A seismic wave is the traveling vibration that radiates out from a break or shift in the Earth's crust. There are two main categories of seismic waves: body waves, which occur below ground, and surface waves, which are waves that roll across the surface of the Earth. Within the body wave category, there are primary and secondary waves. A primary wave (a P wave, or compressional wave) moves the fastest and can penetrate solids, liquids and gases. Secondary waves (S waves, or shear waves) only move through solids. They stop at the liquid layer of the Earth's core.
The ratio of speed between the primary and secondary body waves is fairly consistent; P waves usually travel 1.7 times faster than S waves. You can use this information to determine your distance from the earthquake's focus. You'll need a seismograph to register the presence of the waves and to know exactly when each wave arrived. Then you can calculate how much time passed between the arrival of each wave. You can apply this information to a special chart to determine how far each wave must have traveled to register as it did on the seismograph. The body waves constitute the primary seismic activity of an earthquake, but surface waves tend to have the most impact on human civilizations.
Surface waves (L waves, or long waves) are the seismic waves that cause most of the damage associated with earthquakes. L waves are the slowest moving seismic waves; they tend to arrive toward the end of an earthquake.
When soil or other sediment isn't packed tightly, it can begin to conduct waves like a liquid. When soil acts like a liquid, it's called "liquefaction." Any structures built on loose sediment are at great risk for collapse if anything causes the sediment to move. Even a relatively small earthquake can cause buildings to collapse if they are built on loose soil. Liquefaction is also a common cause of mudslides and landslides during an earthquake. The loose earth just breaks away from the solidly packed ground.
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