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Published on November 3rd, 2009 | by Yellow Magpie6
Earthquakes: The Deadliest Of All Shockwaves
Chilean Earthquake (2010) Photo By Claudio Nunez Creative Commons ShareAlike Licence
A room suddenly grows dark, ripped of its man-made supply of electricity. The walls of the room start to emit faint groans as plaster dust is deposited from an ever-cracking ceiling.
A claustrophobic sense of dread grips the occupants as they try in vain to nobly suppress their rising panic. The denial of mortality is firmly cast aside as they realise that they caught up in nature’s true fury, an Earthquake.
Few acts of nature are as a powerful or awesome as earthquakes. Caused by shockwaves from deep inside the Earth’s crust, they have the potential to decimate skyscrapers and cause great upheavals in landmasses. Some are so powerful that they can be felt from the other side of the planet.
What Causes Earthquakes?
The word earthquake is used to describe a seismic event. So under this definition, an earthquake can be a natural event or caused by people’s actions. Earthquakes can be caused by a multitude of different sources such as volcanic eruptions.
Nevertheless, the vast majority of them occur because of movement in tectonic plates which are faults in the Earth’s crust. These seismic waves travel through the ground at speeds of up to 20 times faster than the speed of sound. If a sufficiently large earthquake occurs out at sea, a tsunami or tidal wave may result.
Earthquakes can also bring about landslides and volcanic eruptions. The focus of an earthquake is known as the hypocentre. The epicentre is the area of ground directly above the hypocentre this is where an earthquake’s power is greatest. An earthquake’s power diminishes the farther away the wave travels.
Most Destructive Earthquake
The most destructive earthquake on record in terms of human fatalities happened in 1556 in the Shensi Province in China. As many as 800,000 people lost their lives. A more recent earthquake in 1976 is said to have caused nearly as many deaths in Tangshan, China. Despite this, the exact amount is not known as the Chinese authorities have not revealed the true number of fatalities. Earthquakes can also induce mass panic and fear. People often die in the panic that ensues when an earthquake hits a highly populated area.
The most widely used method of measuring the size of an earthquake is the Richter Scale. The height of the recorded waves on a seismometer, which measures underground vibrations, are used to indicate the magnitude of the earthquake. The intensity of an earthquake is recorded by the Mercalli scale.
However, not all earthquakes of similar magnitude do the same amount of damage. There are many other variables to take into account such as ground conditions (whether the ground is hard or soft) and how far the hypocentre is below the surface.
The Three Faults
Earthquakes occur along three major types of faults. Strike-slip reverse and faults. Areas where the crust is being extended such as when tectonic plates are separating, give rise to normal faulting. Reverse faults occur when tectonic plates are colliding. In such cases the Earth’s crust is being shortened.
Slip-faults are when landmasses or plates rub off one another in a horizontal fashion. One of the most famous areas of Earthquake activity is the San Andreas fault. It is an example of a strike slip fault.
Aftershocks are earthquakes that occur after the initial event called the mainshock. They are always of a smaller magnitude than the first earthquake. If an aftershock does exceed the magnitude of the first shock then it is reclassified as being the mainshock and the previous shock is called a foreshock.
Aftershocks are the result of the Earth’s crust readjusting to the effects of the main shock. Earthquakes have many different effects. Violent shaking can often destroy buildings that are close to the epicentre of an earthquake. The ground can also visibly rupture and can have devastating impact on buildings and structures.
Scientist are getting better at predicting where an earthquake is likely to strike. So-called earthquake storms are proving to be key to forecasting future quakes. Similar to aftershocks, earthquake storms take place years apart.
There have even been a successful prediction in Izmit, Turkey, which went unheeded killing between 20 and 30 thousand people. It has now emerged that there is a fault running through Turkey that is the equivalent of the San Andres in the United States.
Nonetheless, a serious earthquake in Turkey would have much more devastating consequences than its equivalent in the US due to type of buildings in Turkey. Although it isn’t possible to forecast when an earthquake will happen, it is possible to indicate where it is likely to strike. There has been a great improvement in constructing buildings which are able to counter the seismic effects of earthquakes and reduce fatalities.
Earthquakes And Threat Of Fire
Because earthquakes often brake gas and electric powerlines there is a high risk of fires breaking out when ever an earthquake occurs. In 1906, San Francisco was reduced to a smouldering rubble after fires destroyed it in the wake of an earthquake.
Soil liquefaction occurs in sandy, granular material which temporarily lose their strength at a molecular level due to extreme vibrations. When combined with water in an earthquake these materials act as a liquid rather than a solid. Buildings and other structures can fall victim to liquefaction and sink into the temporarily liquid ground.
Earthquakes And Tsunamis
Earthquakes at sea often cause tsunamis or tidal waves. Nevertheless, not all earthquakes will result in a tsunami or tidal wave. Generally, it takes an earthquake greater than magnitude 7 to cause a tsunami. Vibrations and sudden ruptures cause masses of water to become displaced. Waves can reach heights as great as 500 metres (1,700 feet) as was the case in Lituya Bay, Alaska, in July 9, 1958.
Lituya Bay is home to the largest known tidal wave. Earthquakes often bring floods and case cause widespread disease as they leave unsanitary living conditions caused by ruptured sewerage services and lack of food due to crop damage. They are often linked to volcanic activity, this is especially true of the Mount St Helens eruption in 1980.
Check out Yellow Magpie’s Tsunamis: Terrors Of The Oceans for further insight.
It is thought that much of the world is built on tiny faults in the Earth’s crust. Places that were deemed to be unaffected by earthquakes are also at risk. There are constant earth tremors that are seldom felt by people. Even Ireland or Great Britain experience tremors from time to time.
Some animals sensitivity to earthquakes is well-documented. They probably detect small warning tremors that are a signal of a eminent eruption. There have been countless cases of animals saving people because of their keen senses. One such animal that is said to have a close relationship with earthquakes according to legend is the catfish.
Check out Yellow Magpie’s Devastating Mechanisms Behind Earthquakes And Building Safeguards for closer look at shockwaves and earthquake buildings.
Earthshaking Science: What We Know (And What We Don’t Know) About Earthquakes is a good book by Susan Hough. Like all good books on technical subjects it manages to be both comprehensive and readable.
For people living in Ireland or the United Kingdom you can access Earthshaking Science: What We Know (And What We Don’t Know) About Earthquakes here.
For those living in Canada you can obtain Earthshaking Science: What We Know (And What We Don’t Know) About Earthquakes from here.
For Germany: Earthshaking Science: What We Know (and Don’t Know) about Earthquakes.
For France: Earthshaking Science: What We Know (And What We Don’t Know) About Earthquakes.