An earthquake is a sudden shaking or vibration in the Earth caused by sudden release of energy from within the Earth. Most earthquakes result from movement between the large ridged blocks of rock, or plates, that compose the Earth's surface. This movement occurs on faults, the boundaries between blocks of rock.

The churning motion of the hot molten rock beneath the Earth's surface drives the plates, causing them to move toward each other, away from each other, or to slide past each other along faults. It is difficult for one plate to slip against another because of the great forces pressing them together. Consequently, the plates do not slip freely in constant slow motion; instead, they slip in a jerky fashion. Each jerk causes an earthquake. This continuous motion of the Earth's plates is called plate tectonics.

This report illustrates the motion of plates sliding past each other along what is known as a strike-slip fault. If the relative motion on the fault is such that the side opposite your point of reference moves to your right, it is called a right-lateral strike-slip fault. The San Andreas fault in North America is perhaps the most famous strike-slip fault in the world. Other strike-slip faults include the Denali fault in Alaska and the Dead Sea fault in the Middle East.

Before an earthquake, the tectonic forces that drive the plates cause the rock in the vicinity of a fault to distort and bend. Energy is stored in the rock as it deforms, in much the same way as energy is stored in a rubber band as it is stretched. This energy is called elastic energy. When the forces exceed the strength of the rock along the fault, the fault suddenly slips, just as the stretched rubber band snaps back to its original shape when it is let go. The point on the fault at which slip first occurs is the focus (or hypocenter) of the earthquake. The point on the surface of the Earth directly above the focus is the epicenter of the earthquake. The area of slip on the fault grows rapidly outward from the focus and may extend upward to the surface of the Earth.

When the fault slips, the elastic energy stored in the rock is released as seismic energy in the form of seismic waves, or earthquake waves. These waves spread outward from the fault. Close to the earthquake fault, the seismic waves can be strong enough to knock people to the ground. They are weaker the farther one is from the earthquake fault. Consequently, shaking is greatest near the source of the earthquake.

There are two classes of seismic waves: body waves, which travel at high speed through the deeper, denser rock within the body of the Earth, and surface waves, which travel at a slower speed through rock near the Earth's surface. The body waves precede the surface waves. There are two types of body waves: P-waves, which are similar to sound waves, and the slower but more damaging S-waves. P-waves travel about 4.8 to 8.0 km (3 to 5 miles) in one second, while S-waves travel about 3.2 to 4.8 km (2 to 3 miles) in one second. Surface waves are slower still and can cause even more damage due to their greater duration.

Earthquakes can cause severe and widespread damage to weak buildings or structures, or to those located on ground subject to fault breakage, strong shaking, or landsliding. The slip (movement) on the fault may break the surface of the Earth, offsetting roads and tearing apart buildings or pipelines built across the fault. Such damage can be spectacular, but it is limited to the vicinity of the fault.

Most damage results from strong shaking during the passage of seismic waves, which spread out from the fault over a large region. Shaking may be severe enough and long enough to collapse weak buildings, overturn furniture, topple water heaters and storage tanks, and collapse unsafe dams. These effects can result in further damage through fires resulting from broken gas mains and fallen electric wires, the loss of water to fight fires because of broken water mains, oil spills caused by failure of storage tanks, and flooding resulting from dam failure. Shaking can also cause landslides. These in turn can damage buildings, roads, and pipelines built on slide areas or downhill from them. An underwater slide off the delta deposited by a river can cause a seismic sea wave, or tsunami. Such waves can be as large as 30 m (100 feet) high. If they occur when the tide is high, they can sweep inland into a town and destroy harbor facilities and buildings. In some of the largest earthquakes, such waves have stranded fishing boats in the middle of towns a few blocks from the harbor. The animation sequences in the diskette version of this report illustrate each of the above-mentioned effects of earthquakes.

People can do many things to protect themselves and their homes from earthquakes. They can fasten tall heavy objects in their homes to walls so they will not fall in an earthquake. Houses can be inspected to make sure the foundations, walls, and chimney are built to withstand the effects of possible shaking. When choosing where to live, a family can ask city or county officials what areas are subject to earthquake faulting, strong earthquake shaking, or landsliding. Finally, a family can plan what each member will do if an earthquake occurs while at home or while away from home, such as at school or at work.

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