One of earth’s most dangerous natural occurrences, earthquakes are a natural product of plate tectonics. The crust of the earth is not a solid mass of rock, but large pieces that affect each other through physical interaction. The movements of crustal rock cause the slow build-up of stress. The violent release of that stress causes an earthquake. Earthquakes’ intensities range from barely perceptible to disastrous. They are the origin of phenomena such as tsunamis and crustal fissures. Aftereffects such as these can cause great damage and injury to people.
Following a world map that shows the earth’s crustal plate boundaries and the frequency of earthquakes, one can see that earthquakes are most common along those boundaries. As continental and oceanic plates push against and subduct each other, adjacent rocks are bent and stressed. Large areas of these rocks can fracture along lines called faults. The two sides of a fault are pushed in opposite directions, causing them to slide past one another. When stress and pressure along a fault is released, the effect can be devastating. The most well known fault in the United States is the San Andreas Fault. The 1906 earthquake that nearly destroyed San Francisco was caused by slippage along the San Andreas when a two-hundred-seventy-mile section of land on the western side of the fault slid twenty-one feet northward.
When stress causes rock to fail, the energy released manifests as waves. Seismic (from Seismos, the Greek word for earthquake) waves follow the same behavior as water or light waves. There are two distinct types of waves produced by an earthquake, compressional and shear. In a compressional wave, rock particles expand and contract as the waves pass through them. The particles’ line of movement matches that of the wave, making it move quickly. Compressional waves are known as primary, or P, waves because they travel faster through any medium and are the first to arrive at a seismograph station. In the second type of waves, shear waves, the rock particles move at right angles to the wave’s line of direction. This type of movement is called transverse motion and causes the wave to move more slowly than a compressional wave. For this reason, shear waves are also called secondary, or S, waves. Both types of waves move along either a horizontal or vertical axis.
Because seismic waves share properties with other types of waves, they are useful in studying the interior of the earth. Seismic waves move at different speeds through different densities. They travel much faster through denser rock. The density of the crust increases with depth, so the deeper seismic waves travel, the faster they move. Discontinuities are the boundaries between mediums of different densities. When seismic waves travel from one medium to another, they can be refracted. Refraction is the change in direction a wave undergoes when it passes between two separate mediums. In the past, geologists believed the center of the earth, its core, must be solid. Through seismic wave refraction, it is now believed the core is liquid iron. The earth’s primary discontinuity lies at the boundary between the mantle and the core. In the upper mantle, P waves travel at speeds near 8.5 miles per second and S waves near 4.5 miles per second. At a depth of 1,800 miles, P waves suddenly travel 5 miles per hour and S waves 2.9 miles per hour, proving that the core is less dense than the mantle.
Seismologists have discovered another discontinuity, this time within the core. At around 1,300 miles into the core, waves refract, traveling faster which indicates denser material. There is some debate as to whether this is caused by a liquid iron of different chemical composition than the rest of the core, or a solid interior.
P waves usually refract at the core, reenter the mantle and head back toward the surface. S waves, on the other hand, do one of two things: they fade out and disappear or they continue on as P waves. Transverse motion cannot occur in liquid, so when an S wave encounters the core, the direction of its energy can become compressional. When the converted waves exit the core and reenter the mantle, they will form both S and P waves, each continuing toward the surface.
There is a phenomenon in earthquake wave refraction where waves that have encountered the core do not reemerge. When a seismic wave skims the discontinuity between the mantle and the core, and is refracted at an angle between 105 degrees and 143 degrees, it will not make it back to a seismograph at the surface. These 38 degrees are called the Shadow Zone. Waves refracted on either side of this range will appear again at the surface.
When an earthquake occurs, the event can be measured by a piece of equipment called a seismograph. Seismographs are located on the earth’s surface and measure the motion of waves. To properly locate the epicenter of an earthquake, measurements must be taken from the vertical, north-south and east-west axes. The epicenter is the spot on the surface directly above the earthquake. The point within the crust where the earthquake originates is the focus. Seismographs measure the intensity of an earthquake’s waves. The scale used to define those measurements is the Richter scale. The scale is based on logarithms of the amplitude of the seismic waves. The amplitude is the displacement between the highest and lowest points of a wave. Because the scale is logarithmic, each whole number corresponds to a tenfold increase in amplitude. An earthquake with a magnitude of 4.5 is ten times weaker than one with a magnitude of 5.5.
Geologists can use seismographs in a technique called seismic exploration, which studies areas of the earth’s crust. A time and location can be chosen, as well as the intensity of the event. This is particularly useful when searching for natural gas or oil within the earth’s crust. Unfortunately, unlike man-made earthquakes, natural earthquakes are unpredictable.
Earthquakes, like volcanoes, are products of plate tectonics. The earth’s crust is an ever-changing landscape of rock, carved by the motions of crustal plates moving over the molten magma of the mantle. Our understanding of the processes that drive plate tectonics and events such as earthquakes has helped us protect ourselves from their violence and destruction. Geologists are working to discover the means by which we will be able to predict earthquakes, enabling us to prevent catastrophic damage to both human structures and, more importantly, lives.
Sources:
Clayton, Keith, The Crust of the Earth: The Story of Geology. New York, The Natural History Press, 1967.
http://earthquake.usgs.gov/learning/topics/richter.php
