Immediately after an earthquake, the water in the ponds, lakes, and pools of the area will slosh back and forth for a short period of time in reaction to the force of the quake. This is called seiche (pronounced saysh). This effect isn't always limited to the earthquake's immediate area. In 1985, the University of Arizona's pool lost water due to the 8.1 magnitude quake in Mexico.
Before electronics allowed recordings of large earthquakes, scientists built large spring-pendulum seismometers in an attempt to record the long-period motion produced by such quakes. The largest one weighed about 15 tons. There is a medium-sized one three stories high in Mexico City that is still in operation.
There are a few handfuls of major plates and dozens of smaller, or minor, plates. Six of the majors are named for the continents embedded within them, such as the North American, African, and Antarctic plates. Though smaller in size, the minors are no less important when it comes to shaping the Earth. The tiny Juan de Fuca plate is largely responsible for the volcanoes that dot the Pacific Northwest of the United States. The plates make up Earth's outer shell, called the lithosphere. (This includes the crust and uppermost part of the mantle.) Churning currents in the molten rocks below propel them along like a jumble of conveyor belts in disrepair. Most geologic activity stems from the interplay where the plates meet or divide. The movement of the plates creates three types of tectonic boundaries: convergent, where plates move into one another; divergent, where plates move apart; and transform, where plates move sideways in relation to each other.
In a reverse fault, the block above the fault moves up relative to the block below the fault. This fault motion is caused by compressional forces and results in shortening. A reverse fault is called a thrust fault if the dip of the fault plane is small. [Other names: thrust fault, reverse-slip fault or compressional fault]
Normal faults develop in areas where the land is pulling apart or stretching. The tension in the crust increases until the rocks fracture. One block of land slips downward in relation to the block of land on the other side of the fault plane. A normal fault will have a hanging wall and a footwall. The term footwall is derived from miners finding mineral deposits where inactive faults have been "filled in" with mineral deposits.
Strike-slip faults involve motion which is parallel to the strike of the fault--frequently described as a "side-by-side" motion. Strike-slip faults are further described as "right-lateral" (dextral) or "left-lateral" (sinistral) depending if the block opposite the viewer moved to the right or left respectively.
The San Andreas Fault is the most famous fault in the world. Its notoriety comes partly from the disastrous 1906 San Francisco earthquake.
Faults are surfaces along which rocks have fractured and been displaced. There are three major types of faults: strike-slip, normal, and reverse. The tectonic stresses caused by plate motions build up over time and eventually cause breaks in the crust of the Earth along which the rocks sporadically grind past one another. When this happens, earthquakes occur.
What makes the earth shake? The earth's top layer, or crust, is like a jigsaw puzzle. It has seven big pieces and some smaller pieces. The pieces of crust are called plates. They move a little bit all the time. They move so slowly that we never even feel it. Sometimes two plates bump together underground. They might slip past each other or one might slide under the other. Energy is released and travels up to the surface. Everything on the ground shakes.
Volcanoes, earthquakes, tidal waves, hurricanes, flash floods, and forest fires--nature running wild is both spectacular and terrifying. Despite all our modern resources, natural disasters still devastate lives. Every year they kill, injure, or leave homeless millions of people.