What is an Earthquake?

What is an earthquake?

An earthquake is the very literal name of the phenomena of the surface of earth shaking due to seismic activity happening beneath the crust! These seismic waves, created by the Earth’s lithosphere (upper crust and mantle), cause the various tectonic plates that make up the upper-most part of the Earth to shudder which, in turn make the ground we stand on begin to shake violently. There are various different levels for earthquakes and various different circumstances they require to occur, but overall earthquakes are known for being massive destructive forces that have wreaked havoc upon countless countries, islands, cities, and even the moon!

Earthquakes are phenomena that are important to study in order to help prevent the disastrous fallout they can create. But how do earthquakes happen in detail? And what more is there to learn about them? Well, come along with us to learn all about these earthquake facts for kids!

What causes earthquakes?

To answer that, we need to look at what’s going on beneath the Earth’s surface! The Earth is made up of four main layers: crust, mantle, outer core, and inner core. Together, the crust and mantle form the Earth’s lithosphere, which acts as a solid outer shell. This shell isn’t one continuous smooth surface, though, like an eggshell; it’s more akin to an enormous rocky jigsaw puzzle in which some pieces span entire countries and continents. Below the lithosphere is the outer core, a hot metallic liquid made up of mostly nickel and iron. And at the center of the Earth, nearly 2000 miles below the surface, lies the inner core, a dense, molten ball of compacted iron, the temperature of which is believed to be around 9,000 °Fahrenheit.

So, how do earthquakes occur? Well, it’s all about how those different parts of the lithosphere interact with one another. Known as tectonic plates, these huge pieces of rock are constantly changing positions due to movement in the mantle below and the planet’s steady rotation. The constant motion of tectonic plates can cause cracks, known as faults, in the Earth’s crust. Earthquakes occur when the crust moves suddenly on or near one of these fault lines.

Here's how it happens, step by step:

1. Elastic Strain: The first stage of an earthquake is the gradual build-up of elastic strain along a fault-line between two tectonic plates, which occurs over thousands of years. When both sides of the fault move, the elastic strain builds up in the rocks slowly, compressing the rock particles together.
2. Dilatancy: Stage two occurs when the rocks in the Earth are packed together as closely as possible. The rocks must then expand by way of cracking to increase the amount of space they occupy. This process is called dilatancy. As small cracks form, the water inside the pores of the rocks is forced out and air is let in. As a result, the rocks become stronger. The process allows the rocks to hold even more elastic strain, which makes the eventual quake much more forceful.
3. Influx of Water: Stage three occurs when the water that has seeped out of the rocks is forced back in because of surrounding pressure, similar to the way water fills a hole in sand. As the water is forced back in, the rock loses its strength. The rocks are significantly strained by this. The influx of water keeps more cracks from forming, which causes the rocks to stop expanding. The water later eventually serves as a lubricant when the elastic strain that has been building up over time is released.
4. Earthquake: Stage four is the actual earthquake. Because the rocks can no longer resist the elastic strain, a sudden fault rupture occurs. The stored energy in the rocks is now forced out and released in the form of heat and seismic waves. Seismic waves are large waves of energy that flow outward through the Earth's crust, like ripples in a pond. The waves cause a sudden, often violent shaking of the ground.
5. Aftershocks: Stage five is the final stage during which a sudden drop in stress causes small aftershocks, which are smaller earthquakes or ruptures. The aftershocks release the remaining elastic strain. Aftershocks are often unpredictable and can occur years after the initial earthquake. Depending on the size of the main earthquake, the size, and frequency of aftershocks can be significant. Eventually, the strain decreases and allows normal conditions under the surface to return.

How long do earthquakes last?

Most earthquakes typically last for a few seconds, usually 10-30 seconds. It is assumed that a larger earthquake than we have ever experienced may last minutes at a time, but for the most part earthquakes typically last about half a minute or less! This may feel like a lifetime if you find yourself caught in the center of one, but know that it will soon end faster than you expect! After the earthquake, the ground may tremor slightly still for a few more hours, days, or years even. These shudders are called aftershocks!

What is an aftershock?

Aftershocks or aftershock earthquakes are the after effects of past earthquakes. These slight tremors can last for up to years at a time and are in reality the earth’s mantle trying to realign itself after the small fault or crack that caused the initial quake. These aftershocks will never be as extreme as the earthquake which caused them, but can still be quite devastating if left unexpected.

Types of Earthquakes

Like most of the scientific world, earthquakes are oftentimes categorized by specific types and levels. The strongest earthquake you can experience is anything above an 8.0 magnitude. At this level, the earth shakes violently and communities near the center of the earthquake’s origin will most likely be destroyed. Usually, though, people will experience a magnitude of 5.4 and lower.

As for types, there are four in particular that you can usually classify an earthquake as: volcanic, tectonic, collapse and explosion.

• Volcanic: Perhaps the most self-explanatory earthquake type, the Volcanic Earthquake occurs usually in tandem with volcanic activity. When a volcano starts erupting, the ground shakes, and that there is your volcanic earthquake!
• Tectonic: Tectonic Earthquakes occur thanks to the tectonic plates, who may move and scrape against each other to create the tremors we feel on the crust. These geological forces make them the most common types of earthquakes in places that sit upon a lot of different tectonic fault lines, like California!
• Collapse: Unlike volcanic and tectonic earthquakes, Collapse Earthquakes occur underground, in caves and mines. These earthquakes are usually small tremors produced by the explosion or collapse of rocks on the surface.
• Explosion: The final earthquake type is the Explosion Earthquake, which occurs when usually human-made bombs. The detonations cause an earthquake effect.

Most Famous Earthquakes

Earthquakes are devastating natural events that can completely destroy the areas they affect. Here is a list of some of the most powerful and destructive earthquakes throughout history:

• Knossos, Crete, Greece, c. 1500 B.C.E

The first earthquake on this list dates all the way back to Ancient Greece. As this earthquake was so long ago, there is no exact recording of its magnitude or intensity, but it is known to be one of several events that completely leveled the capital of Minoan civilization.

• Pompeii and Herculaneum, Italy, 62 C.E.

Jumping forward a few years to Ancient Italy, these two important Italian cities were struck by the same earthquake in 62 C.E. This earthquake was so devastating that neither Pompeii or Herculaneum had recovered from its aftermath by the time Mount Vesuvius erupted in 79 C.E.

• Shaanxi province, China, 1556

This devastating earthquake that hit China back in the mid 1500s is widely regarded as the deadliest earthquake of all time. The quake caused 830,000 deaths and completely destroyed the Shaanxi province.

• Port Royal, Jamaica, 1692

Prior to the earthquake that struck this very famous British West Indies port in 1692, Port Royal was a popular haven amongst pirates and slave traders of the time. When the earthquake struck, however, 2,000 people were killed and the entire port was pretty much submerged beneath the sea.

• Lisbon, Portugal, 1755

The earthquake that struck Lisbon in 1755 was not restricted to this one city. In fact, this deadly earthquake was felt as far away as the Algiers and triggered a tsunami that reached the Caribbean. The total amount of deaths caused by this earthquake was recorded as 62,000.

• Charleston, South Carolina, U.S., 1886

This earthquake goes down as one of the largest quakes to ever hit the eastern United States. The total amount of deaths recorded for this earthquake was 60.

• Gansu province, China, 1920

Whilst the earthquake that hit the Gansu province in the early 1900s was severe, the majority of the deaths surrounding it were actually caused by the massive landslides that followed. In total, there are 200,000 deaths associated with this earthquake and the following events that it triggered.

• Assam, India, 1950

The massive earthquake that hit Assam in 1950 is marked down as the largest quake ever recorded in South Asia. Despite its size, the death toll for the earthquake was relatively small, with only 574 people losing their lives. This is largely due to the fact that this region along the Indo-Chinese border is pretty sparsely populated.

• Kathmandu, Nepal, 2015

In more recent years, the earthquake that hit Nepal in 2015 measured in at 7.9 on the Richter Scale and killed 9,000 people. This quake was closely followed by two similarly devastating aftershocks, each coming in at 6.6 and 6.7 on the Richter Scale respectively.

How are earthquakes measured?

Earthquakes are measured by scientists using seismographs, or seismometers. They measure the earthquake's size and intensity. These specialist devices measure how strong an earthquake is by picking up vibrations and movement — the seismic waves — in the Earth’s surface. Seismologists (those who study earthquakes) used to use the Richter Scale to measure the earthquake's strength. The Richter scoring system runs from one through 10, with one representing a very minor tremor and 10 indicating an extremely severe earthquake. The worst earthquake ever recorded was a 9.5 on the Richter scale — earthquakes of this size aren't as common as the smaller ones (which are often not even felt by people on the ground) but can cause massive amounts of destruction.

Today, scientists measure earthquakes using the Moment Magnitude Scale which is more accurate than the Richter scale when measuring large earthquakes.

Did you know the term seismology comes from the Greek “seismos” meaning earthquake? The awe-inspiring nature of earthquakes means that you’ll often hear people use the word “seismic” to describe sudden and significant events that have a big impact on our lives, like the outbreak of war or political unrest.

What was the biggest earthquake in history?

The most powerful earthquake ever recorded struck Valdivia, Chile, just after 3 p.m. on May 22, 1960. It left two million people homeless and killed over 1600. The Valdivia earthquake set off a tsunami that was so strong, it reached the coast of New Zealand over 5000 miles away.

Where do earthquakes usually occur?

Earthquakes can happen anywhere in the world, and in fact there are earthquakes every single day, we just don’t feel them usually. The places in the world where you will most certainly experience a major earthquake more than once would be places along fault lines. Fault lines are usually the edge of the tectonic plates and occur when different plates grind against each other. This movement beneath the earth’s surface is what we know as an earthquake, so fault lines in particular tend to generate much more obvious quakes than the center of a tectonic plate might.

There are three known types of faults: normal, reverse, and strike-slip. Normal faults are when one wall of a tectonic plate moves upwards while the other moves downwards, causing the tremors on the surface and developing the fault. Reverse faults form when the Earth’s crust receives a significant amount of pressure. This compression causes rocks and tectonic walls to slip underneath one another, creating tremors and the fault! Finally, the strike-slip fault is formed when two sides of plates or the crust slide past one another, causing a fault and an earthquake. Strike-slip faults are most commonly found in California!

Speaking of California, because of its placement on top of various fault lines, it is perhaps one of the most earthquake-stricken states in the United States!

How to Stay Safe During an Earthquake

There are many ways to protect yourself during an earthquake, though many of them are chalked up to one major tip that everyone who lives in a place where earthquakes are frequent has: always have an earthquake plan! An earthquake plan is a plan developed by a family to ensure they will always be safe during a possible earthquake. Most earthquake plans involve a meeting place for after the quake settles, numbers to call in case people get separated, a few to-go backpacks filled with nonperishable foods, water, and more, and reminders of earthquake safety basics!

But what are those safety tip basics? Well, most West Coast natives may be well aware of them, but for those who aren’t, here are five tips for earthquake safety that will surely save you from any unnecessary danger during a quake!

Five Tips for Earthquake Safety

1. Drop, Cover, and Hold on! These three words are the basis for any earthquake safety tips. The general conceit is if you find yourself dealing with an earthquake you should drop to your hands and knees, cover your head and neck (either beneath a sturdy table or with your arms) and hold on to something that won’t move around with the earthquake. This simple action will protect your head and neck from any unnecessary damage and keep your body from also being thrown around by the tremors!
2. Hide under tables. While in the past many may have suggested hiding beneath a doorway, but modern houses do not often account for making stronger doorways like in the past. A sturdy table is much safer to hide beneath and can protect you from falling debris.
3. If you are outside, stay outside. If you find yourself outside during an earthquake, move slowly and safely towards an open area with trees, utility poles, street lights, or buildings and get close to the ground until the shaking stops. If you find yourself in a city, try and find a secure location away from the dangers of falling glass and rubble.
4. If you’re driving, pull over. When you’re in a car and a quake starts, simply find a safe place to pull over and park until the shaking stops. Your car is probably the safest place to be during a tremor and can protect you from small debris!
5. Finally, if you find yourself in an earthquake, DO NOT PANIC. Panicking is the worst thing you can do! Keep a clear mind and move slowly and surely to the various secure locations you may have around you. And, if in a crowd, never run. Always walk to a safe place.

10 Earthquake Facts for Kids

• The earliest recorded evidence of an earthquake has been traced back to 1831 BC in the Shandong province of China, but there is a fairly complete record of seismic activity starting in 780 BC during the Zhou Dynasty in China.
• There are several million earthquakes annually. According to the United States Geological Survey, there are around 17 major earthquakes measuring above 7.0 on the Richter scale — and one great earthquake measuring above 8.0 — each year. However, experts estimate that there are actually several million earthquakes annually; many go undetected due to their geographical remoteness or small magnitude.
• An earthquake can affect the length of a day. On 11 March 2011, an 8.9 magnitude earthquake which struck northeast Japan altered the distribution of the earth’s mass, causing it to rotate slightly faster, and shortening an earth day by around 1.8 microseconds.
• A Chilean city moved ten feet to the west as a result of an earthquake. During a massive earthquake on Saturday 27 February 2010, measuring 8.8 on the Richter scale, the earth’s crust was ripped so dramatically that the city of Concepción actually moved 10 feet (ca. 3 m) to the west.
• An earthquake caused Mount Everest to shrink. On 25 April 2015, a magnitude 7.8 earthquake inflicted severe damage to the nation of Nepal, causing several Himalayan mountains to shrink, including Mount Everest which shrunk by one inch.
• Historically it was believed that Namazu — a giant catfish — which lives submerged in mud under the Japanese islands, was responsible for earthquakes. Many believed that the catfish would thrash about, resulting in seismic activity when Kashima, a god of thunder, was not watching.
• Other civilizations around the world found their own mythological explanations for earthquakes, too. The Ancient Greeks believed that Poseidon, the god of the sea, caused earthquakes by banging on the earth with his trident when he was angry. Hindu mythology believes that the earth is held in place by eight elephants, which are in turn balanced on the back of a turtle which is standing on a snake’s coils. Any of those animals moving causes an earthquake.
• Animals have been observed changing their behavior prior to an earthquake. Before the Indonesian earthquake and tsunami in 2004, witnesses reported seeing animals and birds heading for the higher ground — and similar things have been reported about many other quakes. Scientists think that animals may sense weak tremors before a quake, or possibly electrical signals triggered by the shifting of underground rocks.
• A British engineer identified the cause of earthquakes. We have British engineer John Michell to thank for identifying the cause of earthquakes, at the beginning of the 18th Century. He was one of the first fathers of seismology, and he propounded his theory that earthquakes and their resulting waves of energy were caused by “shifting masses of rock miles below the surface”.
• The moon also has earthquakes! Moonquakes happen less frequently and have smaller magnitudes than earthquakes on the Earth. It appears they are related to the tidal stresses associated with the varying distance between the Earth and Moon. They also occur at great depth, about halfway between the surface and the center of the moon.

Earthquake Facts for Kids Video

Use this super fun video to further educate and engage your kids while learning about fun earthquake facts and more!

H2 Earthquake Resources for Kids

Earthquakes are cool and all, but how do you keep your students engaged while learning about them? Well, Twinkl is here to help! We have a vast collection of amazing resources, activities, presentations, and more to help teachers and parents alike prepare their kids for whatever school throws at them! Here are some of our favorite Earthquake resources for kids!:

Earthquakes PowerPoint

Introduce your students to what exactly earthquakes are with this handy-dandy Earthquakes PowerPoint! This presentation features beautifully drawn illustrations along with facts and statistics that will interest your students to learn more! Use this as an introduction to a unit on earthquakes and the Earth in general!

Third Grade Earthquakes Text Activity

Perfect for your third grade readers, this Third Grade Earthquakes Text Activity is a great differentiated reader that allows your students to learn all about earthquakes while practicing their reading comprehension! You can use this activity during an English Language course or a Science course with an earthquake unit!

Earthquake Science Experiment

Have a fun earthquake experiment to do with your class using our Earthquake Science Experiment Activity! This great experiment is teacher-made and uses some of the easiest to obtain materials so it is possible for anyone to recreate an earthquake in their own class or home!