Earthquakes

A shaking of the earth's surface.

When the surface of the earth moves in response to heat and pressure from deep inside the earth, the shaking or movement of the earth is called an earthquake. Earthquakes are a very common occurrence. Once every 30 seconds somewhere in the world the ground shakes. The estimates are that there are about 500,000 detectable earthquakes a year, 100,000 of those can be felt, and about 100 of them cause damage. Scientists and kids alike are fascinated by the shaking of the ground. To find out how this shaking happens, we need to see what is going on under the ground.

Let's begin by looking at the structure of the earth. Earth is made of rock that is very thick. It's divided into 3 main sections: crust, mantle, and the core.

The crust isn't a solid, continuous piece that wraps around the Earth like an eggshell. It's actually made up of sections called tectonic plates, which are more like giant puzzle pieces. These tectonic plates are always moving, drifting like huge rafts on the almost-liquid mantle layer below. Sometimes the plates push past each other, causing stress on Earth's crust. Under great pressure, the rock in some places becomes squeezed, stretched, or folded and can break, leading to cracks known as faults. When tectonic plates slip past one another, it causes movement at the faults. As rocks break under this stress they release huge amounts of energy in the form of shock (seismic) waves. An earthquake is what happens when there is a sudden movement of Earth's crust at a fault line. 

Most earthquakes occur at tectonic plate margins as one plate moves under another or where plates are sliding past each other. The type of earthquake depends on the plate movements that occur beneath the earth's surface at these plate boundaries. When one plate is forced over another plate during movement, it is known as a convergent boundary. Mountain ranges are often formed in this way. A divergent boundary is when plates are forced apart from each other. This is common in ocean floors where new oceanic crust is formed. And a transform plate boundary is when the plates slip by each other. It is very hard to imagine rocks bending and folding or even large areas of land sliding under layers of the ground. But this truly does happen and has been happening for millions of years, ever since the earth began.

Earthquakes can happen in many places, but there are some areas where they are concentrated. They occur at or close to the edges of the major tectonic plates. One of these areas along the Pacific Ocean is called the Ring of Fire. About 90% of the world's earthquakes occur there. In the United States, most earthquakes occur in the West.

The location inside the earth where the earthquake starts is called the hypocenter or focus. The location directly above it on the earth's surface is called the epicenter. An earthquake's strongest shaking is often felt near the epicenter. However, the vibrations from an earthquake can still be felt hundreds of miles away from the epicenter.

It is estimated that there are about 500,000 earthquakes in the world each year. About 100,000 of those can be felt, and 100 of them are strong enough to cause some damage. Check out this link to the US Geological Survey to find out where latest earthquakes have been in the United States and in the world.

Earthquakes travel in ripples or waves across the earth and through the layers of the earth. If a person were standing in just the right location and could see the surface of the earth during an earthquake, the ground would actually appear to be moving in a wave. These waves travel in two types--P waves — or primary waves because they travel faster — will be felt first and S waves — or secondary waves — are usually 2 – 3 times larger than the P wave.

P waves move back and forth.  S waves move up and down.  This leads to the characteristic shape of an earthquake on a seismogram with a small P wave followed by a larger S wave. Because the P wave is traveling faster, the time between the P and S waves increases away from the earthquake. As the waves move, they can actually collide into one another and reverse direction, causing additional damage. 

Seismology is the study of earthquakes and a seismologist is the scientist that studies earthquakes. A seismograph is an instrument used to measure earthquake shocks. It records the data using a special pen and paper rolled on a moving cylinder. As an earthquake is recorded, the pen jumps back and forth on the paper showing the intensity of the earthquake. It can record tiny earth tremors thousands of miles away. Hundreds of seismograph stations are located all over the world. Seismographs can be used to find an earthquake's epicenter and its focus underground. They record "P" waves and "S" waves.

Geologists and seismologists use two different scales to measure how strong an earthquake is. The Richter Scale, invented by the scientist Charles Richter in 1934, measures magnitude or the amount of energy released by an earthquake, by measuring how big a shock wave is. Each number on the scale is 10 times more powerful than the number below it. A magnitude 3 earthquake is ten times greater than a magnitude 2 and is easily felt. An earthquake that registers 6 or more is considered a major quake. 

In addition to the Richter scale, scientists also use the Mercalli Intensity Scale to measure the amount and type of damage caused by the earthquake. Usually, the intensity is greater near the center of the quake.

Humans developed many explanations for earthquakes before they had the understanding and instruments to study them scientifically. You can read about some of these ideas about Earthquake Myths and Folklore.

Here's an earthquake legend from the Chickasaw Native Americans: Once a Chickasaw chief was in love with a Choctaw princess. He was young and handsome, but he had a twisted foot, so his people called him Reelfoot. When the princess's father refused to give Reelfoot his daughter’s hand, the chief and his friends kidnapped her and began to celebrate their marriage. The Great Spirit was angry and stomped his foot. The shock caused the Mississippi River to overflow its banks and drown the entire wedding party. (Reelfoot Lake, on the Tennessee side of the Mississippi River, was formed as a result of the New Madrid earthquake of 1812.)

Did you know that there are earthquakes on the moon? They're called "Moonquakes." Get more information on Moonquakes from TIME for kids.

The energy of an earthquake can be felt for many miles. The shockwaves can travel through the surface of the earth, but they can also travel down through the layers of the earth where they bounce off matter inside the earth. Even following an earthquake, aftershocks can repeat over and over for days or months. An aftershock is a smaller quake that follows the original one, but can still continue to cause additional damage to an already troubled area of land. They usually decrease in size and frequency with time.

Earthquake shaking can cause much destruction and loss of life. The ground under buildings can become loose and buildings can fall or sink. Rockslides can occur. Liquefaction, ground displacement, flooding, tsunamis, and fire are all hazards of earthquakes. Learn more about earthquake hazards and hazard maps.

One of the greatest dangers is a tsunami. A tsunami happens when shocks from the earthquake create huge waves that come inland and strike communities along the ocean coast. In the ocean, tsunamis may be only 1m (3 ft) high and can be 90 miles apart. They can travel at 500 mph. When they reach shallow coastal waters they can grow to 35m (115 ft) high and cause massive flooding! Satellites in space watch for tsunamis and transmit images to scientists on Earth. These images help scientists better predict when, where, and how strong a tsunami will be so that people can get away and stay safe. Learn more about tsunamis.

No one can predict earthquakes yet, nor prevent them, but you can protect yourself by preparing for one. One way scientists help is by comparing readings from widely spaced seismographic stations to determine the exact position of the earthquake's origin. This can help warn communities and people about any potential earthquake dangers.

What to do before, during, and after an Earthquake.

The Idaho Geological Survey provides a history of earthquakes in Idaho. Did you know that Idaho ranks fifth in the nation for earthquake hazard, behind California, Nevada, Utah, and Alaska? In 1983, Idaho had one of the strongest earthquakes recorded in the lower 48 states in the past 60 years. You can see the damage and earthquake fault scarp still today outside of Mackay, Idaho.

In March 2020, the Stanley Earthquake, the second largest earthquake to strike Idaho, rolled through the Sawtooth Mountains. Due to liquefaction and some sliding of the land, a beach at Stanley Lake collapsed and "disappeared." Aftershocks continued for several months following the earthquake.