What caused the earthquake?

As the name implies, an earthquake is the vibration of the earth's crust, which may be caused by volcanic eruption, the influence of meteors, underground nuclear tests, mine development and other human activities, but at most it is caused by crustal movement. In fact, our earth is not peaceful. According to the US Geological Survey, there are 3 million earthquakes every year. The earth shakes its body here and there every 1 1 second on average. Of course, most of these 3 million earthquakes were only slight tremors, which was nothing. But shaking half of Asia like yesterday will inevitably shake the whole earth.

Everyone has heard of the "earthquake zone". Why do earthquakes only disturb some areas? This must start with the theory of plate tectonics, which is one of the greatest discoveries in the 20th century. The lithosphere at the top of the earth is not closely connected, but several hard and independent units floating on the asthenosphere, which seem to be broken and not broken, and have relative sliding, and are a "plate".

Just as national boundaries are the most prone to conflict, the place where plates contact is also the most active place in geology. It is conceivable that there are three kinds of relative movements between plates: pulling apart from each other, squeezing when approaching each other, or twisting at the junction of one facing south and one facing north. Accordingly, the crustal response is to produce three different types of faults (defined as four in some literatures): normal faults, thrust faults and strike-slip faults. Although the principles are different, they are * * *, that is, huge energy is ready to go under the distortion of faults, which is most obvious when the friction between faults is large and the faults want to move but are forced to be fixed. This is "where there is oppression, there is resistance".

We continue to imagine that the fire in the crust is getting bigger and bigger. When the internal pressure is finally greater than the friction on both sides of the fault, the crust gains kinetic energy and shakes violently. Back to the question just now, this kind of distortion is most likely to occur at the junction of plates. If a small fault is distorted, it is most likely to form a large-scale "fault zone". The vibration will be passed down from generation to generation, so the vibration is likely to be connected in a short time. On the Qinghai-Tibet Plateau in China, the Eurasian plate and the Indian plate push each other to form a geological active area; The southeast coast, Eurasian plate and Pacific plate have never been willing to coexist peacefully, which is another earthquake-prone area.

In fact, earthquakes don't just happen between plates. /kloc-in the 0/7th century, an earthquake shook many states in the United States with Missouri as the center. It took scientists more than a hundred years to discover that its initiator was actually a fault that had been planned for 600 million years!

Crustal echo seismic wave

If the earth's crust can't use seismic waves to spread angry news, the power of earthquakes will be greatly reduced. Of course, this is just a dream, just as we know that liquid transmits vibration when we see water waves, so does solid. Before introducing two kinds of seismic waves, by the way, distinguish two concepts. When we say that the source of the Wenchuan earthquake is underground 10 km, we mean the vertical distance between the source of contradictions and conflicts in the earth's crust and Wenchuan County, the epicentre of the earth's surface, so you know that the "source" and the "epicenter" are different.

Back to seismic wave, it can be divided into "body wave" and "surface wave", and the former can be divided into "longitudinal wave" and "shear wave". "Primary wave" is a common P wave, whose vibration direction is the same as that of wave propagation, and it can propagate in solid, liquid and gas, so it is named because it reaches the epicenter first because of its fast speed. In the same way, the vibration direction of "shear wave" is perpendicular to the propagation direction, and it can only move forward in solid, and it will stop when it meets liquid. Then it was the "secondary wave" that hit the epicenter, which we called S wave for short.

When the two waves meet on the surface, they are excited to produce surface waves (Love waves), which grow up and have strong amplitude. When it propagates on the surface, it will make the ground shake up and down like water waves, which is the main factor causing strong damage to buildings. Because the speed of S wave is the slowest, we generally feel that earthquakes always occur in a progressive way, that is, the most violent moment is always the last stage.

Although the propagation velocity of P wave and S wave varies with different media, the ratio of their velocities is always 1.7, which is often used to calculate the distance from the source to the epicenter after being recorded by seismograph.

Crust, are you angry or angry?

The news said: "Wenchuan earthquake in Sichuan, magnitude 7.8". When you see this number, what else do you see besides realizing that it is a "great anger"? In fact, the full name of this "7.8" should be called "7.8 on the Richter scale". This standard was tailored by Charles Richter and Beno Gutenberg of California Institute of Technology for the famous California earthquake in 1935. In order to make its value not negative, they deliberately measured at the epicenter 100 km, and recorded an earthquake with a horizontal displacement of 1 micron of magnitude 0 with a Wood-Anderson torsional seismograph.

Of course, the fate of this "level 0" is the same as many "maxima" and "minima" defined in history. At present, the accuracy of seismographs far exceeds that of those years, and it is no longer uncommon to record negative earthquakes. Even if the earth's crust is slightly alive, it will be recorded by seismographs. The Richter scale is a logarithmic value, indicating that the amplitude of earthquake wave of magnitude 8 is ten times that of magnitude 7, and the energy release reaches 3 1.7 times. Of course, there are countless strong earthquakes in the history of the earth, but in the history of the Richter scale, the largest one is only 9.5, and most of them are below 3.

The Richter scale describes the seismic wave energy emitted by the earthquake source, while another standard "intensity" is related to many factors such as the depth of the earthquake source, epicentral distance, azimuth, geological structure, soil properties and so on. So an earthquake has only one magnitude, but the intensity in different areas is different.

This is the McCully intensity scale in Roman letters. This standard is quite dependent on subjective standards, such as "how many people in the room can feel it when it is completely still" is degree III; The maximum XII degree is "extensive damage to buildings, landslides and ground fissures, and changes in topography". This is why scientists can report the magnitude immediately after the earthquake, but the intensity of different places can't be summarized until they have time to go to different places to ask witnesses.

Earth, we will not wait for death.

If I lived a hundred years ago, even a geologist could not write such an article, which shows how much scientists' understanding of earthquakes has improved. However, it is somewhat disappointing that human beings are still unable to predict earthquakes.

Natural and sensitive seismographs can sense the pioneers of seismic waves. At this time, we can shout that an earthquake is coming, but it is basically useless unless you can arm the county in a few minutes. Geologists and meteorologists can also see the general trend of plate activity and fault activity, but it is conceivable that this prediction is even more inaccurate than the weather forecast of an island in the sea. What can be called accurate prediction now is only for aftershocks. Scientists can learn from the different forms of aftershocks summarized in previous earthquakes and the analysis of local topography.

(There are more than 2,000 words of nonsense ahead, but we still have to honor our promises ...) What scientists should do most is to protect themselves, help people around them when they can, and say something great, which is also a contribution to reducing human casualties in natural disasters. The following (selected according to the national conditions of China) is taken from the earthquake prevention manual of California Fire Department.

During the earthquake:

1. If you are indoors, hide under the table. If there is no table, stay close to the inner wall until the vibration stops. Stay away from windows (glass sometimes flies far), bookcases, refrigerators, mirrors and other hanging objects. If you must move your position, you'd better always keep your head protected and sheltered. Don't take the elevator.

2. If you are outdoors, don't go anywhere, stay away from trees, signs, tall buildings and telephone poles, and be careful of bricks and glass that may fly from buildings.

If you are driving, stop and stay in the car until the vibration stops. But be careful not to park under overpasses or wires.

If you are in the theater, sit still and protect your head with your arms. Don't go out in order until the vibration stops.

5. Be careful not to make children afraid and worried, and try to avoid darkness and loneliness.

6. If there is a fire, close the door as much as possible between the fire source and you, and block the door with something like a quilt. Let firefighters know where you are, make a phone call or hang obvious objects on the window, but don't break the heart. If there is heavy smoke outside, close the window.

After the earthquake:

1. Check yourself and others, first aid is preferred.

2. Stay calm and stabilize others.

3. Small broken glass.

4. Put out the fire, check the gas, water and wires, and turn off the switch if there is any damage. If the gas leaks, don't use anything that can cause Mars (including buttons) and open the window.

If there is a radio, turn on the radio and listen to the first aid announcement (I wonder if we have it? )

6. Don't try to enter the damaged building.

7. If there is no water reserve, it is best to find a way to filter tap water with chemicals or filters. If there is no running water, use toilet water to store water, not toilet water, which may have been polluted.

8. If your child has phobia after the earthquake, try to talk to him, exchange your feelings, tell your child that they are safe, always hold your child, spend more time with him at night, and allow your child to be sad and complain about small things, such as losing toys. Sometimes, adults may have greater psychological difficulties, talk more with their relatives and pay more attention to their health.

Usually:

1. Remember the emergency telephone number and inform all family members.

2. First-aid drugs are often: anti-inflammatory drugs (including external use and internal use), analgesics, antidiarrheal drugs, eye drops, ear, nose and water drops, hydrogen peroxide, skin disinfection spray, bandages, medical gloves, masks, ice packs or hot packs (if necessary, friction can change the temperature), cotton, adhesive tapes and splints.

3. Life emergency items: dry food, canned food, cash, drinking water (very important, because the urban water supply will be damaged in the earthquake, and one person can guarantee at least 19 liter, including your pet), fire extinguishers (although this is not in line with China's national conditions, I still think it is very important), compasses, emergency lights, tents, tarpaulins, ropes, matches, and so on.