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Earthquakes
Earthquakes release energy. The energy released is evident in the shaking ground that accompanies an earthquake, in the destruction of property, and in changes in the landscape.
The Earth's interior is very hot. The heat is continually being produced and redistributed, causing motion within the Earth and on its surface. Even with that intense heat source, the rocks that make up the surface of the Earth are difficult to move. When they do, they often break suddenly, causing an earthquake.
The place where the rocks break is called the earthquake focus. As the rocks "snap," energy travels away from the focus, just as ripples travel on the surface of a pond. There are two main types of waves that are generated by an earthquake, body waves and surface waves, each traveling with a different speed and with different motions. Within body waves, there are two types: P-Waves and S-Waves. The P-Wave ( Primary Wave, or Compressional Wave) is faster than, the S-Wave (Secondary Wave or Shear Wave). There are also two types of surface waves: Rayleigh Waves and Love Waves. Surface waves are slower than body waves. Usually, surface waves are observed at large distances (several hundreds of miles) and they cause the most damage in an earthquake.
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| P-Waves are compressional waves, particles travel along the path of propagation: | S-Waves are shear waves, particles travel perpendicular to the direction of propagation: |
Near the earthquake focus, buildings may vibrate, hanging lamps sway and unstable objects fall over. More surprisingly, we can detect extremely small ground motion caused by an earthquake that is very far from us, even on the opposite side of the Earth, by using sensitive instruments called seismographs. A seismograph consists of a seismometer and a recording system. The seismometer detects ground motion from the different seismic waves generated by the earthquake. This motion is recorded by the system either digitally or on an analog medium (such as: paper, or smoked paper).
If we analyze the waves recorded by the seismometer, we can learn a great deal about earthquakes and the Earth's structure. One of the most important things we would like to know is where the earthquake occurred. Modern seismograph systems are used to detect and locate thousands earthquakes each year.
To learn more about global and regional distribution of earthquakes you can follow the links along the menu on the left side of this page. We recommend that you follow the links in numerical order. If you wish, you may also start QUEST (a special mapping tool) from the link under Tools on the top of this page and explore on your own.
Discovery Topics > Earthquakes <
