Source: NOVA "Earthquake!"
In 1755, when a deadly earthquake struck the city of Lisbon, no one understood what could cause the ground to shake as it did. Observations of residents indicated that the quake had delivered two distinct types of vibrations. Since that time, scientists have developed and used a device called a seismometer to better understand seismic waves. Today, seismometers help scientists predict earthquakes. This video segment adapted from NOVA describes the history of seismology.
Earth's tectonic plates are in constant motion and, because the plates are not all moving in the same direction, the boundaries where they meet are often violent places. Boundaries where two plates move away from each other, usually found on the ocean floor, give rise to spectacular undersea volcanoes. In places where plates collide, huge slabs of crust buckle to create mountains, or one slides above and the other below to trigger violent volcanic eruptions on Earth's surface. Along other boundaries, plates slide haltingly past each other, causing earthquakes as they bind and then suddenly break loose.
Moving tectonic plates produce unfathomable force. Wherever this force exceeds the strength of the rock that makes up the plate, the rock will fracture and shift. The area where these fractures occur is called a fault. Plate movement pushes rocks on either side of a fault to move past each other, albeit slowly. Most of the time, friction keeps the rocks from moving, and the fault is said to be "locked." When the fault is locked, stress in the rocks increases until the rocks fracture and slip into new positions. This causes energy to be released in the form of vibrations, called seismic waves.
Seismic waves travel in all directions away from the point where a fracture occurs. This point, called the focus, is usually several kilometers below the surface. The location at the surface directly above the focus is the earthquake's epicenter.
Earthquakes generate three prominent types of seismic waves. The first two, called primary waves and secondary waves, or P and S waves, are detailed in this video segment. P waves cause rock particles to be compressed and decompressed in the same direction the wave is traveling. S waves cause rock particles to move at right angles, i.e., up and down or side to side, relative to the direction the wave is traveling. The third type, called surface waves, travels slowly over Earth's surface in a motion similar to ocean waves. These are the slowest moving of the three waves, but they are also the most damaging because they cause the surface and anything on it, including people, buildings, and water, to rise and fall dramatically.
Scientists use instruments called seismometers to measure seismic waves. Using these tools, they can determine both the magnitude of the vibration — by the height of the line on the seismogram — and the vibration's frequency — by the distance between the peaks or valleys on the graph. Depending on its configuration, a seismometer can record either vertical or horizontal motion.
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