerals.

IntensityDescription and Damage
IUsually nobody even feels it.
IIWeak shaking. Only a few people may feel it, especially on upper floors or buildings. Hanging objects may swing gently.
IIIWeak shaking. People can feel it indoors, especially on upper floors or buildings. Many people do not recognize it as an earthquake. Vibrations are similar to the passing of a truck.
IVLight shaking. Most people feel it indoors, and some may feel it outdoors. There are vibrations. Windows, dishes and doors rattle. Walls may make cracking sounds.
VModerate shaking. Almost everyone feels it. It wakes sleeping people up. Liquids may be spilled. Doors swing. Windows and dishes are broken. Pendulum clocks may stop or change rate.
VIStrong shaking. Everyone feels it. It is frightening. People walk unsteadily. Trees shake. Pictures fall off walls. Furniture moves or overturns.
VIIVery strong shaking. It is difficult to stand. Weak buildings are damaged. Loose bricks, stones and tiles fall. Some chimneys are broken. Furniture is broken.
VIISevere shaking. Many buildings are damaged, and some may collapse. Chimneys, columns, monuments and walls fall. Tree branches are broken. There may be cracks in wet ground and on steep slopes.
IXViolent shaking. Even strong buildings are damaged. Some buildings completely collapse, some shift off foundations. There are cracks in the ground. Underground pipes get broken.
XExtreme shaking. There are large landslides. Most buildings are destroyed with their foundations. Bridges, dams and dikes are destroyed. Water gets thrown on banks of canals, rivers and lakes. Railway rails bent slightly.
XIRailway rails bent greatly. Underground pipes are completely destroyed.
XIINearly total damage. Objects are thrown into the air. Large rock masses get displaced. Whole cities may be flattened.
Earthquake

Foreshocks and Aftershocks

Before and after a large earthquake, there will likely be smaller earthquakes. The ones that happen before are called foreshocks. The ones that happen after are called aftershocks. The large, main earthquake is called the mainshock.

When and Where Do Earthquakes Occur?

Earthquakes can happen anywhere. Most of them occur along the edges of the plates of Earth’s crust. One great earthquake belt is known as the “Ring of Fire”. This is an area around the edges of the Pacific Ocean. Most large earthquakes occur in the Ring of Fire. Another belt is in the Mediterranean region between Europe and North Africa. In addition, earthquakes can occur at any time of the year and at any time of the day or night. 

Can Earthquakes Be Predicted?

Unfortunately, earthquakes cannot be predicted. However, many large earthquakes have foreshocks that tell a large earthquake is coming. In addition, scientists have observed some physical changes that happen before some earthquakes in the environment around their epicentres. These changes include deformation in the crust in the fault zones, and an increase in the volume of rocks. With continuous research, scientists may one day be able to predict earthquakes. 

Earthquakes Effects

Earthquakes can do severe damage to buildings, bridges, pipelines, railways, dams, and other structures, and they can lead to deadly fires. Earthquakes can also cause serious changes at Earth’s surface. These changes include changes in the flow of groundwater, landslides, and mudflows. 

Adding to all this, some quakes that occur underwater or near oceans can cause huge, deadly waves called tsunamis. The violent shaking of the seafloor produces waves that spread over the ocean’s surface. When these waves reach the shore, they destroy and wipe out the coastal regions. 

How to Stay Safe If an Earthquake Strikes?

Earthquakes can be very dangerous, and they are unpredictable. So, it is important to know some simple safety rules to follow in case of an earthquake.

Preparing Before an Earthquake

  • Make an emergency plan with your family members, and plan where to meet if you get separated.
  • Know where your electricity, gas, and water switches are and how to shut them off.
  • Store large, heavy, and breakable objects on low shelves.
  • Practice how to “Drop, Cover, and Hold On” with each member of your household.

What to Do During an Earthquake?

  • Drop, Cover, and Hold On: DROP down to the ground, COVER your head and neck with your arms, and HOLD ON to any sturdy item you can until the shaking stops.
  • If you cannot drop to the ground, try to sit so the earthquake doesn’t knock you down.
  • Crawl under a sturdy piece of furniture, if you can, for more protection.
  • Stay away from glass, windows, outside doors and walls, and other items that could fall.
  • If you are in bed, turn face down and cover your head and neck with a pillow.
  • If you are outdoors, stay outdoors away from buildings. Do not try to run inside.
  • If you are inside, stay and do not run outside or to other rooms.
  • If you are in a car, stay in it and set the parking brake until the shaking stops.

After an Earthquake

  • Earthquakes are sometimes followed by aftershocks. So, be ready to Drop, Cover, and Hold Onagain if you feel an aftershock. Aftershocks can go on for weeks or even months.
  • Check your home for damage, and get everyone out if the building is unsafe.
  • Do not enter any damaged buildings.
  • If you are in a damaged building, go outside and quickly move away from the building.
  • If you are trapped, try calling or texting for help. Bang on a pipe or use a whistle to help others know where you are. Cover your mouth with your shirt.
  • If you are in an area that may experience tsunamis, go inland or to higher ground right after the shaking stops.
  • Follow instructions from public safety officials.

10 of the Largest Earthquakes by Magnitude in History

1. Valdivia Earthquake (22 May 1960) – 9.4-9.6

It is the most powerful earthquake ever recorded. It struck in Chile, and it lasted for about 10 minutes. Not only that, but the earthquake also caused tsunami waves that affected southern Chile, Hawaii, Japan, the Philippines, eastern New Zealand, southeast Australia, and the Aleutian Islands. Estimates said that between 1000 – 6000 people were killed. 

2. Great Alaska Earthquake (27 March 1964) – 9.2

It is the most powerful earthquake in U.S. history. The earthquake lasted for about 4.5 minutes, and it caused a large tsunami and a massive underwater landslide. 139 people were killed: 15 as a result of the earthquake itself, 106 from the subsequent tsunami in Alaska, 5 from the tsunami in Oregon, and 13 from the tsunami in California. 

3. Sumatra Earthquake (2004) – 9.1-9.3

Also known as the Indian Ocean Earthquake. Its epicentre was off the west coast of northern Sumatra, Indonesia. It was an undersea earthquake that lasted between eight and ten minutes, and it caused the whole planet to vibrate. The earthquake reached a Mercalli intensity up to IX in some areas. This earthquake caused massive tsunami waves that killed at least 225,000 people across a dozen countries.

4. Tōhoku earthquake (2011) – 9.1

Also known as the Japan Earthquake. It was a massive undersea earthquake that occurred off the coast of Japan. The earthquake caused massive tsunami waves that were up to 40.5 metres (133 feet) high. It was one of the deadliest in human history. This resulted in 15,894 deaths, 6,152 injured, and 2,562 people went missing.

5. Kamchatka, Russia Earthquake (1952) – 9.0

It was a huge earthquake that struck off the coast of Kamchatka Peninsula, in the far east of Russia. The earthquake caused huge tsunami waves that went up to up to 15 metres (50 feet) high. It caused huge damage to the Kamchatka Peninsula and the Kuril Islands. Around 10,000 to 15,000 people were killed.

6. Chile Earthquake (2010) – 8.8

It occurred off the coast of central Chile, and lasted for about three minutes. The shaking caused some buildings to collapse in many cities, including the capital, Santiago. The earthquake also caused a tsunami that destroyed several coastal towns in Chile. Around 525 people were killed. 

7. Ecuador-Colombia Earthquake (1906) – 8.8

It occurred off the coast of Ecuador, near Esmeraldas (a coastal city in northwestern Ecuador). The earthquake caused a destructive tsunami that killed at least 500 people on the coast of Colombia.

8. Rat Islands Earthquake (1965) – 8.7

This earthquake triggered a tsunami of over 10 metres on Shemya Island. However, it caused very little damage because of its distant location. 

9. Assam-Tibet Earthquake (1950) – 8.6

This earthquake occurred at Xizang-India border region. Many buildings were destroyed, and between 1.500 and 3.000 people were killed. The earthquake also caused large landslides that blocked the Subansiri River. This caused a huge wave to break out after some days that overflowed many villages and killed around 530 people.  

10. Sumatra Earthquake (2012) – 8.6

It was an undersea earthquake in the Indian Ocean near the city of Aceh, Indonesia. Right after the shaking, authorities feared that the earthquake would cause a tsunami, and they issued warnings, but these warnings were then cancelled. The earthquake occurred very far from any inhabited place, and it caused a small tsunami that was not destructive. There were around 10 deaths, most of them caused by panic or heart attack.

Earthquake

10 of the Deadliest Earthquakes in History

1. Shaanxi Earthquake, 1556

The deadliest earthquake ever recorded in human history. It was a massive earthquake at magnitude 8 in Shaanxi province in northern China. The pre-modern buildings and structures were unable to withstand such a powerful earthquake. There were also great landslides because of the quake. This earthquake resulted in killing or injuring around 830,000 people.

2. Antioch Earthquake, 115

The most damaging earthquake in Turkey. It had a magnitude of 7.5 and a maximum intensity of XI (Extreme) on the Mercalli intensity scale. It destroyed over two-thirds of Antioch, and the damage extended to surrounding areas. It also caused a tsunami that damaged the coast of Lebanon. The total number of deaths is around 260,000 people. 

3. Antioch Earthquake, 526

It struck Syria and the city of Antioch in the Byzantine Empire, with a magnitude of 7.0. The earthquake caused severe damage to many of the buildings in Antioch, but most of the damage was by the fires that broke out after the earthquake. More than 250,000 people died in this earthquake. 

4. Tangshan Earthquake, 1976

With a magnitude of 7.5, this earthquake nearly destroyed the city of Tangshan, located east of Beijing. There was an aftershock that struck 16 hours later with similar magnitude, and it was equally destroying. The reports said that around 240,000 people were killed and at least 700,000 people were injured. 

5. Haiyuan Earthquake, 1920

It occurred in Haiyuan County, Ningxia Province, China, with a magnitude of 7.8 on the Richter scale. This earthquake caused rivers to change course and caused great landslides. It destroyed at least seven Chinese provinces, and a town was completely buried under a landslide. Around 230,000 people were killed. 

6. Aleppo Earthquake, 1138

It struck Aleppo, a city in northern Syria. There was a small foreshock that made many people flee to the countryside before the main earthquake. On the next day, the mainshock occurred, and it caused severe damage. Many buildings collapsed, including the city walls. It is said that around 230,000 people were killed. 

7. Sumatra Earthquake 2004

Also known as the Indian Ocean Earthquake. Its epicentre was off the west coast of northern Sumatra, Indonesia. It was an undersea earthquake that lasted between eight and ten minutes, and it caused the whole planet to vibrate. The earthquake reached a Mercalli intensity up to IX in some areas. This earthquake caused massive tsunami waves that killed at least 227,000 people across many countries.

8. Haiti Earthquake, 2010

With a magnitude of 7.0, this earthquake occurred on the West Indian Island of Hispaniola, comprising the countries of Haiti and the Dominican Republic. The earthquake was followed by more than 50 aftershocks. It caused extreme damage, and most buildings collapsed. Around 200,000 people were killed. 

9. Great Kantō Earthquake, 1932

It was an earthquake with a magnitude of 7.9 that struck the Tokyo-Yokohama metropolitan area in Japan. It lasted between four and ten minutes. More than half of the brick buildings collapsed. Many hundreds of thousands of houses were either destroyed or burned in the fire that was caused by the quake. It also caused a tsunami that destroyed 155 houses and killed 60 people. The total number of deaths was more than 140,000. 

10. Ashgabat Earthquake, 1948

This earthquake struck Ashgabat in Turkmenistan and some nearby villages. With a magnitude of 7.3, this earthquake caused extreme damage. Almost all brick buildings collapsed, and concrete structures were so damaged. The total number of deaths reached about 110,000 persons. 

Quick Facts

  1. Most earthquakes last for less than a minute.
  2. Earthquakes can occur in any kind of weather.
  3. The Ring of Fire is the most seismically and volcanically active zone in the world.
  4. Earthquakes are sometimes called earth tremors.
  5. When two or more of Earth’s tectonic plates are pushed together, they could form large mountain ranges. For example, the Himalayas and the Andes were formed by movement of tectonic plates.

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