An earthquake is the vibration, sometimes violent, of the Earth's surface that follows a release of energy in the Earth's crust. This energy can be generated by a sudden dislocation of segments of the crust, by a volcanic eruption, or even by manmade explosions. 


Figure 1: A Fault 
Most destructive earthquakes--the kind which people generally have in mind when they think about earthquakes, and those of the greatest human and scientific significance--are caused by the sudden dislocation of large rock masses along geological faults within the earth's crust. These are known as tectonic earthquakes.

Figure 1: A Fault

Faults 
A fault is a fracture within some particular rocky mass within the earth's crust. The depth and length of faults vary greatly. Some faults can be many miles long. Earthquakes are caused by active faults, that is, faults along which the two sides of the fracture move with respect to each other. So, elaborating somewhat on what we said above, an earthquake is caused by the sudden movement of the two sides of a fault with respect to another. 

Faults are divided into three main groups, depending on how they move. 

Normal faults 
These occur in response to pulling or tension: the overlying block moves down the dip of the fault plane. 
Thrust (reverse) faults 
These occur in response to squeezing or compression: the overlying block moves up the dip of the fault plane. 
Strike-slip (lateral) faults 
These occur in response to either type of stress: the blocks move horizontally past one another. 

Figure 2: Fault Types

Elastic Rebound


The occurrence of tectonic earthquakes can be explained by the theory of elastic rebound, which was first advanced by H. B. Reid following his observations of ground displacement both before and after the great San Francisco earthquake of 1906. 

Over the course of time, one can observe that the two sides of an active fault are in slow but continuous movement relative to one another. (This movement is known as fault slip.) The rate of this movement may be as little as a few inches or so per year. The movement of these two sides of the fault with respect to one another cannot be an entirely smooth, easy type of movement. We can infer the existence of conditions or forces deep with the fault which resist this relative motion of the two sides of the fault. This is because the motion along the fault is accompanied by the gradual build-up of elastic strain energy within the rock along the fault. The rock stores this strain like a giant spring being slowly tightened. 

Eventually, the strain along the fault exceeds the limit of the rocks at that point to store any additional strain. The fault then ruptures--that is, it suddenly moves a comparatively large distance in a comparatively short amount of time. The rocky masses which form the two sides of the fault then "snap" back into a new position. This snapping back into position, upon the release of strain, is the "elastic rebound" of Reid's theory. 

Most importantly, the rupture of the fault also results in the sudden release of the strain energy that had been built up over the years. The most important form which this suddenly released energy takes is that of seismic waves.

The time it takes for particular area to reach the limit of elastic strain of course varies greatly, but it usually long compared to the typical human life span, varying anywhere from a few decades to a few thousand years. Incidentally, the average time it takes for earthquakes of a given magnitude to re-occur along a fault is known as recurrence interval for that fault. 

Even if a fault zone has recently experienced an earthquake, there is no guarantee that all the stress has been relieved. Another earthquake could still occur.

Seismic Waves

A we said above, once a the fault has actually ruptured, a great quantity of energy is released in the form of vibrations within the earth's interior. These vibrations are known as "seismic waves." 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 cause the entire planet to quiver or ring like a bell or tuning fork. 

It is actually the seismic waves caused by the sudden release of energy as a fault suddenly slips that creates most of the destructive effects which we generally say are caused by earthquakes. What people experience as an "earthquake" is very rarely the actual rupturing of the fault. Rather, they experience a variety of effects, which are often termed earthquake hazards, that are actually caused by seismic waves which have travelled a great distance from the fault before reaching humankind and human works. If they are still carrying enough energy when they reach us, these seismic waves can be destructive to manmade structures in a variety of ways. 

Earthquake Hazards

Strong ground motion 
The most destructive of all earthquake hazards is caused by seismic waves reaching the ground surface at places where human-built structures, such as buildings and bridges, are located. When seismic waves reach the surface of the earth at such places, they give rise to what is known as strong ground motion. Strong ground motion causes building and other structures to move and shake in a variety of complex ways. Many buildings cannot withstand this movement and suffer damages of various kinds and degrees. Most deaths, injuries, damages and economic losses caused by earthquakes result from strong ground motion acting upon buildings and other man-made structures not capable of withstanding such motion. It is for this reason that it is often said, "Earthquake don't kill people, buildings do." 

Damage to buildings also causes a variety of secondary effects which can be greatly destructive. To take just one example, fires are often created as a result of earthquake-caused building damage in urban areas. These post-earthquake fires have on some occasions in the past reached catastrophic dimensions. The Great San Francisco earthquake of 1906 and the Kanto Japan earthquake of 1923 were followed by fires which were among the largest peacetime fires in human history. 

Ground Failure 
Strong ground motion is also the primary cause of damages to the ground and soils upon which, or in which, people must build. These damages to the soil and ground can take a variety of forms: cracking and fissuring of the ground; softening and weakening, sinking, settlement and subsidence; surface fault displacement. 

One of the most important types of ground failure is known as liquefaction. Liquefaction takes place when loosely packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking. Liquefaction occurring beneath buildings and other structures can cause major damage during earthquakes. For example, during the 1989 Loma Prieta, California 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. 



Landslides are another type of ground failure that are often highly destructive. During the 1964 Anchorage, Alaska earthquake, landslides triggered by very strong local ground motion devastated the Turn again Heights residential development and many downtown areas in Anchorage. 

Tsunamis Another important class of earthquake hazards are tsunamis, which are generated by earthquakes which have occurred beneath the ocean floor. Tsunamis are immense sea waves. "Tsunami" is actually a Japanese word meaning "huge wave". Japan is one of the most seismically active countries in the world and has experienced many earthquake and tsunamis. 

These waves travel across the ocean at speeds as great as 597 miles per hour and may be 15 meters 49 feet high or higher by the time they reach the shore. 

Number of Worldwide Earthquakes Annually 
One million or more earthquakes are detected by sensitive seismographs on earth every year. By analyzing the records of all earthquakes, we learn that small earthquakes are much more frequent than larger ones. However, over 50,000 of those earthquakes are large enough to be felt by people each year (Table 1). 

Seismic Waves and Earthquakes

During fault ruptures which cause earthquakes, the sudden breakage and movement along the fault can release enormous amounts of energy. Some of this energy is used up in cracking and pulverizing the rock as the two blocks of rock separated by the fault grind past each other. Part of the energy, however, speeds through the rock as seismic waves. These waves can travel for and cause damage at great distances. Once they start, these waves continue through the earth until their energy is used up.

Types of Seismic Waves 

There are two basic types of seismic waves, and they travel at different speeds through the earth. The faster P waves move by alternately compressing and expanding the rock. The particles move back and forth in the same direction the wave is travelling. You can see this kind of wave action in the coils of a Slinky toy or some other loose spring. P waves can travel through solids, liquids, or gases. When P waves travel in air, they are called sound waves. In most rock types, P waves travel between 1.7 and 1.8 times as fast as the second kind of seismic waves, S waves. 

BACK