Why Earthquake Occurs
CAUSES AND TYPES OF EARTHQUAKE
There is an earth model supported by the data obtained as a result of geological and geophysical studies on the internal structure of the earth. According to this model, there is a lithosphere (lithosphere) formed in the outer part of the earth, approximately 70-100 km thick. The continents and oceans are located in this stone sphere. The belt between the lithosphere and the core, with a thickness of 2,900 km, is called the Mantle. It is accepted that the core under the mantle consists of a mixture of Nickel-Iron. It is known that the temperature of the ground increases as the surface is deepened. From the fact that transverse earthquake waves cannot propagate in the core of the earth, it is concluded that the core must be a liquid medium.
Although the mantle is generally solid, it contains local liquid environments as it goes deep from the surface.
Under Taşküre, there is a soft Upper Mantle called Asthenosphere. Forces formed here, especially due to convection currents, the stone crust is fragmented and divided into many “Plates”. Convection currents formed in the Upper Mantle are attributed to the high temperature caused by radioactivity. As the convection currents rise upward, they cause stresses in the stone and then the formation of plates by breaking weak zones. There are currently as many as 10 large plates and many smaller plates. Together with the continents resting on these plates, they float like a raft on the Asthenosphere and move at a speed that humans cannot feel relative to each other.
Where convection currents rise, the plates diverge from each other and form the mid-ocean ridges in the hot magma that emerges from here. Friction and compression occur in the areas where the plates touch each other, one of the friction plates sinks into the Mantle and melts and forms the wear zones. This sequential event caused by convection currents continues under the background.
The boundaries of these plates, where the plates that make up the earth’s crust rub against each other, compress each other, overlap, or fall under each other, appear as places where earthquakes occur in the world. The vast majority of earthquakes in the world occur on narrow belts at the plate boundaries where these plates force each other.
Above, we said that the “Plates” forming the earth’s crust are in motion due to the convection currents in the Asthenosphere and therefore they repel each other or open from each other, and the zones where these events take place from the earthquake zones.
There is a frictional force between the two plates pushing each other or going under the other, which prevents movement. For a plate to move, this frictional force must be eliminated.
A movement occurs when the frictional force is exceeded between a plate being pushed and another plate. This movement takes place in a very short unit of time and is shocking. Eventually, earthquake (shaking) waves emerge, which can spread very far, and these waves spread by shaking the environments they pass through and decreasing their energy as they move away from the direction of the earthquake. In the meantime, land fractures, sometimes visible, extending for kilometers and called FAY, may occur on the earth. These fractures sometimes cannot be observed on earth, they may be hidden by surface layers. Sometimes a fault that originated from an old earthquake and reached the surface but was covered over time can play again.
The explanation of the occurrence of earthquakes in this way and under the name of “Elastic Back-Tab Theory” was made by the American Reid in 1911 and has been proved by being tested in laboratories.
According to this theory, when the energy stored elastically by the unit deformation accumulation formed gradually at any point, depending on time, reaches a critical value, it overcomes the friction force existing along the fault plane and creates the relative movements of the rock blocks on both sides of the fault line. This event is a sudden displacement movement. These sudden displacements are caused by the release and discharge of the unit deformation energy accumulated at a point, in other words, by the transformation into mechanical energy and consequently by the breaking and tearing movement of the ground layers.
Rocks can’t break without a unit of displacement accumulation beforehand. This unit creates displacement movements, convection currents formed in the upper mantle in the earth’s crust, which appears to be motionless, the rocks can resist until a certain deformation and then break. Earthquakes occur as a result of these ruptures. After this event, some or all of the stresses and energy accumulated since time away from the rocks are removed.
Elastic back tabs (throws) occur mostly on both sides of the fault and in the opposite direction in faults that occur during this earthquake event.
Earthquakes can be of different types depending on the reasons for their occurrence. Although most of the earthquakes in the world occur in the way described above, other types of earthquakes also occur for other natural reasons, albeit in small amounts. Earthquakes resulting from the movement of the plates described above are generally referred to as “TECTONIC” earthquakes and these earthquakes mostly occur at the boundaries of the plates. 90% of the earthquakes on the earth fall into this group. The earthquake in Turkey is also mostly tectonic earthquakes. The second type of earthquake is the “VOLCANIC” earthquake. These are formed as a result of the eruption of volcanoes. It is known that such earthquakes occur with the explosions caused by the gases formed as a result of the physical and chemical events during the emergence of the molten material in the depths of the earth. These are also local as they are related to volcanoes and do not cause significant damage. Some of the earthquakes in Japan and Italy fall into this group. because there is not this type of active volcanoes, earthquakes in Turkey.
Another type of earthquake is the “COLLECTION” earthquake. These cavities underground (caves), galleries in coal mines, and cavities formed as a result of melting in salt and gypsum land are formed by the collapse of the ceiling block. Their sensation areas are local and their energy is low, they do not cause much damage. It is known that large landslides and meteorites falling from the sky also cause minor tremors.
After the Deep Sea Earthquakes, the focus of which is at the bottom of the sea, waves occur in the seas up to the shores and sometimes cause great damage to the coasts, which are called (Tsunamis). In Japan, where sea earthquakes were common, 30,000 people died in 1896 from the Tsunami.