The earth is made of three sections: the outer part or crust, the mantle, and the core. The crust makes up only a very thin section of the earth, sort of like the peel of an apple compared to its flesh and core. The crust is made up of primarily solid rock, as is the mantle. The core makes up the highest percentage of the earth's mass, although the mantle is only slightly smaller. As far as scientists can tell, the outer part of the core is made of molten rock, but the hotter inner core is under so much pressure that it's a solid!
According to the plate tectonics theory, the earth's crust is made of large, continent-sized plates. A fault is a break in the crust caused by plates moving next to each other. Normal faults occur when one side of the fault slides down from the surface. Reverse faults happen when one side is thrust upward. A strike-slip fault happens when plates move sideways against each other. (The San Andreas Fault in California is a strike-slip fault.) See an illustration of all three fault types here.
When two plates hit each other hard enough, they cause an earthquake. The focus of an earthquake is the section in the crust where two plates collide; the epicenter is the point on the surface directly above the focus. Primary waves (p-waves) are the first wave of an earthquake; they travel through the earth's crust, mantle, and core and can be recorded by seismographs on the other side of the world. Next come s-waves, which are slower and only travel through the solid mantle and crust. L-waves travel through the crust and are the strongest waves, with a motion similar to ocean waves. These waves cause the most damage.
You can use a slinky to demonstrate what the movements of p-waves and s-waves look like. With a person holding each end, stretch out the slinky about half way. Now, have one person give his or her end a hard push forward. A compression should form at that end of the slinky and move up the coil, similar to the push-pull movement of a p-wave. S-waves, on the other hand, move back and forth with an undulating motion. Wiggle one end of the slinky. What happens? This side-to-side movement is similar to an s-wave's movement.
In 1935 Charles Richter developed a system to measure the magnitude--or amount of energy released--of an earthquake. Each whole number on the Richter scale indicates a tenfold increase in amplitude (greatness in size). Thus, a 7.5 earthquake on the Richter scale actually has ten times the amplitude of a 6.5 earthquake. There is no upper limit on the Richter scale, meaning that it could be used to measure earthquakes of a ten or more magnitude if one ever occurred. The most devastating earthquakes we know of are 8 or 9 on the Richter scale.
There's another measurement used for earthquakes: the Modified Mercalli Scale is used to measure intensity, or how strong the effects of the quake are. The intensity varies based on position relative to the epicenter of the earthquake, so one earthquake does not have a set number from the scale assigned to it as with the Richter scale. Intensity is measured in Roman numerals I-XII. For a list of effects at each level, visit this Mercalli scale site.
In addition to earthquake magnitude and intensity, scientists measure seismic waves, or movements in the earth's crust. Special machines called seismographs record movement in the earth, including earthquakes that are so low in magnitude that people cannot feel them.
You can make a simple seismograph to demonstrate to your kids how this machine works. Fill a 2-liter soda bottle with water and use wire to suspend it about 1' above the surface of a table, using a sturdy stick or ruler set across stacks of heavy books. (The bottle should hang freely between the books.) Tape a sheet of paper with aluminum foil underneath it to the table, beneath the bottle. Next, roll a felt-tip pen in a piece of paper, and tape it loosely enough for the pen to slide up and down. Tape this roll to the side of the bottle so that the pen's tip touches the paper. Shake the table back and forth, gently at first and then a little harder. (Don't move the table's legs; shaking is enough.) When you're done, examine the paper. What kind of record is there of the 'quake'?
If it isn't confusing enough with so many things to measure, there's one more method for determining magnitude. The Moment Magnitude scale is more accurate than the Richter scale for measuring large earthquakes. In the world's worst recorded earthquake--the 1960 one in Chile--the magnitude on the Richter scale was 8.5, but on the Moment Magnitude (Mw) scale it was 9.5.
At 5:12 am on April 18, 1906, a huge tremor shook the San Francisco area. This foreshock, which collapsed wooden buildings and brick chimneys, was quickly followed up by a quake with an epicenter near San Francisco. The quake lasted for 45-60 seconds, with violent shocks throughout and an estimated 7.7-8.25 magnitude. The intensity was as much as IX at distances up to 60 miles to the east of the fault line. Buildings crumpled, several thousand people were killed and many times that number wounded, and 250,000--more than half the population--were left homeless. Fires raged through the city for days afterward.
The earthquake was caused by the North American plate and the Pacific plate crashing against each other. The gap or offset caused by the earthquake was as wide as 21 feet in some places along the San Andreas fault line and the rupture in the earth along the fault line was about 290 miles long, stretching north up the California coast. Areas on softer ground suffered much worse effects than areas founded on solid rock. Effects from the quake were felt throughout California and as far north as Oregon and east to Nevada.
For more information, including contemporary photos and eyewitness accounts, visit the Virtual Museum of the City of San Francisco's exhibit and the Exploratorium's exhibit.
The first tool used for measuring seismic activity was invented in China in 132 AD. Chang Hang made a bronze urn that had a highly sensitive pendulum inside which would detect movement in the earth. For an alarm, the urn had figures of dragon heads at its top. Each head held a ball in its mouth, which would be pushed out during seismic activity by a mechanism within the urn, and would fall into the open mouth of a frog figure below it. Chang Hang claimed that he could tell the direction that seismic activity came from based on which balls fell.
Simple seismometers were in use during the 1700s and following. Then, in 1855, volcanologist Luigi Palmieri invented a mercury seismograph to detect trembling of the earth around Italy's Mt. Vesuvius. He figured out that explosions were often foretold by seismic activity. Later in the century, the seismograph model was improved by the English scientists John Milne, James Ewing, and Thomas Gray, who were working in Japan at the time. Gradually the seismograph became an international instrument. Charles Richter used American instruments in the 1930s that did not measure exactly the same as other models, but now seismographs have become standardized so that the measurements are calculated the same way everywhere.
Tsunamis. A tsunami (say it 'tsoo-nah-me') is a powerful wave caused by plate movement, not by wind and weather like other waves. An earthquake under the ocean floor can cause tsunamis that move at speeds of around 600 miles per hour. However, a tsunami doesn't really become dangerous until it reaches shallow water around land. There the pace of the wave is forced to slow down, causing the wave's energy to turn to a different route and sometimes build up into a wall of water as much as 130 feet tall. Other tsunamis only cause strong currents as they rush up on shore, tearing at the beach and bringing debris with them.
The Giant Wave. In 1958, after an 8.0-magnitude earthquake in Alaska, part of a mountain slid down into Lituya Bay and caused a 1700-foot wall of water to spurt up. The subsequent tsunami washed out trees and vegetation on either side of the bay and swept away one boat. Amazingly, the people on two other boats survived.
Ring of Fire. The zone where the Pacific plate touches adjoining plates is known as the 'Ring of Fire'. This zone of high volcanic and seismic activity runs along the coasts on both sides of the Pacific Ocean, from New Zealand to Alaska to Chile.
Earthdin and earthquave both mean 'earthquake' (and are obsolete now). Seism and tremblor are also synonyms for earthquake.
The root seism in seismograph, seismology, and related words come from the Greek word meaning 'to shake'.