An earthquake is the shaking and moving that happens when rock moves beneath earth's surface. When earth's plates move, they create powerful forces that squeeze, push, and pull the rocks in earth's interior. These forces are called stress. Forces that act on rock can change its shape and size. Stress adds energy to rocks.
There are three types of stress, shearing, tension, and compression. They work over millions of years to change the shape of rocks. Some rocks become thin and crack, while others bend slowly.
Stress that pushes lots of rock in two opposite directions is called shearing. Shearing causes rocks to break and crack.
Stress that pulls on the crust is called tension. Tension stretches rock so that it becomes thin in the middle. Tension happens where two plates are moving apart.
Compression squeezes a rock until it bends or breaks. When one plate pushes another plate, it can compress it like a giant trash compactor.
When anything changes in the shape or size of Earth's crust, it is called deformation.
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A fault is a break in the crust where pieces of crust go past each other. When stress builds up, it makes a fault. The rocks that create faults can move up, down, or sideways. Faults usually occur along plate boundaries, where the force of plate motion compresses, pulls, or shears the crust so much that the crust shakes. There are three types of faults; strike-slip faults, normal faults, and reverse faults. Faults can create mountain ranges.
Shearing creates strike-slip faults. In a strike- slip fault, the rocks on the sides of the fault slip past each other sideways in an up-or-down motion.
In a normal fault, the fault is at an angle so one block of rock lies above the fault while the other block lies below. The above block is called a hanging wall, and the below wall is called the footwall. When something moves, the hanging wall slips down.
A reverse fault has the same structure as a normal fault, but the blocks move in opposite directions. Parts of the Appalachian Mountains in the eastern United States were formed by reverse faults.
Over a long period of time (several million years) faults could change a flat plain into a high mountain range. When normal faults lift up a block of rock, a fault block mountain forms. Where two plates move away, tension forces create many normal faults. Folds are bends in the rock that form when compression shortens or thickens part of earth's crust. A fold in the crust that bends upward is called an anticline. A fold that bends downward is called a syncline.
The focus is the point beneath the surface where the rock that is under stress breaks, starting an earthquake. The point on the surface directly above the focus is called the epicenter. Seismic waves are vibrations that travel through Earth's interior carrying the energy released during an earthquake. Seismic waves carry the energy released during an earthquake away from the focus to the surface. The energy of the seismic waves that reach the surface is the biggest at the epicenter. The types of soil around the epicenter can affect how much the ground shakes.
There are three types of seismic waves. They are: Primary waves (P waves), Secondary waves (S waves), and surface waves. They go out from the focus; P waves first, then S waves. Surface waves develop at the surface from P and S waves.
P waves are earthquake waves that compress and expand the ground like an accordion. P waves are the only waves that can move through liquids.
S waves come second. They are earthquake waves that vibrate sideways or in an up-and-down motion. When S waves reach the surface, they shake structures very hard.
When P waves and S waves reach the surface, some of them turn into surface waves. Surface waves move slower than P or s waves, but can cause more damage. Surface waves make the ground roll.
To record and measure seismic waves, scientists use instruments called seismographs. They record ground movements made by seismic waves. Today, scientists use electronic seismographs that work the same way as old-fashioned mechanical seismographs. Electric seismographs changes ground movements into a signal that can be recorded and printed.
There are at least 20 different instruments used to measure and rate earthquakes. Each one has its strengths and weaknesses. The Richter scale is the most commonly used scale. It is a rating of the size of seismic waves as measured by a type of seismograph. The Richter scale gives accurate measurements for close-by earthquakes, but it doesn't work well for far away earthquakes.
Geologists use seismic waves to locate an earthquake's epicenter. Since P waves come first and S waves come second, Geologists measure the distance between the arrival times of the waves. The greater the depth of the earthquake is, the greater the time difference.
0-2: The earthquake is recorded by instruments, but not felt by people.
2-3: The earthquake is felt slightly by a few people.
3-4: People feel tremors. Hanging objects swing.
4-5: Earthquake causes some damage, walls crack, dishes and windows may break.
5-6: Furniture moves, earthquake seriously damages weak buildings.
6-7: Furniture may fall, strong buildings are damaged, walls and buildings may collapse.
7-8:Many buildings are destroyed, underground pipes break, wide cracks appear in the ground.
8 and above: Total devastation, including to buildings, bridges; ground wavy.