How Earthquakes occur

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Introduction: I have done a work about earthquakes. Tried to explain what an earthquake really is. Many researchers and experts have through the years tried to figured out what it is that starts an earthquake. And this is in cautious terms what i have come up with.

An earthquake is the vibration of the Earth´s surface that follows a release of energy in the Earth´s crust. This large energy can be generated by a sudden dislocation of segments of the crust, by a volcanic eruption, or even by manmade explosions.
Most destructive quakes, however, are caused by dislocations of the crust.
The crust may first bend, and then when the stress exceeds the strength of the rocks, break and "snap" to a new position. In the process of breaking, vibrations called "seismic waves" are generated.
These waves travel outward from the source of the earthquake along the surface and through the Earth at varying speeds depending on the material through which they move.
Some of the vibrations are of high enough frequency to be audible, while others are of very low frequency. These vibrations sometimes cause the entire planet to quiver.

A fault is a fracture in the Earth´s crust along which two blocks of the crust have slipped with respect to each other. Faults are divided into three main groups, depending on how they move. Normal faults occur in response to pulling or tension; the overlying block moves down the dip of the fault plane. Thrust (reverse) faults occur in response to squeezing or compression; the overlying block moves up the dip of the fault plane. Strike-slip (lateral) faults occur in response to either type of stress; the blocks move horizontally past one another.
Most faulting along spreading zones is normal, along subduction zones is thrust, and along transform faults is strike-slip.

Geologists have found that earthquakes tend to reoccur along faults, which reflect zones of weakness in the Earth´s crust. Even if a fault zone has recently experienced an earthquake, however, there is no guarantee that all the stress has been relieved.
Another earthquake could still occur. In New Madrid, a great earthquake was followed by a large aftershock within 6 hours on December 6, 1811.
Furthermore, relieving stress along one part of the fault may increase stress in another part; the New Madrid earthquakes in January and February 1812 may have resulted from this phenomenon.

The focal depth of an earthquake is the depth from the Earth´s surface to the region where an earthquake´s energy originates
Earthquakes with focal depths from the surface to about 70 kilometers are classified as shallow.
Earthquakes with focal depths from 70 to 300 kilometers are classified as intermediate. The focus of deep earthquakes may reach depths of more than 700 kilometers. The focuses of most earthquakes are concentrated in the crust and upper mantle. The depth to the center of the Earth´s core is about 6,370 kilometers, so event the deepest earthquakes originate in relatively shallow parts of the Earth´s interior.

The epicenter of an earthquake is the point on the Earth´s surface directly above the focus. The location of an earthquake is commonly described by the geographic position of its epicenter and by its focal depth.


Earthquakes beneath the ocean floor sometimes generate immense sea waves or tsunamis ("huge wave"). These waves travel across the ocean at speeds as great as 960 kilometers per hour and may be 15 meters high or higher by the time they reach the shore. During the 1964 Alaskan earthquake, tsunamis engulfing coastal areas caused most of the destruction at Kodiak, Cordova, and Seward and caused severe damage along the west coast of North America, particularly at Crescent City, California. Some waves raced across the ocean to the coasts of Japan.

Liquefaction, which happens when loosely packed, water-logged sediments loose their strength in response to strong shaking, causes major damage during earthquakes.
During the 1989 Loma Prieta earthquake, liquefaction of the soils and debris used to fill in a lagoon caused major subsidence, fracturing, and horizontal sliding of the ground surface in the Marina district in San Francisco.

Harry Fielding Reid said after studying the fault trace of the 1906 earthquake, that he postulated that the forces causing earthquakes were not close to the Earthquake source but very distant. Reid´s idea was that these distant forces cause a gradual build up of stress in the earth over hundreds or thousands of years, slowly press the earth towards each other underneath our feet. Eventually, a pre-existing weakness in the earth--called a fault or a fault zone--can not resist the strain any longer and fails catastrophically. This is something like pulling a rubberband gradually until the band snaps. This theory is known as the "elastic rebound theory."

Interesting facts: In USA is earthquakes very common, and also is Japan och china. But in japan and china most people have accept the great danger with earthquakes. Earthquakes is the naturephenomenon that have killed most humans all over the world. From 1900 to 1979 2,7 millions people have been killed in earthquakes all over the world. Earthquakes occur, in difference from example overfloods and hurricanes, without almost no warning at all, even if experts is on their way to find predictmethods. Big ear...