Under what circumstances do tsunamis usually occur?

First of all, nature

[b] Tsunami is a powerful and destructive wave. When there is an earthquake on the seabed, the sea water fluctuates violently due to the power of seismic waves, forming powerful waves, moving forward and flooding the coastal areas one by one. This is the so-called tsunami.

Tsunami is called "Tsunami" in many western languages, and the word comes from Japanese "Tsunami", that is, "waves by the harbor" ("Tianjin" means "harbor"). This also shows that Japan is a country that is often hit by tsunamis. At present, human beings can only prevent or reduce the losses caused by sudden disasters such as earthquakes, volcanoes and tsunamis through observation and prediction, but they can't stop them from happening.

Tsunamis are usually caused by undersea earthquakes, with a focal point less than 50 kilometers below the seabed and a magnitude of 6.5 on the Richter scale. The wavelength of the tsunami is greater than the maximum depth of the ocean, and its spread near the seabed will not be greatly hindered. No matter how deep the ocean is, waves can pass through. The propagation speed of tsunami in the ocean is about 500 to 1000 kilometers per hour, and the distance between two adjacent waves may be as far as 500 to 650 kilometers. When the tsunami wave enters the continental shelf, the wave height suddenly increases because of its shallowness, and the wave height rolled up by this wave movement can reach tens of meters.

The waves caused by earthquakes are different from those on the sea. Generally, waves only fluctuate in a certain depth of water layer, and the water body fluctuation caused by earthquake is the fluctuation from the sea surface to the whole water layer on the seabed. In addition, submarine volcanic eruptions, landslides and artificial underwater nuclear explosions can also trigger tsunamis. In addition, the meteorite impact will also trigger a tsunami, and the "water wall" can reach 100 feet. Moreover, the tsunami caused by meteorites can occur in any water area, not necessarily in the earthquake zone. However, the tsunami caused by meteorites may only happen once in a thousand years.

Tsunamis are very different from waves or tides generated by wind. The breeze blows across the ocean, forming relatively short waves. The corresponding flow is limited to shallow water. Strong winds can save the vast ocean, roll up waves more than 3 () meters, but can't shake the deep water. Tides sweep the globe twice a day. The current it generates can go deep into the seabed like a tsunami, but the tsunami is not caused by the gravity of the moon or the sun, but by underwater earthquakes, volcanic eruptions, meteorite impacts, or underwater landslides. The speed of deep-sea tsunami waves can exceed 700 kilometers per hour, which can easily keep pace with Boeing 747 aircraft. Although it is fast, the tsunami is not dangerous in deep water. In the open ocean, the length of a single wave below a few meters can exceed 750 kilometers. The tilt of the sea surface caused by this action is very subtle, and such waves usually pass inadvertently in deep water. Tsunamis pass through the ocean quietly and unconsciously, but if they are accidentally in shallow water, they will reach catastrophic heights.

Whistle is a very destructive wave. Earthquakes, volcanic eruptions, underwater collapses and landslides can all cause tsunamis.

When the earthquake occurred, the seabed strata broke, and some strata suddenly rose or sank, which caused the whole water layer from the seabed to the sea surface to "shake" violently. This "jitter" is very different from the waves you usually see. Generally, waves only fluctuate near the sea surface, and the depth involved is not large, and the amplitude of waves decays rapidly with the water depth. The sea water "jitter" caused by earthquake is the fluctuation of the whole water body from the bottom to the sea surface, which contains amazing energy.

The height of the wild waves caused by the tsunami can reach 10 meters to tens of meters, forming a "water wall". In addition, the tsunami has a large wavelength and can travel thousands of kilometers with little energy loss. For the above reasons, if the tsunami reaches the shore, the "water wall" will rush to the land, posing a serious threat to human life and property.

cause

Tsunami is a catastrophic wave, usually caused by an undersea earthquake, with a focal point less than 50 kilometers below the seabed and a magnitude of 6.5 or above on the Richter scale. Underwater or coastal landslides or volcanic eruptions may also trigger tsunamis. After an impact, the shock wave travels far away in an expanding circle on the sea surface, just like a pebble falling into a shallow pool. The tsunami wavelength is greater than the maximum depth of the ocean, and the orbital motion is not greatly hindered near the seabed. No matter how deep the ocean is, waves can travel through it.

Destructive waves caused by underwater earthquakes, volcanic eruptions or underwater collapses and landslides are called tsunamis. Destructive earthquakes and tsunamis only occur when there are vertical faults and the Richter scale is greater than 6.5. When the submarine earthquake causes seabed deformation, the water body near the deformation zone fluctuates greatly, resulting in tsunami.

The speed of tsunami propagation is directly proportional to the depth of its movement. In the Pacific Ocean, the speed of tsunami propagation is generally 200-300 kilometers per hour to 1000 kilometers or more. Tsunamis do not cause disasters in the deep sea, and even sailboats are hard to detect such fluctuations. When a tsunami occurs, it is safer on the high seas.

Once the tsunami enters the continental shelf, the depth becomes shallow sharply and the wave height increases sharply, reaching 20 to 30 meters. This huge wave will bring a devastating disaster.

Before the tsunami hit, why did the tide suddenly retreat away from the beach, and the sea rose again after a while?

In most cases, the phenomenon of sea level decline occurs because the wave trough of tsunami shock wave reaches the coast first. The trough is the lowest part of the wave. If it lands first, the sea will definitely fall. At the same time, the tsunami shock wave is different from ordinary waves and has a large wavelength. Therefore, it will take quite a while for the peak to come after the trough falls.

In addition, if this happens near the epicenter, it may be caused by another reason: when the earthquake occurred, the seabed ground rose and fell in a large area. At this time, the sea water near the earthquake zone also fluctuates, thus forming a tsunami.

classify

Tsunamis can be divided into four types. That is, storm surge caused by meteorological change, volcanic tsunami caused by volcanic eruption, landslide tsunami caused by submarine landslide and earthquake tsunami caused by submarine earthquake. According to the materials provided by the Seismological Bureau of China, the earthquake tsunami is a strong disturbance of seawater caused by the drastic fluctuation of seabed topography when an earthquake occurs on the seabed. Its mechanism has two forms: "falling" tsunami and "rising" tsunami.

"Descending" tsunami: Some tectonic earthquakes caused a large-scale sharp decline in the seabed crust. At first, the sea water rushed to the space where it suddenly sank, and there was a large area of sea water piled up on it. When the submerged seawater encounters resistance at the bottom of the sea, it turns back to the sea surface to produce compression waves, forming long waves and spreading around. The tsunami formed by this downward submarine crustal movement first appeared abnormal ebb tide along the coast. 1960 Chile earthquake and tsunami belong to this type.

"Uplift" tsunami: Some tectonic earthquakes caused the submarine crust to rise sharply in a wide range, and the seawater also rose with the uplift area, and large-scale seawater accumulation appeared above the uplift area. Under the action of gravity, the seawater must maintain an equipotential surface to achieve relative balance, so the seawater diffuses from the wave source area to the surrounding areas, forming a turbulent wave. The tsunami wave formed by this uplifted submarine crustal movement first showed an abnormal climax on the coast. 1The tsunami triggered by the earthquake of magnitude 7.7 in the Sea of Japan on May 26th, 983 belongs to this type.

Roaring waves

According to the theory of modern plate structure, Chile is a subduction zone where the Pacific plate collides with the South American plate, and it is located in the volcanic activity zone around the Pacific Ocean. This special geological structure leads to the extremely unstable surface of Chile. Since ancient times, volcanoes have been erupting, earthquakes have been repeated, tsunamis have appeared frequently, and disasters have often come. 2 1 In the early morning of May 26th, a strong earthquake, which is rare in the world earthquake history, suddenly occurred on the seabed near puerto montt, Chile. Large and small earthquakes lasted until June 23rd, and there were 225 earthquakes of different magnitudes in 1 several months. There are more than a dozen earthquakes of magnitude 7 and above, and 3 earthquakes of magnitude 8 and above.

harm

Shortly after the violent vibration, huge waves roared, pushing them to wither and rot, crossing the coastline and fields, and quickly attacking cities and villages on the shore. In an instant, people disappeared in the waves. All the facilities and collapsed buildings in the port were swept away by the rough waves. Afterwards, the beach was a mess, full of broken wood and human and animal bodies. The disasters brought by earthquakes and tsunamis to human beings are enormous. At present, human beings can only prevent or reduce the losses caused by sudden disasters such as earthquakes, volcanoes and tsunamis through prediction and observation, but they cannot control their occurrence.

Yu Fujiang, deputy director of the Marine Environment Forecasting Office of the Marine Environment Forecasting Center of the State Oceanic Administration, said that China is located on the west coast of the Pacific Ocean, with a continental coastline of 1.8 million kilometers. However, because the coast of Chinese mainland is blocked by Ryukyu Islands and Southeast Asian countries, the continental shelf is wide. After the transoceanic tsunami enters this sea area, the energy decays rapidly, which has little impact on the mainland coast.

Because seismic waves travel along the crust much faster than seismic tsunami waves, tsunamis can be predicted in advance. However, tsunami prediction is more difficult than earthquake detection. Because the seabed topography is too complex, it is difficult to accurately measure the deformation of the seabed.

65438-0964 A global tsunami early warning system coordination group was established internationally. Due to the frequent occurrence of tsunamis in the Pacific Ocean, the tsunami early warning system is very developed. After the earthquake 15 minutes, the Pacific Tsunami Warning Center issued early warning information from Honolulu Branch to 26 countries participating in the joint early warning system. If there is an early warning system in the Indian Ocean, perhaps people can make better use of the precious time from the earthquake to the tsunami landing on the Indian Ocean coast.

record

China scholars found that in 47 BC (that is, in the early years of the Western Han Dynasty) and in AD 173 (in the second year of Xiping in the Eastern Han Dynasty), there were records of the tsunami in Laizhou Bay and Huangxian County in China. These records are widely quoted by foreign scholars and are considered as the earliest two tsunami records in the world. The tsunami area in the world is roughly consistent with the earthquake zone. There are about 260 recorded destructive tsunamis in the world, averaging about once every six or seven years. Earthquakes and tsunamis around the Pacific Ocean account for about 80%. The earthquakes in the Japanese archipelago and its adjacent waters account for about 60% of the Pacific earthquakes and tsunamis, and Japan is the country suffering from the most severe earthquakes and tsunamis in the world.

8. Tsunami early warning system

Earthquakes can trigger tsunamis, so how can the earthquake monitoring system provide tsunami warning letters? In earthquake-prone areas around the world, such as the Pacific coast and the Indian Ocean coast, there should be a perfect earthquake monitoring network.

Recently caused a large-scale tsunami.

On June 26th, 2004, 65438+February 26th, 2004, a submarine earthquake of magnitude 9 occurred in the waters near Sumatra Island, Indonesia. The tsunami hit Sri Lanka, India, Thailand, Indonesia, Malaysia, Bangladesh, Maldives, Myanmar and the east coast of Africa, killing more than 300,000 people. The exact number of deaths has been impossible to count. See the 2004 Indian Ocean Earthquake.

1In July 1998, two submarine earthquakes of magnitude 7.0 occurred in Papua New Guinea, causing about 2 100 deaths.

1992 A tsunami occurred in Nicaragua in September.

1On August 25th, 883, dutch east indies volcano erupted, which triggered a tsunami and killed 36,000 people.

Record the location and date of noise source, and the number of deaths caused by wave height (meters) in the affected area.

Vancouver island1700 65438+1October 26th? Northern California to vancouver island and Japan?

Portugal1755165438+10 month16 60,000 in western Europe, Morocco and West Indies.

Ryukyu islands1771April 24th 85 Ryukyu islands 1 1, 94 1.

Sunda strait1883 August 26th 35 Java and Sumatra 36000.

Japan Sanlu 1896 30 Japan 27, 122

Aleutian Islands1946 April 1 32 Aleutian Islands, Hawaii and California 165