February 17, 2004

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What's Shaking Title

     Earth Clipart 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.


     Cross Section of 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 and PBSonline, The Savage Earth, 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.

     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.
Ring of Fire map from WNET and PBS online, "Savage Earth"

     Seismology is the study of earthquakes and a seismologist is the scientist that studies earthquakes.

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.

History of Seismology

An early seismograph


Map of world Seismograph Stations


Something about waves.

P waves
and S waves

     Did you know that there are earthquakes on the moon? They're called "Moonquakes."Get more facts from the USGS Earthquake Hazards Program. FAQ's about Earthquakes
     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.
Frog Myth from China
     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 dangers are tsunamis. 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!
Tsunami animation from WNET and PBS online, "Savage Earth"
Predicting and Preparing

    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.

     What to do before during and after an Earthquake

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. Read about them at:

     To see the most recent earthquake activity around Idaho visit:

     This Franklin Institute Website, Earthforce, will lead you to much more information about earthquakes.

Plates of the Earth
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