Earth Quake

The Nature of Earthquakes

Shaking motion of an earthquake is the result of a sudden release of energy. An earthquake occurs when stress, building up within rocks of the earth's crust, is released in a sudden jolt. Rocks crack and slip past each other causing the ground to vibrate. The slippage emits large amounts of energy in the form of waves that travel through the interior of the earth and across the surface, similar to the waves emanating from a stone dropped into a still pond. An excellent animated story on earthquake waves has been developed by PBS.

Cracks along which rocks slip are called faults; these may break through the ground surface, or remain deep within the earth. San Andreas fault, which stretches for more than 900 km along the coast of California, is shown on the figure below. This fault lies along the boundary between the Pacific Plate and the North American tectonic plate (tectonic plates are discussed in the next section). Activities of this fault have caused some of the major earthquakes in the United States and the world, such as the 1906 San Francisco earthquake (magnitude 8.3 on the Richter scale, killed 700 people and left 250,000 people homeless).

Depending on the relative layout of the rock blocks that move relative to each other in an earthquake and the direction of their relative movements, there are several types of faults. Some of the most common types of faults are: normal fault, reverse fault and strike-slip fault.

Normal Fault.   In a normal fault, the block above the fault moves down relative to the block below the fault. This fault motion is caused by tensional forces and results in extension.

Reverse Fault.    In a reverse fault, the block above the fault moves up relative to the block below the fault. This fault motion is caused by compression forces and results in shortening. A reverse fault is called thrust fault if the dip of the fault plane is small.

Strike-Slip Fault.   In a strike-slip fault, the movement of blocks along a fault is horizontal. The fault motion of a strike-slip fault is caused by shearing forces.

e a r t h q u a k e   d a m a g e

Earthquake damage depends on what area is hit. If an unpopulated region is struck, there will be low loss of life or property. If it hits a large city, there may be many injuries and destruction. Many of the areas at risk on the Ring of Fire are largely populated. Major earthquakes hitting those areas today could produce terrible damage.

Most global cities have at least tripled their populations in the last hundred years, so a major earthquake could collapse skyscrapers, factories, and power plants. Millions of people could feel the shock waves, not only of the initial tremor, but of the aftershocks, tsunamis, landslides, floods, and social effects.

Earthquakes have the power to uproot trees and send them crashing into buildings. They can trigger landslides and avalanches, and cause flooding and tsunamis. Human structures are also at risk. It is interesting to note that tall buildings will sustain the least damage if they are located directly at the epicenter. This is because they can withstand the up-and-down motion of P-waves. S-waves, on the other hand, occur far away from the epicenter, and cause the greatest stress by shaking buildings from side to side. These buildings are often knocked off their foundations.


Buildings with thick, heavy walls do not resist shock waves very well. Violent earthquakes often cause structures to collapse, burying people underneath. Brick buildings are the most vulnerable. Chimneys and heavy roof tiles are often shaken off during the tremors, and can crash into bystanders or passerbys.

Earthquake Facts

1. 1.Many natural events such as volcanic eruptions or meteor impacts can cause an earthquake, but the majority of naturally-occurring earthquakes are caused by movements of the earth's plates.
2. 2.20 constantly moving plates make up the surface of the earth. As the plates move they put force on themselves and each other, which, when large enough, causes the crust to break. When the break occurs, the stress is released as energy which moves through the Earth in the form of waves, which we feel and call an earthquake.
3. 3.According to the U.S. Geological Survey, more than three million earthquakes occur every year. That's about 8,000 a day or one every 11 seconds! The vast majority of these quakes are extremely weak.
4. 4.Almost 80 percent of all the planet's earthquakes occur along the rim of the Pacific Ocean, called the "Ring of Fire," a region that encircles the Pacific Ocean and is home to 452 volcanoes, that’s over 75% of the world's active and dormant volcanoes.
5. 5.Each year the southern California area has about 10,000 earthquakes. Most of are so small that they are not felt. If there is a large earthquake, however, the aftershock sequence will produce many more earthquakes of all magnitudes for many months.
6. 6.The largest recorded earthquake in the United States was a magnitude 9.2 that struck Prince William Sound, Alaska on March 28, 1964.
7. 7.The largest recorded earthquake in the world was a magnitude 9.5 in Chile on May 22, 1960.
8. 8.When the Chilean earthquake occurred in 1960, seismographs recorded seismic waves that traveled all around the Earth. These seismic waves shook the entire earth for many days.
9. 9.Usually, it's not the shaking ground itself that claims lives -- it's the associated destruction of man-made structures and the instigation of other natural disasters, such as tsunamis, avalanches, and landslides.
10. 10.An undersea earthquake in the Indian Ocean triggered a series of devastating tsunamis on December 26, 2004. The tsunamis struck along the coasts of most landmasses bordering the Indian Ocean, killing more than 225,000 people in eleven countries, and inundating coastal communities with waves up to 100 feet high.
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