Earth quackes

Earthquakes

 Earthquakes are a shaking of the ground. Some are slight tremors that barely rock a cradle. Others are so violent they can tear down mountains.

Earthquack

 

 

 

 

 

 

 

Small earthquakes 

small earth quaks may be set off by landslides, volcanoes or even just heavy traffic. 

Big earthquacks

Big earthquakes are set off by the grinding together of the vast tectonic plates that make up the Earths surface. Tectonic plates are sliding past each other all the time, but sometimes they stick. The rock bends and stretches for a while and then snaps. This makes the plates jolt, sending out the shock waves that cause the earthquake’s effects to be felt far away. Tectonic plates typically slide 4 or 5 cm past each other in a year. In a slip that triggers a major quake they can slip more than 1 m in a few seconds. In most quakes a few minor tremors (foreshocks) are followed by an intense burst lasting just one or two minutes. A second series of minor tremors (aftershocks) occurs over the next few hours. 

Hypocentre

The starting point of an earthquake below ground is called the hypocentre, or focus.

Epicentre

The epicentre of an earthquake is the point on the surface directly above the hypocentre. Earthquakes are strongest at the epicentre and become gradually weaker farther away. 

Earthquack zones

Certain regions called earthquake zones are especially prone to earth-quakes. Earthquake zones lie along the edges of tectonic plates. A shallow earthquake originates 0-70 km below the ground. These are the ones that do the most damage. An intermediate quake begins 70-300 km down. Deep quakes begin over 300 km down. The deepest-ever recorded earthquake began over 720 km down. 

Major cities of world in earthquack zones

 Many major cities are located in earthquake zones, such as Los Angeles, Mexico City and Tokyo. Severe earthquakes can collapse buildings and rip up flyovers. When freeways collapsed in the 1989 San Francisco quake, some cars were crushed to 0.5 m thick. The 1906 earthquake in San Francisco destroyed 400 km of railway track around the city. Some of the worst earthquake damage is caused by fire, often set off by the breaking of gas pipes and electrical cables. In 1923,200,000 died in the firestorm that engulfed Tokyo as an earthquake 
 The complete collapse of overhead freeways upset domestic charcoal stoves. Z5 a major danger in severe earthquakes. In the Kobe earthquake of 1995 and the San Francisco earthquake of 1989 some of the worst damage was to buildings built on landfill - loose material piled in to build up the land. The earthquake that killed the most people was probably the one that hit Shansi in China in 1556. It may have claimed 830,000 lives. 

Earthquack in tangshan

The most fatal earthquake this century destroyed the city of Tangshan in China in 1976. It killed an estimated Europe centred on Lisbon, Portugal, in 1755. It destroyed the city, killing 100.0 or more people. It probably measured 9.0 on the Richter scale  and was felt in Paris. 

Prediction of earthquack

 • One way to predict earthquakes is to study past quakes. 
• If there has been no earthquake in an earthquake zone for a while, there will be one soon. The longer it has been since the last quake, the bigger the next one will be. 

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 • Seismic gaps are places in active earthquake zones where there has been no earthquake activity. This is where a big earthquake will probably occur.
 • Seismologists make very accurate surveys with ground instruments and laser beams bounced off satellites (see earthquake measurement). They can spot tiny deformations of rock that show strain building up. 
• A linked network of four laser-satellite stations called Keystone is set to track ground movements in Tokyo Bay, Japan, so that earthquakes can be predicted better. 
 • The level of water in the ground may indicate stress as the rock squeezes groundwater towards the surface. Chinese seismologists check water levels in wells.
 • Rising surface levels of the underground gas radon may also show that the rock is being squeezed.
 • Other signs of strain in the rock may be changes in the grounds electrical resistance or its magnetism. 
 • Before an earthquake dogs are said to howl, chickens flee their roosts, rats and mice scamper from their holes and fish thrash about in ponds. Some people claim to be sensitive to earthquakes. .