1in the early morning of Wednesday, April 3, 906, residents within 700 miles of the west coast of the United States (that is, Oregon and California from Coos Bay to Los Angeles) were awakened by the earth's vibration in their sleep. The earthquake in San Francisco is particularly strong. According to the police who were on the streets of San Fan City when the earthquake occurred, "I suddenly heard the roar of the earth, and the streets and alleys in front of me were shaking up and down, as if the waves were coming to me." At that time, there was no systematic standard for measuring earthquake intensity, so it was impossible to estimate its intensity. It was not until the Richter scale 1935 appeared that this problem was solved. According to scientists' later estimation, the earthquake that occurred in Los Angeles in 1906 was about 7.8 on the Richter scale. After the earthquake that day, the fire caused by the earthquake swallowed up the city mercilessly. It is estimated that more than 700 people died; 250,000 people are homeless; 28,000 buildings were destroyed. The economic loss caused by the earthquake is about $5 billion.
Historical photos 1906 photos after the earthquake.
The San Francisco earthquake will be remembered forever, not only because of its amazing destructiveness, but also because its occurrence time cannot be accurately predicted by scientists. At that time, scientists had already understood the fault structure near the Santandrea Mountains and were familiar with the theory: "When the crust breaks, one side of the crust moves up or down, which will prompt the other side to move in the opposite direction, thus triggering an earthquake." When geographers first explained earthquakes at that time, they could not clearly explain the energy level of earthquakes. The scale and scope of the interval that caused the earthquake in Sanfan City are unheard of. It spans about 300 miles, from San Juan Bautista in San Benito County to the south of San Francisco and the north of the upper reaches of Mattler River in Hubert County, and then extends westward to the sea. The explanation of the causes of earthquakes has been studied for 60 years, which is derived with the development of plate theory.
Plate theory is one of the great discoveries of modern science, which describes that the earth's surface is composed of several huge plates, which support the continents to drift around the earth quite slowly. When the two plates act, it will bring great natural disasters to mankind. For example: volcanic eruption, earthquake, etc. Volcanic eruptions and earthquakes can cause tsunamis. Since 1960s, the earth plate theory has been recognized by scientists. The following article introduces us to the basic knowledge of seabed structure and earth structure. From this, we can understand the causes and principles of crustal movement; Know what emergency measures should be taken when an earthquake occurs.
From earthquake research to continental drift theory
As early as190s, a geographer named John Milne taught in Imperial College London, Tokyo. Together with his colleagues, he invented the first seismograph which can accurately measure the intensity of earthquakes, and it can record data about earthquakes. But just a few years later, an unexpected fire not only destroyed Minnie's home and laboratory. And burned the earthquake data that Minnie had collected for more than ten years, but he was not discouraged by it. After he returned to his hometown of England, Minnie continued to study earthquakes. By the beginning of19th century, Minnie had a set of methods to study earthquakes. Minnie established a network of 27 measuring instruments to measure earthquakes in many parts of the then Kingdom of Great Britain. Until Minnie died in 19 13, 40 observation stations had been established all over the world to monitor the occurrence of earthquakes.
Seismographs are used to measure earthquakes and their vibration intensity caused by sudden movement of earth faults. There are many forms of vibration: up and down earthquakes, left and right earthquakes, and earthquakes before and after. First, seismologists should be able to distinguish two kinds of seismic waves moving at different speeds. P wave (main wave): the fastest and the first to reach the seismograph. It is a wave conducted in the form of back-and-forth vibration, and its linear conduction state can be drawn in the chart. S wave (secondary wave) is slow, and the vibration wave is serpentine and moves at a specific phase angle. The distance from the monitoring station to the epicenter of the earthquake and the distance from the underground source to the surface can be calculated by the time interval between the arrival of P wave and S wave. By measuring the distance between the three monitoring stations, the exact location of the earthquake can be marked on the map by triangulation.
1In the 1990s, British geologist John Minnie devoted himself to the invention of seismograph.
(This picture is provided by the Archives of Science and Society Pictures)
The seismic network established by Minnie pioneered the remote seismic survey and made great contributions to science and human society. Soon seismologists were surprised to find that seismographs can not only measure the intensity of earthquakes, but also let us know the internal structure of the earth. By the end of World War I, scientists had learned about the structure of the center of our planet by studying earthquake waveforms. There is no unified conclusion about whether the core is liquid or solid, but the core is surrounded by high-density rock layers and mantle. The mantle is 30 miles from the crust surface.
Aside from the above background knowledge, let's meet a German meteorologist who put forward a bold idea about the nature of the earth's surface. 19 15, Alfred Wegener published a book entitled "The Origin of the Continent and the Ocean". In his book, he pointed out: "The prominent part of Brazil's territory coincides with the depressed part of Southwest Africa's territory, so Brazil and Southwest Africa were integrated at first, and then gradually separated. Another evidence of continental drift theory is the discovery of middle pterosaur fossils (a kind of lizard) that lived in eastern and western America and western Africa 2.7 billion years ago. At that time, most scientists explained: "It is possible that there are land bridges between continents, and these land bridges are slowly submerged by seawater. However, Wei Gena holds a different view. He believes that the discovery of pterosaur fossils in Africa and America at the same time is because the two continents were connected 500 years ago, and the plates drifted away, taking away their fossils respectively. Wei Gena called the continent formed before continental drift Pan-ancient land or United ancient land.
At that time, meteorologists were not sure what drove these huge plates to move. Wei Gena explained that the continental drift was the result of the centrifugal force of the earth's rotation and the attraction of the sun and the moon to the earth. However, after research and debate by many influential geophysicists, it is found that this assumption is not valid, and the interaction of the three forces is too small to drift the continent. Subsequently, the explanation of continental drift theory took a turn for the better in 1929. Holmes, a supporter of this theory, found that in the crust and lower mantle, the force generated by convection between hot rocks is enough to drift the continent; That is, the hard rock material in the mantle is heated, making it less dense and moving to the surface, where it cools down, and then the cycle repeats again. Due to the lack of mechanism evidence, the theory of continental drift was rarely accepted at that time.