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 just what are earthquakes and what causes earthquakes in this lesson!
An earthquake is a shaking, rolling, or sudden shock of the surface of Earth.
An earthquake, also known as a seismic event, is a powerful shaking, rumbling, or sudden jolt beneath the Earth’s surface. Picture yourself standing on a massive rug, and someone unexpectedly yanks it out from under you! That’s what experiencing an earthquake can feel like. These awe-inspiring natural occurrences take place when colossal blocks of Earth’s crust, referred to as tectonic plates, collide, glide past one another, or drift apart.
You might be pondering, “What exactly are tectonic plates?” Excellent question! Tectonic plates are gigantic, jigsaw-like slabs that form Earth’s outer layer or lithosphere. These plates are continuously on the move, albeit at a pace too slow for us to notice. However, when they do shift, they generate an immense amount of energy that travels through the Earth as seismic waves, resulting in an earthquake.
Earthquakes can be both thrilling and intimidating, but they play a crucial role in shaping and transforming our planet. So, the next time you hear about an earthquake or feel the ground quiver, remember that you’re witnessing the astounding power of nature’s forces at work!
Why not check out our tectonic plate activity:
The Earth is made up of four layers: inner core, outer core, mantle, and crust. The mantle and crust are 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.
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.
Did you know that not all earthquakes are the same? That’s right! There are various types, each with its own unique characteristics and what causes earthquakes. Let’s check out these fascinating ground-shaking events and how they differ from one another.
Tectonic earthquakes are the most common type of earthquake experienced around the world. These earthquakes happen when tectonic plates – the massive pieces of Earth’s crust – shift and move, causing the rocks within the crust to break. The intense energy released during these movements results in the ground shaking and trembling that we know as an earthquake.
Volcanic earthquakes are a special type of tectonic earthquake that occurs due to a combination of volcanic activity and tectonic forces. When molten rock, or magma, moves beneath the Earth’s surface, it can create pressure and cause the surrounding rocks to fracture. This movement, along with the shifting tectonic plates, can trigger these unique ground-shaking events.
Collapse earthquakes are smaller earthquakes that take place in underground spaces such as caverns and mines. These quakes happen when the ground above these hollow areas collapses or caves in. Although collapse earthquakes are usually less powerful than other types of earthquakes, they can still cause significant damage if the collapsed cavities are located beneath buildings or other structures.
Explosive earthquakes are man-made events that occur as a result of the detonation of nuclear or chemical devices or weapons. These explosions release a tremendous amount of energy, causing the ground to shake and create earthquake-like effects. Unlike the other types of earthquakes, explosive earthquakes are not caused by natural processes.
The epicenter of an earthquake is a crucial point on Earth’s surface that lies directly above the location where the rocks cracked or shifted deep within the Earth. This spot helps scientists understand where the earthquake originated and which areas might have experienced the strongest shaking.
Meanwhile, the focus of an earthquake, also known as the hypocenter, is the actual site where the rocks crack or slip underground. The focus is often situated miles beneath the Earth’s surface, hidden from view but responsible for the tremors we feel above ground. When an earthquake occurs, seismic waves radiate outwards from the focus, traveling through the Earth and causing the ground to shake and vibrate.
Understanding the difference between the epicenter and focus is essential for grasping the mechanics of earthquakes. The focus represents the underground source of the earthquake, while the epicenter is the point on the surface directly above it. By studying these two key components, scientists can gain valuable insights into the causes, locations, and impacts of these awe-inspiring natural phenomena.
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.
There are two basic types of seismic waves: Body Waves and Surface Waves.
They are the waves that travel through the Earth. They are classified into two types:
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.
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.
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:
Also called ground roll. They cause the ground to shake in an elliptical pattern. This motion is similar to that in ocean waves.
They cause the surface to move horizontally from side to side perpendicular to the direction of travel. They move faster than Rayleigh waves.
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.
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.
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:
| Magnitude | Class | Description and Damage |
| 3.0 – 3.9 | Minor | May be felt. |
| 4.0 – 4.9 | Light | Could shake your house. Some people may not notice. |
| 5.0 – 5.9 | Moderate | Stuff may fall off of shelves. There may be minor damage. |
| 6.0 – 6.9 | Strong | May cause damage to buildings and other structures. Walls in some houses may crack and windows break. |
| 7.0 – 7.9 | Major | Serious damage. Weaker buildings will collapse, and cracks will occur in bridges and on the street. |
| 8.0 – 8.9 | Great | There will be significant damage. Large cracks will appear in the Earth. Sometimes whole cities could be flattened. |
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 one location to another depending on how near it is to the epicenter. This means that there can be multiple intensity measurements as opposed to one magnitude measurement.
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.
| Intensity | Description and Damage |
| I | Usually nobody even feels it. |
| II | Weak shaking. Only a few people may feel it, especially on upper floors or buildings. Hanging objects may swing gently. |
| III | Weak 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. |
| IV | Light 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. |
| V | Moderate 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. |
| VI | Strong shaking. Everyone feels it. It is frightening. People walk unsteadily. Trees shake. Pictures fall off walls. Furniture moves or overturns. |
| VII | Very strong shaking. It is difficult to stand. Weak buildings are damaged. Loose bricks, stones and tiles fall. Some chimneys are broken. Furniture is broken. |
| VII | Severe 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. |
| IX | Violent shaking. Even strong buildings are damaged. Some buildings completely collapse, some shift off foundations. There are cracks in the ground. Underground pipes get broken. |
| X | Extreme 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. |
| XI | Railway rails bent greatly. Underground pipes are completely destroyed. |
| XII | Nearly total damage. Objects are thrown into the air. Large rock masses get displaced. Whole cities may be flattened. |
Foreshocks are smaller earthquakes that take place before the main earthquake, or mainshock occurs. These preliminary tremors can sometimes provide scientists with clues about an impending larger seismic event. However, predicting the exact timing and magnitude of a mainshock based on foreshocks remains a challenge for seismologists.
The mainshock is the largest and most powerful earthquake in a series of seismic events. This primary tremor is the one that typically causes the most damage and receives the most attention from scientists, emergency responders, and the public.
Aftershocks are the smaller earthquakes that follow the mainshock. These subsequent tremors can continue for hours, days, or even months after the main earthquake. Aftershocks are typically less powerful than the mainshock but can still cause additional damage to weakened structures and pose risks to people in affected areas.
By understanding the differences between foreshocks, mainshocks, and aftershocks, we can better appreciate the complex and dynamic nature of earthquake sequences.
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.
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 epicenters. 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 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.
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.
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.
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.
Also known as the Indian Ocean Earthquake. Its epicenter 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.
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 meters (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.
It was a huge earthquake that struck off the coast of the 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.
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.
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.
This earthquake triggered a tsunami of over 10 metres on Shemya Island. However, it caused very little damage because of its distant location.
This earthquake occurred at the 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.
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 canceled. 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.
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.
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.
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.
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 a 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.
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.
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.
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 of up to IX in some areas. This earthquake caused massive tsunami waves that killed at least 227,000 people across many countries.
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.
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.
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.
Why not check out some more natural disaster articles: Hurricanes, Volcanoes, Tsunami and Tornadoes.
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