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Earthquakes - Two of the most devistating disasters ever were Earthquakes |
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Main Articles -
Casualty by Natural
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Written by Armageddon Online Articles
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June 04, 2007 |
What is an Earthquake? An earthquake is a trembling or shaking movement of the Earth's surface. Earthquakes normally result from the movement of faults, quasi-planar zones of deformation within its uppermost layers. Basically earths's tectonic plates shift every so often, and the areas on the fault lines are the ones that feel it's results.
Earthquakes occur every day on Earth, but the vast majority of them are minor and cause no damage. Large earthquakes can cause serious destruction and massive loss of life through a variety of agents of damage including fault rupture, vibratory ground motion (i.e., shaking), inundation (e.g., tsunami, seiche, dam failure), various kinds of permanent ground failure (e.g. liquefaction, landslide), and fire or hazardous materials release. In a particular earthquake, any of these agents of damage can dominate, and historically each has caused major damage and great loss of life, but for most earthquakes shaking is the dominant and most widespread cause of damage. Where do most earthquakes occur? The earth is made of layers, divided into the crust, mantle, and core. The crust is the Earth’s surface, a thin, hard layer of rock, broken into many pieces. Each of these pieces is known as a crustal plate. Some form continents, others the ocean floor, but they are always moving. According to the theory of plate tectonics, the earth’s crust is made of six major plates and nine smaller ones that lie on the mantle, a thicker, denser layer of hot, soft, molten rock. These plates float around within the mantle, in a hot, soft zone known as the asthenosphere.
The core is made up of even hotter rocks below the mantle, and currents of burning rock rise up through the mantle. These currents spread out once they hit the bottom surface of the crust. This behavior tends to tear the crust, pulling the apart, grinding some plates against others, colliding them into one another. Continental drift (when major plates are slowly but steadily moved apart) also contribute, carrying plates until they collide. It is through these collisions that mountain ranges are also formed. This movement of our dynamic planet produces earthquakes and volcanoes.
Earthquakes occur most frequently (about 95% of the time) at the point where two plates scrape against one another. When these two plates move against each other, the crack is known as a fault. A famous example is the 700-mile-long San Andreas Fault running up the length of California (United States). When plates jam against one another, stress builds up in between. When the pressure becomes too great, the bends and snaps free with a jerky motion. This sudden motion is an earthquake.
| | The "Ring of Fire" is an arc stretching from New Zealand, along the eastern edge of Asia, north across the Aleutian Islands of Alaska, and south along the coast of North and South America. It is composed over 75% of the world's active and dormant volcanoes. (See the full USGS MAP here)
- In South America the Nazca plate is colliding with the South American plate. This has created the Andes and volcanoes such as Cotopaxi and Azul.
- In Central America, the tiny Cocos plate is crashing into the North American plate and is therefore responsible for the Mexican volcanoes of Popocatepetl and Paricutun (which rose up from a cornfield in 1943 and became a instant mountains).
- Between Northern California and British Columbia, the Pacific, Juan de Fuca, and Gorda plates have built the Cascades and the infamous Mount Saint Helens, which erupted in 1980.
- Alaska's Aleutian Islands are growing as the Pacific plate hits the North American plate. The deep Aleutian Trench has been created at the subduction zone with a maximum depth of 25,194 feet (7679 meters).
- From Russia's Kamchatka Peninsula to Japan, the subduction of the Pacific plate under the Eurasian plate is responsible for Japanese islands and volcanoes (such as Mt. Fuji).
- The final section of the Ring of Fire exists where the Indo-Australian plate subducts under the Pacific plate and has created volcanoes in the New Guinea and Micronesian areas. Near New Zealand, the Pacific Plate slides under the Indo-Australian plate.
What kind of damage can earthquakes cause? The method for measuring the shaking effect and damage is what is known as the Richter Scale. The Richter magnitude scale is a mathematical technique used to quantify the size of earthquakes. Developed in 1935 by Charles Richter in collaboration with Beno Gutenberg, both of the California Institute of Technology, the Richter scale assigns a single number to quantify the size of an earthquake. Here's what the scale looks like.
(Section of collapsed freeway after the 1989 Loma Prieta earthquake) - Less than 2.0 - Microearthquakes, not felt.- About 8,000 per year
- 2.0-2.9 - Generally not felt, but recorded - No damages - About 1,000 per year
- 3.0-3.9 - Often felt, but rarely causes damage.- 49,000 per year (estimated)
- 4.0-4.9 - Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. - 6,200 per year(estimated)
- 5.0-5.9 - Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings.- 800 per year
- 6.0-6.9 - Can be destructive in areas up to about 100 miles across in populated areas.- 120 per year
- 7.0-7.9 - Can cause serious damage over larger areas. Skyscrapers as risk. 18 per year
- 8.0 or greater - Can cause serious damage in areas several hundred miles across. Building structures collapse - skyscrapers as SERIOUS risk. Average 1 per year
The Great China Earthquake of 1556 The worst natural disaster in recorded history, at least in terms of lives lost, was caused by an earthquake in Hausien in the Shensi Province of China in 1556. The earthquake devastated 98 counties and eight provinces of Central China. The destruction spanned an area of 500 miles, and in some counties the average death toll was 60 percent of the population. An estimated total of 830,000 people lost their lives, most of them from the collapse of poorly constructed houses. The magnitude of this earthquake has been estimated to be from 8.0 to 8.3 on the Richter scale.
The Great 1906 San Fransisco Earthquake At almost precisely 5:12 a.m., local time, a foreshock occurred with sufficient force to be felt widely throughout the San Francisco Bay area. The great earthquake broke loose some 20 to 25 seconds later, with an epicenter near San Francisco. Violent shocks punctuated the strong shaking which lasted some 45 to 60 seconds. The earthquake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada. In the public's mind, this earthquake is perhaps remembered most for the fire it spawned in San Francisco, giving it the somewhat misleading appellation of the "San Francisco earthquake". Shaking damage, however, was equally severe in many other places along the fault rupture. The frequently quoted value of 700 deaths caused by the earthquake and fire is now believed to underestimate the total loss of life by a factor of 3 or 4. Most of the fatalities occurred in San Francisco, and 189 were reported elsewhere.
December 26, 2003 - Bam Iran Tragedy hit Iran on 26 December 2003 when a major earthquake registering 6.5 on the Richter scale hit its south-eastern province of Kerman at 05:28 (local time). The area most affected was the ancient city of Bam where more than 43,000 people were killed, an estimated 30,000 injured and up to 75,000 left homeless, according to official estimates.
More Resources:
The U.S. National Earthquake Information Center
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Last Updated ( July 13, 2009 )
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