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=Earthquakes= By Joseph Cook

What causes an earthquake?[[image:sanfrancisco_earthquake_630px.jpg width="385" height="208" align="right" caption="San Francisco following the devastating earthquake of 1906."]]
According to the United States Geological Survey, earthquakes are "... caused by the sudden release of accumulated strain by an abrupt shift of rock along a fracture in the Earth or by volcanic or magmatic activity, or other sudden stress changes in the Earth." (Natural Hazards Gateway) During an earthquake, energy is released in two forms: heat and seismic waves. Seismic waves are "a form of elastic energy that causes vibrations in the material that transmits them." (Tarbuck, 304) These vibrations of earth, if produced by a large earthquake, can be devastating to both life and infrastructure.

Geologist have only in recent years discovered the driving force behind earthquakes. This origin was revealed following the great San Francisco earthquake of 1906, when it was discovered that there were horizontal surface displacements along a portion of the San Andrais Fault. During the earthquake, the Pacific plate moved approximately 15 feet northward, passing the North American Plate. This plate movement is part of what Johns Hopkins University professor H. F. Reid called, elastic rebound— the springing back of rocks that have been under great tectonic stress to their original stressless form. (Tarbuck, 305-6)

Tectonic stress is an important variable in the creation of an earthquake. The elsastic rebound theory involves a four-part process in which (A.) the rock on the fault line begins in its original position, (B.) there is a buildup of strain that deforms the rock, (C.) the strain becomes too much for the rock to handle and there is a rupture of the rock that releases the stored-up energy out as seismic waves, and finally, (D.) the rock springs back to its original undeformed position.

That said, the springing back of rock does not necessarily bring about an immediate end to the destruction caused by earthquakes. Particularly devastating earthquakes are followed by the occurance of aftershocks— "a smaller earthquake that follows the main earthquake." (Tarbuck, 704) While they are generally much smaller in scale than the first quake, there can be many of them, possibly tens of thousands that occur in the months and years following a major seismic event. Similarly, some earthquakes are preceded by foreshocks— "small earthquakes that often precede a major earthquake." (Tarbuck, 707) These can occur anywhere from days to years before the major event.

Where do earthquakes occur?
To put it quite simply, earthquakes occur all over the world. However, their exact location is determined by either the plate edges or fault lines on which they occur. In the United States, this happens most often in California— the most notable occasions being the San Francisco earthquake of 1906, during which 3,000 people perished, and the San Francisco Bay area earthquake of 1989, in which 62 people perished. The damages from the latter exceeded $6 billion. The United States has also experienced major earthquakes in Missouri (1811-12), South Carolina (1886), and more recently Alaska (1964).

While these earthquakes were devastating for the American people and economy, many of the worst recorded earthquakes have occurred in other parts of the world. The most notable of these is the earthquake that occured in Shensi, China in 1556. This was arguably the worst natural disaster that has ever occured, killing 830,000 people. Others that have occurred in places such as Lisbon, Portugal (1755), Messina, Italy (1908), Tokyo, Japan (1923), Peru (1970), Tanshan, China (1976), Iran (1990), Latur, India (1993), Sumatra in the Indian Ocean (2004) and most recently Haiti (2010), have resulted in tens-to-hundreds of thousands of deaths.

[|Click here to watch footage of the 2010 Haiti earthquake as it began to occur.]

Mitigation and Earthquakes
According to the Federal Emergency Management Agency (FEMA), "Mitigation is the effort to reduce loss of life and property by lessening the impact of disasters. This is achieved through risk analysis, which results in information that provides a foundation for mitigation activities that reduce risk, and flood insurance that protects financial investment." (FEMA: Mitigation)

While it is nearly impossible to prevent an earthquake from occuring, proper mitigation can aid society in such a way that we become better prepared to handle it. For this reason, the U.S. Government has set up the National Earthquake Hazards Reduction Program (NEHRP) whose sole purpose is to "... develop, disseminate, and promote knowledge, tools, and practices for earthquake risk reduction—through coordinated, multidisciplinary, interagency partnerships among the NEHRP agencies and their stakeholders—that improve the Nation’s earthquake resilience in public safety, economic strength, and national security." (NEHRP Vision & Mission)

This type of mitigation can have a positive impact on infrastructure and its ability to survive an earthquake. In light of recent earthquakes in Haiti and Chili, the NEHRP assembled a number of experts together on June 8, 2010 in order to discuss the state of seismic building codes in the United States. (NEHRP News) While this won't prevent a natural hazard from occurring, it could potentially minimize the destruction caused by one. Likewise, mitigation cannot prevent the effects that an earthquake will have on the environment. However, it can leave people better prepared to deal with the environmental damage caused by the quake so that the environment can be restored more quickly. Earthquake prediction, while not always definitive, can aid in the protection of not only people, but also any endangered wildlife that may reside near the epicenter. Finally, it important to remember how closely a society's environment and infrastructure are tied together. The more well-prepared people are to protect their infrastructure, the less that destruction will occur to their environment as a result of the infrastructure being damaged during an earthquake.

Gas fires are an example of this type of environmental destruction occurring as a result of the infrastructure. In such a case as this, NEHRP should look closely at how to protect gas lines from being effected by an earthquake. One solution is the installation of seismic gas shutoff valves which "... shut off [a] structure's gas supply in the event of an earthquake large enough to damage gas lines." (Earthquake Guard) These valves should be required in locations where there is a high risk of earthquakes.