Earthquakes are studied in a specialty of physics (geophysics) and geology called seismology. There are two causes of earthquakes. There is a minority, the underground activity caused by a volcano in the process of erupting. However, most earthquakes are caused by the movement of tectonic plates. These are large plates that, fitting together like a puzzle, make up the lithosphere (the solid surface layer of the earth). Parts of these plates cannot be seen because they are located at the bottom of the seas, oceans and frozen areas. Other parts are continental surfaces and islands. Therefore, most plates contain part submerged and part emerged. These large plates move and their interaction, when they approach or separate, gives rise to the so-called geological activity of the earth.
This movement can be: convergent, when they approach and when a plate sinks under another plate (subduction), divergent, when they separate and transform, when they move laterally with respect to each other. These movements give rise to mountain ranges, volcanoes, earthquakes, etc. Today we will talk about earthquakes on earth, although it is not the only place in the solar system where there is seismic activity.
The plates are moving on an ocean of magma that is below them, in a process of accommodation that began many millions of years ago. Normally the movements are slow and imperceptible. However, in some cases the plates collide with each other preventing their displacement. In this process, tension energy begins to accumulate. When this energy reaches very high values it breaks the boundaries of the plates. A part of this released energy is what produces earthquakes.
Areas where plates exert these kinds of forces, which produce huge cracks, are called faults. On earth there are certain areas where these faults exist and it is precisely in them where 90% of all earthquakes occur. Therefore only 10% occur away from plate boundaries.
hypocenter and epicenter
When there is news of earthquakes, two pieces of information are usually given. One, the hypocenter, which is the focus of the earthquake. It is the deep point where energy is released. The earthquake is called superficial when the hypocenter is at a depth of less than 70 km, intermediate if it is between 70 and 300 km, and deep when it is more than 300 km. The other datum is the epicenter, which is the point of the surface above the hypocenter. It can be located on dry land or under the sea. The most intense movement of an earthquake is located near the epicenter, but the elastic waves generated by the earthquake can be detected at great distances.
Power dissipation is an instant process. Therefore, today, earthquakes cannot be predicted. There are several scales to assess its magnitude. One of the most common is Richter's. In it, for values less than 3.5, it is detected by devices, but not by people. Between 3.5 and 5.4 it is detected by people and can cause some minor damage. 5.5 to 6.0 can cause minor damage to buildings. From 6.1 to 6.9 can cause significant damage. From 7.0 to 7.9 it deals serious damage and above 8.0 it is devastating. This scale is of the logarithmic type, that means that, for example, 4 is not twice 2, it is 100 times two. That is why small numerical differences represent enormous growth in the effects.