Definition :
It is earthquakes that affects the earth's crust and spread in waves through the vast areas of it. And the earth`s crust always suffers from movements due to seismic instability of its interior, but this is usually persistent tremors of vulnerability so that we do not feel it , just monitoring devices and sensors recognize them.
Causes of earthquake :
1-The cracks and breaks in the crust of the earth because of the balance disorder. And the balance of the earth's crust upsets as a result of the sweep of huge amounts of material by the village-borne erosion and deposition in the seas and oceans.
2-Movements of molten rocks through the earth's crust or underneath .
Types of earthquakes :
1-Volcanic earthquakes: they occur associated with volcanic activity, and the rush of molten rock material from the underground to the surface, for example, the eruption of volcanoes of the Hawaiian Islands from earthquakes with a lot of violence and force,Also when the volcano erupted Krakata In (Indonesia)it Lasted a lot of destruction and sabotage, it exploded, causing violent tremors caused sea water in the form of massive waves raid on the plains located in the nearby islands,this destroyed homes and displaced many residents, and caused heavy losses for the inhabitants of the islands of Sumatra and Java and other islands nearby.
However, most of the seismic tremors that occur because of volcanic activity are in fact domestic shocks do not arise in large areas, and many of the volcanic eruptions are accompanied by weak shocks.
2-Tectonic earthquakes: this type occurs in areas that is being undermined and hit by the break-up and this type is common frequent. It is concentrated particularly in the surface crust at depths of up to 70 km.
3-Plutonic earthquake: (relative to Pluto, god of the earth for the Greeks)
It`s very deep from the ground ,When some earthquakes were recorded at a depth of 800 km in the east of Asia.
* The last two types of earthquakes occur because of movements in the earth's crust and below it. There are a lot of convincing evidences which suggest that most major earthquakes occur due to sudden violent pressure in the earth's crust, resulting in cracking and movement of the layers along the lines of the old breaks already exist.
The epicenter and internal center of the earthquake :
The forces of the earthquake is not equal at the surface of the earth but it becomes the get peaked at a point on the Earth's surface is called the center surface and beneath it in the portrait orientation is another point is the point of generating and called the center of procedure of the earthquake, which arise vibrations ripple up in a vertical orientation to the center surface, and spread in diffrent directions to all other parts of the body of the Earth.
The effects of earthquakes :
Seismic shocks vary in the degree of strength, there is the weak that is hardly felt, and the destructive, which causes significant losses in the built-up area. And their effects can be summarized in the following points.
1- May cause a shift of parts of the earth's crust in the horizontal and vertical directions.
2- Can raise or lower parts of the seabed, as happened in the Gulf of Sajami Japan in 1923 when some parts of it rose (about 250 m) and other parts decreased (about 400 m).
3- They can raise or lower coastal areas, as happened in Alaska (in 1899).
4- May cause landslides as happened in north China in the years 1920 and 1927.
5- Earthquakes that occur in the bottom of the ocean arrise waves of destructive winds that cause destruction on the coasts that are exposed to them.
6- The earthquakes that occur in areas with many residents destroy constructions and causes an excessive loss of life.
