Earthquakes on the planet are recorded frequently. Most people only become aware of some of them. Basically, about that earthquake, from which many people died and more or less significant damage was done. Seismic disasters can occur anywhere in a very large area populated by billions of people. Unfortunately, the threat cannot be averted. You can only reduce the tragedy of the consequences..
Earthquake, general information, earthquake sources, intermediate, deep-focus and shallow-focus earthquakes, types of earthquakes, signs of an emerging earthquake.
Specialists are able to assume that tremors are likely to occur in a particular place soon. But no one will say what kind of strength they will be, where exactly they will happen and when. Scientists will answer these questions with approximate terms, approximate place and power with a significant range of values.
An earthquake is usually accompanied by numerous sounds of different intensities, depending on how far away from the source of the cataclysm is the one who hears them. So, near the epicenter, you can hear sharp, sharp sounds that at a certain distance become like peals of thunder or the roar of an explosion.
. Underwater earthquakes lead to the formation of giant waves – tsunamis. Much more dangerous for people and settlements than tremors. In general, the results of powerful earthquakes to some extent resemble the consequences of a nuclear explosion.
The place of birth of the earthquake is the dark and hot depths of the earth’s interior, inaccessible to direct observation and measurement. The one who thinks that the manifestation of the underground element is purely geological is mistaken. That is, it is connected with the structure and composition of the earth’s crust. In fact, earthquakes are a geophysical phenomenon associated with the physical properties and physical processes of the planet as a whole, both in solid shells and in liquid and gas media.
Earthquakes are one of those terrifying natural phenomena (along with hurricanes, tornadoes, storms, volcanic eruptions, floods and fires) that pull a person out of greenhouse conditions and make you remember that he is just one of the inhabitants of the planet and, along with many others, equally helpless and defenseless like animals, insects, etc..
Earthquake foci, intermediate, deep focus and shallow focus earthquakes.
An earthquake is not just ground vibration. A certain source of energy in the upper layers of the globe produces seismic waves that diverge in all directions (not in concentric circles). Despite the fact that the source of natural earthquakes is a certain volume of rocks, it is more convenient to conditionally determine it as a point (earthquake focus, focus, hypocenter, rock movement point) from which waves propagate.
Focus is able to be at different depths. With natural disasters deep enough. Or very close to the surface if tremors are artificially caused, such as a nuclear explosion. The projection of the focus on the earth’s surface directly above it is called epicenter earthquakes.
A very interesting question is related to the possible depth of focus. It may seem surprising, but they are sometimes located hundreds of kilometers inside the planet. These are, for example, the territories in which ocean troughs pass:
Andes Mountain Range in South America.
Tonga Islands, Samoa.
New Hebrides Archipelago, Indonesia.
As a rule, in these zones an earthquake rarely occurs at depths of over 200 km. However, sometimes the hypocenter is located 700 km below the surface of the planet. Earthquakes that have been born within 70-300 km are considered intermediate. And with greater depth – deep focus. Such an earthquake occurs not only in the Pacific region, but even in Central Asia (the Hindu Kush mountain system), Romania (the Carpathians), the Aegean, Spain.
An earthquake whose hypocenters are located directly below the earth’s surface is called shallow focus. Just this type of earthquake leads to the most large-scale destruction. Their share in the total energy released during the cataclysms is 3/4.
More often after such earthquakes, subsequent shocks of lesser strength (aftershocks) are recorded, which can last several hours or even months. Moreover, their number during strong earthquakes can be quite large. Before some earthquakes, there are preliminary shocks from the same source, called foreshocks. It is assumed that they can predict the main blow.
Types of Earthquakes.
Relatively recently, many believed that since an earthquake occurs at great depths and is not accessible to direct observation, its nature will remain covered in darkness. To the credit of modern science, it can be said that for the most part, the nature of earthquakes and their main properties are easily explained from the perspective of physical theory.
This is a description of the phenomenon, and not an explanation of the processes occurring inside the planet, with this, not everything is clear. According to it, an earthquake is a natural result of the Earth’s continuous geological transformation.
The bulk of earthquakes occur on the outskirts of tectonic plates, which suggests global geological “players” that form mountains, rift valleys, underwater mountain ranges and deep gutters.
An earthquake is initiated precisely by these forces, whose nature is not yet clear. However, they must be caused by temperature inhomogeneities inside different layers of the globe, which are formed, firstly, due to the release of heat into the environment. And secondly, due to the influx of heat as a result of the decay of the radioactive elements contained in the rocks.
Further, it is appropriate to characterize different types of earthquakes based on the features of their occurrence. The most common are tectonic earthquakes, which occur when a break occurs in rocks under the influence of any geological factors.
A tectonic earthquake attracts the closest attention of scientists, since it can tell more about the bowels of the planet than anything else. And besides, it brings to humanity the bulk of all the misfortunes from this kind of natural phenomena.
However, tremors are able to be caused by other events. For example, they accompany volcanic eruptions. Moreover, the misconception that earthquakes are certainly associated with volcanic activity still exists..
Ancient Greek philosophers, who could observe the activity of Mediterranean volcanoes and the widespread occurrence of earthquakes in their region, were also fond of this idea. It was impossible not to connect these two facts, and they are actually related. But only as a special case of the action of global tectonic forces on rocks does not at all have to arise together.
Sometimes there is another type of earthquake – landslide. These are quite small tremors occurring in places where underground voids or mine workings are present. The direct catalyst for earthquakes is the collapse of the roof of a mine or cave.
Earthquakes associated with “mountain impacts” also occur when stresses accumulating in the mining zone lead to a sharp separation of the huge masses of rocks from their bottom, which results in seismic waves. A similar earthquake is characteristic of the territories of Canada and South Africa. Of significant interest are landslides associated with the development of large landslides.
It is impossible not to mention artificial earthquakes, the culprit is a person carrying out conventional or nuclear explosions. So, underground nuclear explosions of the last few decades led to quite noticeable seismic activity in different places of the planet. If a nuclear device detonates in a deep well, a colossal amount of nuclear energy is released.
The pressure increases by a fraction of a second to values that are thousands of times higher than atmospheric pressure, and the temperature at the epicenter rises by millions of degrees. In this case, the surrounding rocks evaporate. A spherical cavity several meters in diameter is formed, which becomes larger as the boiling rock evaporates. And the shock wave penetrates the rocks around the sphere with subtle cracks.
The main object of seismology research that provides key information is seismic waves. It is possible to study not only an earthquake from them, but also the structure of the Earth. Locate minerals and identify explosions (e.g. nuclear).
Outside the area covered by cracks (hundreds of meters), rock compression triggers the seismic wave divergence mechanism. When the 1st wave reaches the earth’s surface, the soil rises and, if there is a sufficient amount of energy, the rocks can “shoot” with the formation of a funnel. Seismic waves from very powerful nuclear explosions propagated through the inner regions of the planet and were recorded at distant seismic stations.
Moreover, the amplitude corresponded to the strength of real seismic waves from an earthquake of 7 magnitudes on the Richter scale. Such waves were felt even in cities far from the place of the explosion..
Signs of a looming earthquake.
First of all, specialists at seismic activity tracking stations can tell a lot about changes in the speed of longitudinal seismic waves. The equipment is designed to accurately determine the time of arrival of waves. In addition, if the surface of the earth suddenly tilts, then this will also become more than suspicious. And before the earthquake, radon gas may be released.
An important role in predicting the imminent earthquake is played by the electrical conductivity of the rocks in the risk zone. As it turned out during laboratory experiments, it noticeably changes in a water-saturated rock before it is destroyed by high pressure. Fluctuations in the general background of seismic activity is probably the most interesting parameter. But also the most uncertain in terms of specific conclusions.
An earthquake develops in the following stages. First, under the influence of global tectonic movements, elastic deformation accumulates for some time. This is the name of the special effect of tectonic forces when the body is able to restore shape when the influence on it ceases.
During this period, seismic indicators do not go beyond the standard values. Then, cracks develop in the rocks of the earth’s crust in the fault zones. Which ultimately lead to a change in the volume of the material due to shear deformation.
In the process of crack opening, the velocity of longitudinal waves passing through a similar expanding region decreases. The surface is lifted. Gas is released, electrical conductivity increases. Possible increase in the number of small earthquakes per unit time.
Further, water penetrates into the pores and microcracks from the surrounding rocks, exacerbating the instability. During the filling of cracks with water, the first weak seismic waves begin to propagate faster, the soil stops rising, gas stops being released, and electrical conductivity continues to increase. Then an earthquake occurs immediately followed by aftershocks.