If a seismogram records P-waves and surface waves but not S-waves, the seismograph was on the other side of the Earth from the earthquake because those waves cannot travel through the liquid core of the earth. The data are then kept digitally on a computer. Modern seismometers record ground motions using electronic motion detectors. Seismograms contain information that can be used to determine how strong an earthquake was, how long it lasted, and how far away it was. At this time, seismologists have not found a reliable method for predicting earthquakes.A seismograph produces a graph-like representation of the seismic waves it receives and records them onto a seismogram. The currently accepted method is the moment magnitude scale, which measures the total amount of energy released by the earthquake. Over the past century, scientists have developed several ways of measuring earthquake intensity. In an earthquake, body waves produce sharp jolts, while rolling motions of surface waves do most of the damage in an earthquake. Surface waves are the slowest of all seismic waves, traveling at 2.5 km (1.5 miles) per second. Surface waves travel along the ground, outward from an earthquake’s epicenter. P-waves travel through solids, liquids, and gases. The two types of seismic waves described in “Plate Tectonics,” P-waves and S-waves, are known as body waves because they move through the solid body of the Earth. Seismologists use seismic waves to learn about earthquakes and also to learn about the Earth’s interior. The energy from earthquakes travels in seismic waves, which were discussed in the chapter “Plate Tectonics.” The study of seismic waves is known as seismology. The distance between waves from crest to crest (or trough to trough) is its wavelength. The height of a wave from the center line to its crest is its amplitude. Every wave has a high point called a crest and a low point called a trough. Physics shows us that energy is always transmitted in waves.
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