Earthquakes

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Description or situation

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Overview

Most of us know what an earthquake is however not everyone has experienced such an event. Earthquakes are events that come in many sizes and have a variety of characteristics which sets each one apart from the next. Experiencing an earthquake can be excieting and often fun or life changing and deadly depending on the severity and impact. An entirely new perspective can be acquired for the phenomena for first time experiences. Earthquakes can be frightening to say the least and have effects on personal levels of security as we experience the reality associated with living on unstable ground. On average, ... the best case scenario will leave us with something to talk about for the next few days. The worst case scenario can thrust a regional community into chaos and lawlessness often resulting in displacement, hunger and death. The following information is relative to the effects associated with various levels of ground shaking earthquakes which survival minded people may consider being prepared for.

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What is an Earthquake?

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An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. In its most general sense, the word earthquake is used to describe any seismic event,... whether natural or caused by humans, ...which generates seismic waves.  An earthquake's point of initial rupture is called its focus or hypo-center. The epicenter is the point at ground level directly above the hypo-center. Seismic waves are sent outward from the epicenter and felt as rolling and rumbling often changing the very landscape depending on the intensity. Earthquakes come in all sorts of sizes with a variety of characteristics worthy of measuring. Because of that fact a system of measurement has been applied to define the severity of each event. Earthquakes are measured using observations from seismometers. Earthquakes can be recorded by seismometers up to great distances, because seismic waves travel through the whole Earth's interior. The absolute magnitude of a quake is conventionally reported by numbers on the moment magnitude scale (formerly named the Richter scale, magnitude 7 causes serious damage over large areas). The magnitude felt by humans is reported using the modified Mercalli intensity scale. The moment magnitude is the most common scale on which earthquakes larger than approximately 5.0 are reported for the entire globe. The more numerous earthquakes smaller than magnitude 5.0 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the Richter magnitude scale. These two scales are numerically similar over their range of validity. Magnitude 3.0 or lower quakes are often imperceptible or weak and magnitude 7.0 and higher potentially cause serious damage over larger areas, depending on their depth. The largest earthquakes in historic times have been of magnitudes slightly higher than 9.0, although there is no limit to the possible magnitude. The most recent large earthquake of magnitude 9.0 or larger was a 9.0 magnitude earthquake in Japan in 2011 (as of March 2014), and it was the largest Japanese earthquake since records began. Intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake is, the more damage to structures it causes, all else being equal. The bottom line in describing an earthquake is equivalent to that of an eye opening experience, especially for first timers. [Source].

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Below is a video provided to enlighten those who may not be familiar with earthquakes.  

Courtesy of Frank Gregorio, educator.

This HD dramatic video choreographed to powerful music introduces the viewer/student to earthquakes. It is designed as a motivational "trailer" to be shown in classrooms by Earth Science and Physical Science teachers in middle school, high school and college as a visual Introduction to the power of Earth's tectonic crust.

 

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Types of earthquakes

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There are three main types of faults, all of which may cause an inter-plate earthquake:

Normal, Reverse thrust & Strike-slip

 

Normal and reverse faulting are examples of dip-slip type earthquakes, where the displacement along the fault is in the direction of dip and movement upon them involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other; transform boundaries are a particular type of strike-slip fault. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip. Reverse faults, particularly those along convergent plate boundaries are associated with the most powerful earthquakes, mega-thrust earthquakes, including almost all of those of magnitude 8.0 or more. Strike-slip faults, particularly continental transforms, can produce major earthquakes up to about magnitude 8.0. Earthquakes associated with normal faults are generally less than magnitude 7.0 For every unit increase in magnitude, there is a roughly thirty-fold increase in the energy released. For instance, an earthquake of magnitude 6.0 releases approximately 30 times more energy than a 5.0 magnitude earthquake and a 7.0 magnitude earthquake releases 900 times (30 × 30) more energy than a 5.0 magnitude of earthquake. An 8.6 magnitude earthquake releases the same amount of energy as 10,000 atomic bombs that were used in World War II. This is so because the energy released in an earthquake, and thus its magnitude, is proportional to the area of the fault that ruptures and the stress drop. Therefore, the longer the length and the wider the width of the faulted area, the larger the resulting magnitude. The topmost, brittle part of the Earth's crust, and the cool slabs of the tectonic plates that are descending down into the hot mantle, are the only parts of our planet which can store elastic energy and release it in fault ruptures. Rocks hotter than about 300 degrees Celsius flow in response to stress; they do not rupture in earthquakes. The maximum observed lengths of ruptures and mapped faults (which may break in a single rupture) are approximately 1000 km. Examples are the earthquakes in Chile, 1960, Alaska, 1957, Sumatra, 2004, all in subduction zones. The longest earthquake ruptures on strike-slip faults, like the San Andreas Fault (1857, 1906), the North Anatolian Fault in Turkey (1939) and the Denali Fault in Alaska (2002), are about half to one third as long as the lengths along sub-ducting plate margins, and those along normal faults are even shorter. [Read more / source].

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Relevant data

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Earthquakes away from plate boundaries

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What are the effects of an Earthquake?

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The effects of an earthquake are certainly not limited to experiencing a little rumbling under our feet.  Shaking and ground rupture are the main effects created by earthquakes, typically resulting in more or less damage to buildings and other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from the epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation. Ground rupture is a major risk for large engineering structures such as dams, bridges and nuclear power stations and requires careful mapping of existing faults to identify those which are likely to break the ground surface within the life of the structure. Most earthquake-related deaths are caused by the collapse of structures and the construction practices play a tremendous role in the death toll of an earthquake. There are secondary effects which can alter the face of the earth and change the course of our lives.

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Secondary effects are largely associated with the following

Landslides and avalanches: Data from 40 historical world-wide earthquakes were studied to determine the characteristics, geologic environments, and hazards of landslides caused by seismic events. This sample of 40 events was supplemented with intensity data from several hundred earthquakes in the United States to study relations between landslide distribution and seismic parameters. Fourteen types of landslides were identified in the earthquakes studied. The most abundant of these were rock falls, disrupted soil slides, and rock slides. The greatest losses of human life were due to rock avalanches, rapid soil flows, and rock falls. [Source].

Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid. In soil mechanics the term "liquefied" was first used by Hazen[ in reference to the 1918 failure of the Calaveras Dam in California. He described the mechanism of flow liquefaction of the embankment dam as follows: If the pressure of the water in the pores is great enough to carry all the load, it will have the effects of holding the particles apart and of producing a condition that is practically equivalent to that of quicksand ... the initial movement of some part of the material might result in accumulating pressure, first on one point, and then on another, successively, as the early points of concentration were liquified. The phenomenon is most often observed in saturated, loose (low density or non-compacted), sandy soils. This is because a loose sand has a tendency to compress when a load is applied; dense sands by contrast tend to expand in volume or 'dilate'. If the soil is saturated by water, a condition that often exists when the soil is below the ground water table or sea level, then water fills the gaps between soil grains ('pore spaces'). In response to the soil compressing, this water increases in pressure and attempts to flow out from the soil to zones of low pressure (usually upward towards the ground surface). However, if the loading is rapidly applied and large enough, or is repeated many times (e.g. earthquake shaking, storm wave loading) such that it does not flow out in time before the next cycle of load is applied, the water pressures may build to an extent where they exceed the contact stresses between the grains of soil that keep them in contact with each other. These contacts between grains are the means by which the weight from buildings and overlying soil layers are transferred from the ground surface to layers of soil or rock at greater depths. This loss of soil structure causes it to lose all of its strength (the ability to transfer shear stress) and it may be observed to flow like a liquid (hence 'liquefaction'). [Source].

Fires resulting from earthquakes: Fires, which are often associated with broken electrical and gas lines are one of the common side effects of earthquakes. Gas is released from broken lines exposed to an electrical spark can result in many fires. To complicate things broken water lines make it difficult to fight those fires. Fire was responsible for 90% of all the damage in the San Francisco earthquake of 1906.  [Source].

Tsunami's: For sure, one of the most dangerous effects of an earthquake is a Tsunami. Tsunamis are giant waves that can cause floods and in some cases can reach up to 100 feet in height or more. These deadly waves can strike areas a great distance from the epicenter. Tsunamis often result from sub-sea faulting of ocean floors sending seismic shocks through the water creating large waves of low amplitude but of long period, moving at 500-700 mph. Historical records, geological data and myths have suggested that tsunami's have been estimated as high as 1000 feet or more reaching inland to over 600 miles. [Source].

Floods: Flooding can come from many sources such as broken water main pipes, dams that fail due to an earthquake and earthquake-generated tsunamis. When an earthquake breaks a dam or levee along a river, the water from the river or the reservoir floods the area, damaging buildings and maybe sweeping away or drowning people. Small tsunamis, called seiches occur on lakes shaken by earthquakes and are usually just a few feet high. These small tsunamis are capable of destroying houses and uprooting trees. Additionally, earthquakes can alter the course of a river and can even cause it to flow in the opposite direction for a short time (this happened to the Mississippi River in the late 1800's). [Source].

Human impacts: Population growth and increasing urbanization in earthquake-prone areas suggest that earthquake impacts on human populations will increase in the coming decades. Inconsistent reporting across data sources suggests that the numbers injured and affected are likely underestimates. Findings from a systematic review of the literature indicate that the primary cause of earthquake-related death was trauma due to building collapse and, the very young and the elderly were at increased mortality risk, while gender was not consistently associated with mortality risk. Conclusions. Strategies to mitigate the impact of future earthquakes should include improvements to the built environment and a focus on populations most vulnerable to mortality and injury. Earthquakes were responsible for an estimated 1.87 million deaths in the 20th century with an average of 2,052 fatalities per event affecting humans between 1990 and 2010. [Read more / source].

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The following is a list of earthquakes in history

  wikipedia   Contents

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  Find  "solutions to earthquakes" 

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Video's

San Andreas Earthquake movie trailer

World of Discovery - Earthquakes: The Terrifying Truth

Earthquake Live Videos of The Action

EARTHQUAKE Video footage from camera in Crashing Building

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Resources

http://en.wikipedia.org/wiki/Earthquake

https://www.thetech.org/exhibits/online/quakes/overview/

http://earthquake.usgs.gov/earthquakes/

http://gsabulletin.gsapubs.org/content/95/4/406.abstract

 http://www.sms-tsunami-warning.com/pages/earthquake-effects#.VUqeWZNUXNM

http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/earthquake_effects.html

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3644288/

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