Earthquake and tsunami

An American president once said in a speech: "American students have a vast world to study in China. They learn from China how to monitor and predict earthquakes. " A vice president of the United States once said in a speech: "My hope is to broaden my horizons with the help of China people. From treating burns to earthquake prediction, researchers in China are in a leading position in some key areas. " A science and technology adviser to the President of the United States also said, "China has more experience in earthquake prediction than other countries. Therefore, we have a lot to learn from China. "

The earthquake monitoring and prediction that they praised, admired and asked to learn refers to the successful alarm of the Haicheng earthquake in Liaoning. At 9: 30 on February 4, 1975, the prediction opinions of possible earthquakes were put forward. 10: 30, the government issued an earthquake warning notice. 19: 36, when the magnitude 7.3 earthquake occurred, the masses had tried their best to evacuate, transfer or avoid, saving hundreds of thousands of lives.

It is the first time in the history of human science and technology to successfully monitor and predict the Haicheng earthquake and issue an early warning notice in time, which is also an important step for human beings to move from passive to active on the road of preventing earthquake disasters.

So, how is earthquake monitoring carried out? Or take Haicheng as an example:

Long-term monitoring stage: explore the possibility of earthquakes for more than three years. For example, during the southern Liaoning 1970 ~ 1973 period, it was recognized that this area was located at the junction of the Pacific plate, the Siberian plate and the North China plate, and there was a structure called "Tanlu deep fault", which may be a factor that gave birth to seismic activity. Therefore, it is necessary for seismic stations to adhere to long-term observation.

Mid-term monitoring stage: explore the distribution of earthquake risk areas in one or two years. For example, the analysis of monitoring data in southern Liaoning 1973 to 1974, Yingkou, Jinzhou, Dalian and other places shows that this area is in a region with high probability of destructive earthquakes.

Short-term monitoring stage: track, monitor and analyze the seismic activity within several months. For example, during southern Liaoning/KOOC-0/974/KOOC-0//KOOC-0/~/KOOC-0/975/KOOC-0/,a large number of macro-seismic anomalies (geomagnetic observation, gravity observation, seismic wave observation, groundwater observation, topographic change observation, etc.) occurred. ) were found in Dandong, Benxi and Jinzhou respectively. After comprehensive judgment, earthquake monitoring will soon change from short-term stage to short-term stage.

Impending earthquake monitoring stage: monitoring, analyzing and tracking the upcoming earthquake in a few weeks. For example, from late October to early February of 65438+ 19751in southern Liaoning, the multi-parameter comprehensive anomaly increased sharply, and the change range was quite large, and the foreshocks were quite concentrated. From February/Kloc-0 to February 3, geophysical and geochemical anomalies such as geomagnetism, geoelectricity, ground tilt and radioactive radon in groundwater increased sharply, well water bubbled, became turbid and smelly, and even a large number of abnormal biological activities appeared, such as hibernating snakes crawling out of caves and freezing, and domestic animals and tigers and monkeys in parks were restless. These phenomena provide a basis for imminent earthquake prediction and government alarm.

Earthquakes are caused by the vibration of fault structures caused by local stresses in the crust and lithosphere. Precursors are inevitable, just like a thick branch always makes a creaking sound before bending and breaking, but it makes a loud click when breaking. In addition to vibration waves, there are a series of other physical (chemical) phenomena that can be observed, such as geomagnetic, geoelectric, geoacoustic, geolight and groundwater changes.

Earthquakes cause ground vibration, which generally gives people the feeling of bumping up and down (vertical movement) and then shaking left and right (horizontal movement). The former is called longitudinal wave (also called P wave), which moves like a big earthworm crawling, with its body stretching and contracting, and its propagation speed on the crust is 5 ~ 6 kilometers per second; The latter is called shear wave (also called S wave), and it moves in the same way as a stone falling into a lake. The water surface ripples round and round, and the propagation speed on the earth's crust is 3 ~ 4 kilometers per second. Because of the different propagation speeds, they first feel the longitudinal wave jumping up and down, and then feel the horizontal shaking of the shear wave, and there is a time interval between them. With the length of this time interval, we can estimate the distance of the source. The long interval indicates that it is far from the source; A short interval means that it is close to the source. Rough calculation, you can multiply the seconds of this time interval by 8 kilometers per second, and you can estimate the distance from your location to the source. For example, when an earthquake occurs in a certain area, when the interval between the up-and-down motion and the horizontal motion felt by the ground in Beijing is 20 seconds, it can be immediately judged that the earthquake occurred in an area 0/60 kilometers away from Beijing/Kloc (as was the case during the Tangshan earthquake). Knowing the distance of the earthquake, we can choose appropriate defense or avoidance measures.

The internal structure of the earth

(1) The crustal strata are in a horizontal state before being stressed.

(2) Crustal strata are stressed and bent.

③ Earthquakes are caused by faults and dislocations in the crust and strata.

If there is a big earthquake under the sea, some will cause a big tsunami. Just like a huge rock falling into a river or lake, the water body vibrates violently. The waves of submarine earthquakes are constantly expanding in the ocean and spreading far away. The wavelength is tens to hundreds of kilometers, the period is 2 to 200 minutes, and the speed can reach 800 kilometers per hour, which is equivalent to the speed of a jet airliner. Waves 10 meters or even more than 20 meters high move forward like a wall.

1960 On May 23rd, a major earthquake struck off the coast of Chile, which triggered a tsunami and reached Japan within 24 hours. The wave was 3 meters high, and 109 the ship sank, causing hundreds of casualties. On the way to Hawaii, the tsunami threw more than 10 tons of basalt blocks beyond 100 meters.

On February 26th, 2004, in 65438, a great earthquake occurred on the seabed near Indonesia. China's earthquake observation is 8.7, and the United States is 8.9, which is equivalent to the energy of 3,000 ~ 6,000 atomic bombs. The tsunami caused huge losses to many countries along the Indian Ocean 10, and about 300,000 people were killed.

At present, more than 50 seismic stations in Pacific coast 12 countries monitor marine earthquakes and tsunamis and can make timely predictions. In the future, the Indian Ocean region will inevitably receive attention.

The tsunami washed ashore a few minutes to several hours later than the submarine earthquake, so there is a chance for tsunami warning and escape. Many animals are more sensitive than people. The TV broadcast introduced the Indian Ocean tsunami, and there were almost no casualties among animals in neighboring countries, so they could escape quickly with different keen senses in advance.

An article in the Popular Science newspaper said that at dawn on February 26, 2004, at 65438, a major earthquake struck Sumatra, Indonesia, and the elephants at the seaside in Corey, Thailand began to sing "excitedly". Although the calm was quickly restored, it sang again about an hour later and ran to the high slope opposite the coast, so the tourists riding on the elephant's back were lucky to escape the tsunami disaster. So why did the elephant know that the tsunami was coming?

According to the Daily News, Professor Hideo Chiyama of Shibuya University of Technology in Japan said: Tsunamis will emit low-frequency "sounds" that humans can't hear. Humans can only hear sounds with a frequency of 20-20000 Hz, but elephants can make sounds below 20 Hz and talk to other elephants about 10 km away. When Professor Hideo Shan Zhi measured the sound of elephants in the park, he also measured the extremely low frequency sound of 8 ~ 12 Hz. He said: "The frequency of sound that an elephant can hear is greater than the frequency range it emits, so the elephant can hear quite low sound." At the same time, he also pointed out that elephants can feel the vibration of the ground and the agitation of the air in advance, and then hear the sound of the tsunami, so they instinctively panic and run to high places to avoid.

1 1989, another earthquake occurred in San Francisco.