Destructive Power of an Earthquake: What Happens?

Destructive Power of An Earthquake: What Happens?

By Mr Ghaz, March 10, 2011

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Destructive Power of An Earthquake: What Happens?

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Core facts

As earthquake is any motion of the ground, no matter what the cause. They range from being imperceptible to being so violent that they can raze cities. No part of the world is exempt from earthquakes.

Earthquakes happen as a result of the release of energy when rocks come under pressure, mainly from plate movements, shift or fracture.

A few earthquakes are triggered by meteorite impacts, man-made explosions, or the weight of water, for instance behind a dam.

Earthquakes produce three main types of seismic waves primary (P), secondary (S) and surface (L).

Earthquake prediction, the best defence against their effects, is still an inexact science.

Where Earthquakes Happen

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Most earthquakes occur at plate boundaries, 70 per cent occur around the Pacific, including the San Andreas Fault region. Earthquakes also occur within continents, such as Tangshan in 1976. This map shows the regions with the greatest concentration of earthquake activity.

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Mountain Regions around the Mediterranean, through Turkey and Iran to the Himalayas.

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The Pacific Rim: Japan, the Philippines, western Alaska, California’s San Andreas Fault, Chile, and several island chains.

Ocean Ridge System:Winding around the globe. Frequent low-intensity quakes.

What Happens?

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When rocks can no longer absorb the build-up of pressure they break or shift along fault lines, releasing the pent-up energy as waves of vibrations radiating out from the center, or focus, of the failure. We feel the surface waves as an earthquake, which can last just a few seconds or as long as four minutes. The intensity of the earthquake and the damage it wreaks are measured on the Richter and Modified Mercalli scales.

Mexican Quake

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In 1985 vibrations from a mild earthquake 400km (250miles) away were amplified by the rock on which Mexico City is built, causing extensive ground movement and damage.

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Shanshi China, 1556, Highest known death toll – 830 000 people died. China has the worst record for earthquake deaths.

Libson Portugal, 1755, Worst in Europe – 8.7 on Richter scale. 70 000 people died and the entire city was destroyed. Tremors were felt in North Africa, England and Luxembourg.

Yakatut Bay Alaska, 1899, Greatest recorded vertical movement-the coast rose in places by 14.5m (47 ½ ft).

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San Francisco USA, 1906, Destroyed most of the city, and a huge fire burned for 3 ½ days. Survivors included the celebrities Enrico Caruso and John Barrymore.

Off Chile, 1960, largest earthquake ever recorded-registered 9.5 on the Richter scale.

Alaska, 1964, Longest recorded-4 minutes (Libson in 1755 was a 10-minute series of shocks).

Tangshan China, 1976, Deadliest in the 20th century-8.0 on the Richter scale. 655 000 people died.

Kobe Japan, 1995, Worst damage financially-over US$ 147 billion of damage. 5500 people died.

Destructive Power

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Rhodes Greece, c.225BC, The Colossus, a huge bronze statue of the sun god Helios, one of the Seven Wonders of the World, was toppled.

Alexandria Egypt,, AD 365, The lighthouse (Pharos), one of the Seven Wonders of the World, was toppled in an earthquake that killed 50 000 people.

Port Royal Jamaica, 1692, This pirate haven was virtually destroyed-seen as punishment for the port’s sinful reputation.

Unsen Japan, 1793, Thee whole island and 53 000 inhabitants sank beneath the Satsuma Sea.

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Messina Sicily, 1908, The entire city was destroyed. Only one building survived.

Assisi Italy, 1997, Earthquakes damaged the basilica of St Francis, destroying ceilings and frescoes. St Francis was said to have cured a boy who had been injured in an earthquake.

Earthquake Gods

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In the Bible, Yahweh speaks of the prophet Elijah as an earthquake. In African myth earthquakes are caused by the dead of the underworld. The Greek god of earthquakes was Poseidon, also god of the sea. His Roman counterpart was Neptune. In Siberia the earthquake god is Tuil, who rides a sleigh beneath the Earth.

Fore-and Aftershocks

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Foreshocks are minor precursors that can occur before a major earthquake, and are often used to predict a larger seismic event. Aftershocks are normally weaker shocks that happen days weeks later. They can cause extensive damage to structures already weakened by the initial quake.