Earthquake tremors/waves travel inside the earth and along the surface at great speed. There are three main types:
1. P waves, or compressional waves, shake the ground back and forth in the same direction as the wave is moving. These are very fast and do little damage; they can pass through a building in less than a second.
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2. An S wave, or shear wave, shakes the ground back and forth perpendicular to the direction in which the wave is moving. They arrive after the P waves as they travel more slowly; they can damage buildings.
3. Surface waves travel slowly along the earth’s surface. They move the surface of the earth up and down as well as from side to side. They are extremely devastating because they travel slowly. They are also called Love and Rayleigh waves.
Movement A Wavelength P wave Slinky Movement Wavelength B Direction of wave
S wave Fig. 3 The different types of earthquake waves
Recording and measuring earthquakes 1. Seismographs 2. The Richter scale 3. The Modified Mercalli scale
1. Seismographs
Sensitive instruments called seismographs record earthquake waves. The Global Seismic Network (GSN) is a web of 128 recording stations across the world. These detect earthquakes and help monitor nuclear tests.