Earthquakes
Once every 30 seconds somewhere in the world the ground shakes. The shaking is called an, earthquake. 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. Wow!!! Let's find out why the earth is, "All shook up!"
When rocks are squeezed and heated under great pressure they can bend and fold without breaking, or they also can break and release energy in the form of shock waves. Some shock waves move through the ground before a volcano erupts, or underground where new crust is being formed. Most occur at tectonic plate margins as one plate moves under another or where plates are sliding past each other. You'll need to read more to understand what this means.
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Inside the Earth
Let's first look at the structure of the earth. Earth is made of rock that is very thick. It's divided into 3 main sections: crust, mantle, core. This Animation from WNET will show you these layers.
Huge sections of the crust, called plates, are always moving, some floating like huge rafts on the almost-liquid rocks below! When this happens, under great pressure and heat, the rock in some places becomes squeezed, stretched, or folded and can break. The location where these plates meet or come together is known as a fault.
Moonquakes
Did you know that there are earthquakes on the moon? They're called "Moonquakes." Get more facts from NASA.
Where Do Earthquakes Happen?
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 plates.This area is called the Ring of Fire.
The epicenter of an earthquake is the place on the Earth's surface directly above the focus (more than one are called foci) which is the place inside the Earth where the quake originates. Foci are usually somewhere between the surface and 100 km in depth. Find where the latest earthquakes have been in the United States and in the World.
Measuring Earthquakes
A seismograph is the instrument used to measure earthquake shocks. 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 use two different scales to measure how strong an earthquake is. The Richter Scale measures magnitude, 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 that a magnitude 2 and is easily felt. An earthquake that registers 6 or more is considered a major quake.
The Mercalli Intensity Scale measures the amount and type of damage caused by the earthquake. Usually the intensity is greater near the center of the quake.
Seismology is the study of earthquakes and a seismologist is the scientist that studies earthquakes.
Waves
P-waves travel faster, but S waves are usually 2-3 times larger than the P wave. 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 wave increases away from the earthquake.
See an animation of the P wave here.
Early Explanations
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 at Earthquake Myths and Folklore, from the University of Memphis.
Here's a myth from Mongolia, China. A gigantic frog which carried the world on its back, twitched periodically, producing slight quakes.
Predicting Earthquakes
You can't 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 to warn of tsunami in the Pacific Ocean.
Earthquake Dangers
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 through the layers of the earth where they bounce off of matter inside the earth. Even following an earthquake, aftershocks can repeat over and over for days or months. An aftershock is a smaller quake which 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.
Tsunami
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.
One of the greatest dangers is a tsunami. A tsunami is a huge wave caused by an earthquake. 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!
Earthquakes in Idaho
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? The two largest earthquakes in the lower 48 states in the last 45 years have been in Idaho (1959 and 1983). Both caused fatalities and damage.
Tsunami animation from WNET and PBS online, "Savage Earth"
This Franklin Institute Website, Earthforce, will lead you to much more information about earthquakes.