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  Latest Training
Environmental Geology
Publisher date : July 2008
Chapter title : Earthquake
Data title : Earthquake

The sudden earth shaking resulting from an energy relief at one moment or at a few seconds till a few minutes is said as earth quake, that has a different intensities.

The earthquake has and has caused an abundant damages as financial and human ones. Of world large earthquakes, we can mention to an earthquake dated on 27/Jun/`1976 at Tangshan, China, that about 650/000 persons were killed.

Magnitude of this earthquake was 7/6 Rishter.

Year

Country

Fatalities

Magnitude

1737

Kalkate-India

300/000

?

1755

Lisboon-portugal

70/000

?

1783

Kalabr-Italy

50/000

?

1768

Ekvador

40/000

?

1897

Asama-India

150

8/7

1908

Misen-Italy

120/000

7/5

1920

Ganso-China

180/000

8/2

1923

Koanto-Japan

143/000

8/2

1932

Ganso-China

70/000

7/6

1939

Shilan-Chilli

30/000

7/7

1939

Arzinkan-turkey

23/000

8

1970

Peru

66/000

7/8

1979

Tabas-Iran

12/000

7

1989

Roodbar-Iran

45/000

7/4

1999

Ezmir-Turkey

15/000

6/7

1968

Dashte-Bayaz.Iran

2500

7/4

 The earthquake geographical publication has a special decipline, as their 90% occur at pacific ocean margin and other 10%  occur on Alpine-Himalayan orogenic belt (Mountains chain of Alps, Karpat, Anatoly, Alborz, Zagroos GHendokoosh. Pamirotabat). The plate tectonic model could determine a basic cause of earthquake occurance and seismic regions. According to this model, the earth surface has been formed from one plate with maximum thickness 90km, that this plate is subdivided to a different pieces that each piece is said one plate.

The most important of these plates are as follows:

1-     Orazi plate, 2- Africa plate 3- pacific ocean plate, 4- India ocean plate, 5- S-pole plate, 6- south America plate, 7- North-Americal plate, 8- Naska plate (southern America).

There may be one microplate at the margin of each plate like India microplate, Arabia microplate or Iran microplate.

There is a paste-like part under earth forming plates.

The plates are said as lithosphere(Crust plus upper part of mantle) and the melting part under it is said as mantle.

If we look at the earth inside structure, we will have a parts from outside to inside as follow:

1-     Crust that it is solid

2-     Mantle that it's upper part is said as asthenosphere and this part has a melt property.

3-     Core external part that has a melt property.

4-     Core internal part that has a solid property.

The earth forming plates move to each other resulting from the mantle inside convection current, that these movements are performed by several ways.

1-     Two plates may be far away each other

2-     Two plates may be slide on each other

3-     Two plates maybe near to each other and meet with each othger that at a result of these meets, a large forces become a free which they are the same as forces of earthquake generating.

The speed of opening of oceans bottom crusts and/or annihilating the plates takes a few cm/year.

The continents age is a higher than the oceans age, since the continental plates don't penetrate inside a mantle and don't annihilate, while ocean plates are annihilated by penetrating inside a mantle.

The largest earthquakes occur at border zones of plates, those plates that meet with each other and one goes under another (At inter- ocean rifting place)


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