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Published on: February 11, 2012, by: STW Editor

Earthquakes

An earthquake can be one of the most catastrophic events to strike, whether it be inland or out at sea.

The sheer energy unleashed by an earthquake can cause so much damage it can be hard to comprehend.  But how exactly do these earth tremors happen?  Well, our planet’s seemingly stable surface is made up of enormous pieces of rock, also known as tectonic plates, that are slowly but constantly moving. Those pieces continually collide with and rub against one another, and sometimes their edges abruptly crack or slip and suddenly release huge amounts of pent-up energy. These unsettling events are called earthquakes, and small ones happen across the planet every day, without people even noticing. But every so often, a big earthquake occurs, and when that happens, the pulses of energy it releases, called seismic waves, can wreak almost unfathomable destruction and kill and injure many thousands of people.

CAN WE GET A PIC OF THE TECTONIC PLATES IN HERE??

Tectonic Earthquakes 

Tectonic earthquakes are triggered when the crust becomes subjected to strain, and eventually moves. The theory of plate tectonics explains how the crust of the Earth is made of several plates, large areas of crust which float on the Mantle. Since these plates are free to slowly move, they can either drift towards each other, away from each other or slide past each other. Many of the earthquakes which we feel are located in the areas where plates collide or try to slide past each other.

The process which explains these earthquakes, known as Elastic Rebound Theory can be demonstrated with a green twig or branch. Holding both ends, the twig can be slowly bent. As it is bent, energy is built up within it. A point will be reached where the twig suddenly snaps. At this moment the energy within the twig has exceeded the Elastic Limit of the twig. As it snaps the energy is released, causing the twig to vibrate and to produce sound waves.

Perhaps the most famous example of plates sliding past each other is the San Andreas Fault in California. Here, two plates, the Pacific Plate and the North American Plate, are both moving in a roughly northwesterly direction, but one is moving faster than the other. The San Francisco area is subjected to hundreds of small earthquakes every year as the two plates grind against each other. Occasionally, as in 1989, a much larger movement occurs, triggering a far more violent ‘quake’.

Major earthquakes are sometimes preceded by a period of changed activity. This might take the form of more frequent minor shocks as the rocks begin to move,called foreshocks , or a period of less frequent shocks as the two rock masses temporarily ‘stick’ and become locked together. Detailed surveys in San Francisco have shown that railway lines, fences and other longitudinal features very slowly become deformed as the pressure builds up in the rocks, then become noticeably offset when a movement occurs along the fault. Following the main shock, there may be further movements, called aftershocks, which occur as the rock masses ‘settle down’ in their new positions. Such aftershocks cause problems for rescue services, bringing down buildings already weakened by the main earthquake.

The san Andreas fault is probably one of the most famous of the fault lines, and it lies in the Pacific Basin.  This area is part of what is commonly known as the Ring of fire, and it takes  in a 25,000 mile horseshoe shape. It is associated with a nearly continuous series of oceanic trenches,  volcanic arcs, and volcanic belts and/or plate movements. The Ring of Fire has 452 volcanoes and is home to over 75% of the world’s active and dormant volcanos.

INSERT PIC OF RING OF FIRE

The Ring of Fire

About 90% of the world’s earthquakes and 89% of the world’s largest earthquakes occur along the Ring of Fire.  Countries that lie along these plates are:

Chile, Bolivia, Central America,  North America Cordillera, Mexico, United States, Canada. Russia, Japan, The Philippines, Indonesia, New Zealand and Antarctica.

The next most seismic region (5–6% of earthquakes and 17% of the world’s largest earthquakes) is the alpide belt, which extends from Java to Sumatra through the Himalayas, the Mediterreanean, and out into the Atlantic. The Mid-atlantic ridge is the third most prominent earthquake belt.

 Volcanic Earthquakes 

Volcanic earthquakes are far less common than Tectonic ones. They are triggered by the explosive eruption of a volcano. Given that not all volcanoes are prone to violent eruption, and that most are ‘quiet’ for the majority of the time, it is not surprising to find that they are comparatively rare.

When a volcano explodes, it is likely that the associated earthquake effects will be confined to an area 10 to 20 miles around its base, where as a tectonic earthquake may be felt around the globe.

The volcanoes which are most likely to explode violently are those which produce acidic lava. Acidic lava cools and sets very quickly upon contact with the air. This tends to chock the volcanic vent and block the further escape of pressure. For example, in the case of Mt Pelee, the lava solidified before it could flow down the sides of the volcano. Instead it formed a spine of solid rock within the volcano vent. The only way in which such a blockage can be removed is by the build up of pressure to the point at which the blockage is literally exploded out of the way. In reality, the weakest part of the volcano will be the part which gives way, sometimes leading to a sideways explosion as in the Mt St.Helens eruption.

When extraordinary levels of pressure develop, the resultant explosion can be devastating, producing an earthquake of considerable magnitude. When Krakatoa ( Indonesia, between Java and Sumatra ) exploded in 1883, the explosion was heard over 5000 km away in Australia. The shockwaves produced a series of tsunami ( large sea waves ), one of which was over 36m high; that’s the same as four, two story houses stacked on top of each other. These swept over the coastal areas of Java and Sumatra killing over 36,000 people.

By contrast, volcanoes producing free flowing basic lava rarely cause earthquakes. The lava flows freely out of the vent and down the sides of the volcano, releasing pressure evenly and constantly. Since pressure doesn’t build up, violent explosions do not occur.

Info from geography-site.co.uk
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