An earthquake is a shock or series of shocks caused by a sudden movement of crustal rocks. They are caused by a sudden release of pressure within the Earth’s crust. They may occur at any type of plate boundary. The Earth’s crustal plates do not always move smoothly against each other. Sometimes they may get stuck and this causes pressure or tension to build up. When this pressure is released, a massive amount of energy travels through the earth’s crust as seismic waves causing the ground to shake, sometimes violently. The point within the crust where the earthquake is generated is called the focus and it may be several miles below the surface. The point where the seismic waves reach the surface is called the epicentre. The epicentre is located directly above the focus and is usually where the effects of the earthquake are the worst. The diagram below shows one way in which an earthquake may occur.
An instrument called a seismograph is used to measure and record seismic waves. The amount of energy released during an earthquake is known as the moment magnitude of that earthquake. It is expressed using the Moment Magnitude Scale which is a magnitude scale which has no upper limit. The moment magnitude expresses the total amount of energy released during an earthquake as a number. Experts arrive at this number soon after an earthquake by consulting various seismograph readings.The moment magnitude scale has replaced the older and perhaps more familiar Richter scale.
Another scale used to measure earthquakes is the Mercalli scale. This is a twelve point scale which rates the intensity of an earthquake based on the things which can be observed in the area where it occurred. The Mercalli scale can be seen below.
The Mercalli Scale
One: Not felt except by very few people under especially favourable conditions.
Two: Felt only by a few people at rest, especially on upper floors of buildings.
Three: Felt by people indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations are similar to the passing of a truck. Duration estimated.
Four: Felt indoors by many, outdoors by few during the day. At night, some people are awakened. Dishes, windows and doors are disturbed. Walls make a cracking sound. The sensation is similar to a heavy truck striking building. Standing vehicles are rocked noticeably.
Five: Felt by nearly everyone. Many people are awakened. Some dishes and windows are broken. Unstable objects are overturned. Pendulum clocks may stop.
Six: Felt by all. Many are frightened. Some heavy furniture is moved. There are a few instances of fallen plaster. The damage is slight.
Seven: Damage negligible in buildings of good design and construction and slight to moderate in well-built ordinary structures. There is considerable damage in poorly built or badly designed structures. Some chimneys are broken.
Eight: Damage is slight in specially designed structures but there is considerable damage in ordinary substantial buildings with partial collapse. Damage is great in poorly built structures. Chimneys, factory stacks, columns, monuments and walls are likely to fall. Heavy furniture is overturned.
Nine: Damage considerable in specially designed structures. Well designed frame structures are likely to be tilted. Damage is great in substantial buildings, with partial collapse. Some buildings are shifted off foundations.
Ten: Some well-built wooden structures are destroyed. Most masonry and frame structures are destroyed with foundations. Rails are bent.
Eleven: Few, if any (masonry) structures remain standing. Bridges are destroyed. Rails are bent greatly.
Twelve: Damage is total. Lines of sight and level are distorted. Objects are thrown into the air.
A measure of intensity on the Mercalli scale is arrived at after observing the effects of the earthquake in the affected area as well as interviewing people who experienced it.
Powerful earthquakes are capable of causing incredible damage. Although we understand what causes earthquakes to occur, scientists still are not able to predict the occurrence of earthquakes. The fact that they strike suddenly and without warning makes earthquakes very scary indeed. Watch the videos below which contain much information about earthquakes.
When powerful earthquakes occur beneath our oceans, there is a chance that they might cause tsunamis. A tsunami (also called a tidal wave) is a very large wave usually caused by undersea earthquakes. Tsunamis can travel miles inland and cause incredible damage. Watch the video below, which shows footage of a tsunami which hit Japan on March 11, 2011.