When an earthquake occurs, the energy is released in various forms: mechanical, thermal, energy in the form of electric and magnetic fields, etc. This energy is huge, and to define it in full is quite a challenge. Most of the mechanical energy is used to destroy the rock in the source region of the earthquake, the vertical and horizontal displacement of the adjacent crustal blocks. And only a small fraction of this energy is radiated in all directions into space in the form of seismic waves that spread across the globe. When the waves reach the earth's surface, they generate those tremors, which we perceive as an earthquake.
To characterize the strength of earthquakes used terms such as magnitude, the energy and intensity of the class.
Magnitude (M) of an earthquake is a conventional measure of the energy released from the earthquake in the form of seismic waves. The amplitude of the seismic wave field is for the soil, and the greater the scope of the wave, the greater the magnitude of the earthquake.
The notion of the magnitude imposed in 1935 by the American seismologist Charles Richter, a professor at Caltech in Pasadena. Define the magnitude of Richter in his own words: "The magnitude of any shock is defined as the logarithm expressed in microns, the maximum amplitude of the recording of this push, make the standard short-period torsion seismometer 100 km from the epicenter." In practice, measurements are made at different distances from the epicenter of the various devices. Therefore, to bring the necessary conditions to use the amendment.
In the world there are different scale of magnitude, different way to define them. This is a local magnitude (ML), the magnitude of surface waves (MS), body-wave (mb), on the seismic moment (MW). The maximum value of the magnitude on Richter scale introduced — about 9 units. Minimum earthquake, yet no tangible devices are characterized by a magnitude in the range of 2-3. Smaller magnitude earthquake recorded only sensitive seismic instruments.
Ground motion from earthquakes with magnitudes that differ by one, characterized by seismic amplitude of 10. Thus, we notice without instruments earthquake from barely perceptible to catastrophic, destructive, and different amplitudes of the waves, at least a million times. With the value of seismic energy
liberated when earthquakes comparable energy atomic and hydrogen explosions.
In our country, as in other countries of the former Soviet Union, is used another characteristic magnitude earthquake
equivalent magnitude and called energy class (K).
Energy class of earthquakes vary in the range from 0 to 18-20. On average in the world to convert magnitudes in value to the adoption of energy class formula:
K = 4, 1.8 M.
In turn, the energy class of seismic energy associated with the simple formula:
E = 10To Joules.
Consequently, the magnitude can be related to seismic energy as follows:
4 lg E = 1.8 M.
Intensity — Fundamentally different characteristic strength of the earthquake, installed only tangible tremors in each point on the Earth's surface by the narrative and usually neinstrumentalnoy scale. The intensity of the characteristic earthquake effect on the Earth's surface, it depends on the earthquake magnitude, focal depth and soil conditions, and is measured in points. In seismology, the intensity is synonymous intensities.
The intensity is greater, the focus is closer to the surface, for example, if the earthquake with a magnitude equal to 8, at a depth of 10 km, on the surface, the intensity will XI-XII points, with the same magnitude, but at a depth of 40 — 50 km impact on the surface is reduced to IX-X points.
Seismic scale. Seismic movements are complex, but to classify. There are a large number of seismic scales, which can be summarized in three main groups. Russia has used the most widely used in the world of XII-point scale MSK-64 (Medvedev-Shponhoyera-Karnik), Latin America received X-point scale Rossi-Forel, in Japan — VII-point scale. Assessment of the intensity, which builds on the domestic manifestation of the earthquake, easily distinguishable even inexperienced observer, the seismic scale of different countries is different. For example, in Australia, one of the state compared with those concussions, "the horse rubs against the porch column," Europe is the same seismic effect is described as "the bells start ringing," in Japan appears "inverted stone lantern." In the simplest and most convenient way to experience and observations are presented in a schematized brief descriptive scale (option MSK), which can be for everyone.
|Mark||Manifestation on the surface|
|I||Not felt by anyone, only registered seismic instruments|
|II||There is sometimes people who are in the quiescent state|
|III||There is a few more strong in the rooms on the upper floors|
|IV||Felt by many (especially indoors), at night, some are waking up. Possible the clatter of dishes, glasses rattling, clapping doors|
|V||Feels almost all, many nights awake. Hanging objects swing, cracked window panes and plaster|
|VI||Felt by all, crumbling plaster, light damage to buildings|
|VII||Cracks in plaster and spalling of separate pieces, fine cracks in the walls. Tremors felt in cars|
|VIII||Large cracks in the walls, drop pipes, monuments. Cracks on steep slopes and in moist soil|
|IX||Collapse of walls, ceilings, roofs, some buildings, underground pipeline ruptures|
|X||Collapses of many buildings, the bending rails. Landslides, rock falls, cracks (up to 1 m) in the ground|
|XI||Numerous large cracks in the ground, falls in the mountains, the bridge collapse, only a few buildings retain stability|
|XII||Significant changes in topography, the deviation reaches of rivers, objects thrown in the air, the total destruction of buildings|
Manifestations of the same earthquake at different points are different — from the most intense in the epicentral area to the minimum in the distance. Thus, the magnitude — is a certain value that characterizes the energy in the earthquake, and the intensity — a measure of the strength of seismic shaking at the observation point, which depends not only on the intensity of seismic waves emitted from the source, but also on the distance from the epicenter, focal depth, location point and ground features at this point. This is clearly seen in the figure below.
Magnitude is given in arbitrary units, reflecting the focus of the energy (or the intensity at the focus) earthquakes. Intensity (or intensities) determines the degree of shocks and disturbances on the surface.
Journalists make the mistake of giving the message: "Earthquake 7 points on the Richter scale," since at magnitude 7 on the Richter scale earthquake intensity at the surface, if shallow focus, can make X points, as it were, for example, in Spitak (Armenia) in 1988. If the center is located at a sufficient depth, when a magnitude 7 intensity on the surface can make only VIII points, as it was during the earthquake Zaisan (Kazakhstan) in 1990.
To better navigate the newspaper and television reports on the occurrence of earthquakes, you can use this simple table NV Shabalin.
|Magnitude earthquake on the Richter scale||4.0||5.0||6.0||7.0||8.0|
|The depth of the earthquake, km||3||5.10||5||10||10||20||15||30||25||40|
|The intensity of the tremors on the surface of the scale MSK-64, points||VII||VI||VIII||VII||VIII-IX||VII-VIII||IX-X||VIII-IX||X-XI||IX-X|
Scientists believe that earthquakes stronger than magnitude 9 occur on Earth can not. It is known that each earthquake is a push or a series of shocks that occur as a result of the displacement of rock masses along a fault. Calculations have shown that the size of the earthquake (ie, the extent to which there was a displacement of rock) at the weak, barely perceptible tremors man is measured by several meters. The earthquakes of moderate strength, when there are cracks in the buildings, the size of the source has already reached kilometers. Foci is at the strongest, most catastrophic earthquakes have a length of 500-1000 km and even more and go to a depth of 50 km. At the maximum of the recorded earthquakes on Earth size hearth close to the maximum length of faults known to scientists. Impossible, and further increase the depth of focus, as the earthly substance at depths greater than 100 km in the mantle moves in a semi-molten state. Consequently, such an earthquake as occurred in South-East Asia (Sumatra) in December 2004 and having a magnitude 9 can be considered close to the maximum for his strength. The length of the gap in the focus of the earthquake was estimated at 1200-1300 km, and its width — more than 100 km.
How often earthquakes of such force? Seismology relatively young science. She is a little over 100 years. But she had already accumulated enough data to make some statistical conclusions that are presented in the following table:
There are no data on the frequency of occurrence of earthquakes with a magnitude of 9. But it is clear that they occur less frequently. Since 1900, with the magnitude of events in the world registered only 5.
1. AA Nikonov. Earthquake … Past, Present, and prognosis. M. KomKniga. 2006. 192s.
2. A. Nurmagambetov, MS Kuhn. Physics of the Earth. Almaty: KazNTU. 2007. 224s.
3. J. Gil, H. Shah. Shaky firmament: What is an earthquake and how to prepare for it. Wiley. 1988. 220C.
4. AD Zavyalov. Earthquake off Sumatra / / Nature. 2005. № 5
Vera Ilina, Ph.D. in Physics and Mathematics, Professor Hasan Kaz, Art. Republican teacher refresher course management team of Emergency Situations Ministry of Emergency Situations of Kazakhstan