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 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)

Earthquake intensity:

By before inventing the earthquake recorders, the earthquakes importance has been posed given to a degradation extent caused from it. First time, Robert Mallet drew a intensity curve, as at earthquake on 1875 Italy, he connected a points having a unified damages  to each other  and then is grade curves were drawn, and he named a place having a maximum damages as an earthquake focus that it is the same as an external focus or earthquake external center.

First time, the intensity scale was posed by Italian, Rossi and Swiss, Foret on 1880, that was corrected by Mercalli on 1902. This scale was completed by other persons later, and today, it is an only scale that is used for stating an earthquake. With regards to the names of contributors such as karnik, sponkener, medveder, it is named as MSK scale.

Earthquake magnitude:

The earthquakes magnitude dependes on their energy, as a stronger earthquake has a more intensive vibrations. The earthquake magnitude is stated with Rishter scale that was posed by American Rishter. With magnitude measurement, the caused energy amount of earthquake was easily corrected with Cancani acceleration equivalent (1956).

Description:                                               Acceleration per mm/S2:

It is not felt. It is detected only with seismographs.

Less that 2/5

It is a weak. It is felt by persons at a suitable places or opper stories of buildings.

2/5-5

It is felt as a mild in the home. The hanging bodies shake. The shaking like a shaking resulting from passing a light truck is caused . It may be doesn't detect as earthquake.

5-10

It is an average. The hanging bodies shake. The shaking like to heavy truck is caused.

With feeling like to shaking the wall resulting of hitting a heavy ball is caused. The windows, plates and doors all shake.

The glasses resulting of the shaking cause a sound. The pottery dishes also cause a sound. At higher that intensity 4, the wooden walls also cause a sound.

10-25

It is rather strong. It is fect outside of homes. The earthquake bearing is approximatable. People wake up. The liquids shake, and even discharge from dish. The small unstable objects are moved or reversed. Doors are open and closed. Curtains, and picture frames are moved. Clocks's pendulums stop, move and their move method change.

25-50

It is a strong. All people feel it.

Most fear, and go out. The walking of people become unbalanced. Windows, plates and crystals break down.

The make up and books and etc fall down from shelves. Frames fall down from a wall. The home's farnitures are moved or reversed. The walls loose plasters and face stones break down. The small rings (church and school) ring. The shaking of trees and crucibles is visible or their rustle sound is heared.

50-100

It is very strong. The stand is a difficult. Drivers and motorcyclists recognize it. The hanging objects vibrate. The furniture break.

The Buildings decay of type 2 is occurred and they creak. The loose pipes located on roof break. The loose plaster of walls.

Free bricks of stones, roof  tiles, and also walls that have not well been built and architecture decorations fall down. Some of buildings of Typec crack. Water of dams are waved an mudy resulting from distortion. Small slips and pores are occurred at sand sheets. Large rings of churches ring. Water ditches that have been built a soild but un regular, are decayed.

It is a destructive. It affects on the act of automobiles motor. Buildings of type C are destroyed, and fall down as part. But Buildings of type A are not destroyed. Some of decorative walls and plasterworks fall down.

The stovepipes, plants chimneys, asphalt rocks, elevators chambers and cabinets twist, and wooden houses and ones having weak structural adhesion move on their foundation. The loose walls fall down. The wearied columns also fall down. The branches of trees break, and flow or water temperature of springs and wells changes. At wet lands and steep slope, the fractures are occurred.

250-500

It is very destructive. It results a fear for people. Buildings of type D are destroyed. Buildings of type C are destroyed very much some times, a complete collapse is occurred, and buildings of type B damage hardly.

500-1000

The complete damage occurs at a foundation of wooden buildings, if they have not a good structural adhesion, they will move. Buildings break. Storage tank damage extensively. Underground pipes break.

The clear creaks are occurred in the ground . At alluvial areas eject a sand and mud from a ground. Springs become seismic and a sand craters are occurred.

1000-2500

It is a disastrous. Most of wooden buildings and their foundation are destroyed. Some of wooden buildings having a suitable build and bridges are destroyed. Dames and walls and stone walls along with rivers damage extensively. Large landslides are occurred. Water ejects inside of channels, rivers, lacks and etc. Sands and moods move on beaches and a flat lands horizontally. Railways curve

2500-5000

There is a complete disaster and a complete destruction is occurred. Large stones move. The previous alignment levels are destroyed.

The objects throw at air. It is accounted.

5000-10000

 Two factors affect on the earthquake destructive effect as follows:

1-     Region geological construction and a land type of one region.

2-     Earthquake focus depth.

Periodicity:

A number of earthquakes at the earth is very much, and thousands earthquakes occur every year, that they are studied by seismograph statistically. If we can determine a number and magnitude of earthquakes at one country or region, we can find to an amount of risk at one country or region. The risk possibility means the occurance possibility of one known event (like earthquake) at a known time limit. For determining the risk possibility, it is required an attention to current age and historical statistics of region. For example, from past 1000 years to today at a special area, 8 earthquakes with magnitude more than 7 Rishter have occurred. In fact 1000/8=0/25 year means that there is the occurance possibility of one earthquake with magnitude more than 7 Rishter at each 125 years.

Prediction and risk decline:

Today's, given we know that the earthquakes are occurred at a fault areas and an impact place between previous or current plates, the maps of risk regions have been prepared that also this case has been performed in Iran. For doing this, it is a necessary the field geology on large basic scale and very carefully, and it is a necessary the geological study of regions that have a history of seismicity. As a result, we can prepare maps that determine a risk of earthquake at each region, and determine a magnitude of earthquake possibility at each region. Even with regard to a smaller earthquakes focus located on one region, we can draw the maps of earthquake intensity before.

Today's, geologists in many countries, particularly, seismic countries such as Japan , China, America, and Russia, search a solution for anticipating earthquakes, but they have not so far had a special success, and today, we can not say that it is possible the anticipation of earthquakes, but only seismic regions have been determined.

So far, in some of the world points the anticipation of earthquake has been performed, it has been based on their foreshock that of course, such anticipation of predictions are a correct rarely.

The expansion assumption, one was that at first it was a promising for the prediction of earthquakes very much, but finally, it didn't seem as a clear and suitable solution, because it doesn't apply to at mast of regions. According to the expansion assumption, that it's base is the different experiments related to stones reaction to the applied stresses and pressures, it is occurred a cracks and fractures in the stones resulting from the applied stress and is caused a multiple joints in them, that as a result, their volume increases and of course, the wares passing speed in them changes. On the other words. The earthquake waves speed stones decreases and the electric conduction increases.

These laboratory observations namely expansion assumption, it has derived a suitable anticipations at some of regions, and at other areas, it has no provided a correct answer.

Fore an occurrence of earthquake, the existing Radon gas at minerals having uranium become a free a given the existence of radioactive material at one region, that this subject was proved by Russian people.

The research on the waves speed P and S showed that before an accordance of earthquake, the waves speed P and S ratio changes considerably. This method of earthquake prediction is said VS-VP method, that it was anticipated given those two earthquakes. This has a disadvantage for a large earthquakes that it doesn't provide a correct answer.

Chinese predicated the earthquake dated on Feb.8.at Hayching using a simple seismographs and domestic and wild animals movements, like chicken, cow, snack, rabbit, lions inside zoo and a red fishes and underground water surface change, that it was caused that a human fatalities approached to a minimum, because a city had been discharged before an earthquake.