Academician Chen Xiaofei: What can penetrate the entire Earth to perform CT scans? The answer is seismic waves

Recently, Chen Xiaofei, an academician of the CAS Member and director of the Department of Earth and Space Science of South University of Science and Technology, told us about the research and application results of seismology at the "Shenzhen Civic Culture Lecture Hall" under the title of "Exploring the mysteries of the Earth". The following is the main content of the speech.

Academician Chen Xiaofei

Using seismic waves to perform CT scans on the Earth

The universe contains space and time, with the four directions above and below being the universe, and the past and present being the universe. The universe is an infinitely diverse material world, composed of countless galaxies. A galaxy is a massive celestial system where a large number of stars, interstellar gas, and dust matter gather together.

People have observed about 100 billion galaxies in the universe. According to the Big Bang theory, the universe originated from a groundbreaking explosion. This big explosion occurred approximately 15 billion years ago, and time and space began from then on. After the Big Bang, the universe experienced initial rapid expansion, early rapid expansion, and later local contraction stages.

The Milky Way, where we are located, is a typical spiral galaxy composed of approximately 200 billion stars, nebulae, and interstellar matter, with a diameter of 100000 light-years. The Sun is an ordinary star in the Milky Way, located approximately 30000 light-years from the center of the galaxy. The solar system is composed of the sun, eight major planets, dwarf planets, at least 165 known satellites, and billions of small celestial bodies, with a maximum range of 1 light-year away. The mass of the sun accounts for 99.86% of the total mass of the solar system, so its gravity controls the rotation of other celestial bodies around the sun within the solar system.

Our Earth is located at the most appropriate position in the solar system, 150 million kilometers away from the Sun. There is water, air, and life on Earth, which is our blue planet and our home. Although Earth ranks fifth in volume and mass among the eight major planets, it is the most wonderful and unique.

Let's first take a look at the basic data of the Earth. The Earth is an ellipsoid with a relatively small oblateness, with an average radius of nearly 6400 kilometers. The South Pole and North Pole are slightly different. The circumference of the Earth's equator is 40075 kilometers, with a surface area of over 510 million square kilometers. The mass of the Earth has reached nearly 6 trillion tons, and 65% of the world's land is concentrated in the northern hemisphere, hence the northern hemisphere is also known as the "land hemisphere".

There is a core inside the Earth, which consists of a solid core and a liquid outer core. The radius of the core is approximately over 1200 kilometers, and the radius of the outer core is over 2000 kilometers. The outer layer of the Earth's core is the mantle and crust. If we compare the Earth to an egg, the crust is even thinner than the eggshell.

How do we know the internal structure of the Earth? The basic principle of detecting the internal structure of the Earth is the same as medical imaging, which means conducting a CT scan of the Earth. So what can penetrate the entire Earth to perform CT scans on it? The answer is seismic waves. As is well known, earthquakes can generate seismic waves, and after a major earthquake, seismic waves may spread all over the world, even circling the Earth a few times. In summary, seismic waves can propagate very far. We use the seismic waves generated by earthquakes and employ certain mathematical and physical methods to image the internal structure of the Earth. This is why we can know the structure of the Earth's interior and the approximate radius of the Earth.

In addition, we can also use computers and big data to achieve more refined imaging through seismic waves. For example, the internal structure of the Earth can be made into a three-dimensional structure, and the relative average structure and difference can be enlarged, making it easier to see the problems of the internal structure of the Earth. This is equivalent to using seismological methods to make an enhanced version of CT for the Earth.

Earthquakes are the number one natural disaster in China

The Wenchuan earthquake that occurred on May 12, 2008 was the most destructive, wide-ranging, devastating, and difficult disaster relief earthquake since the founding of New China. According to data measured by the Japan Meteorological Agency, the seismic waves of the Wenchuan earthquake orbited the Earth 6 times. The Wenchuan earthquake caused significant losses, with nearly 70000 deaths and nearly 18000 missing, resulting in direct economic losses exceeding 800 billion yuan.

Earthquakes are the natural disasters that cause the highest number of deaths in China. There are many natural disasters in our country, such as meteorological disasters, geological disasters, marine and forestry disasters. However, overall, earthquake disasters are the largest type of disaster and the largest cause of disasters.

Why would there be an earthquake? Earthquakes themselves are natural phenomena and a manifestation of Earth's activity. The power of earthquakes comes from plate movement. The Earth itself has plate movement, and the speed of plate movement is equivalent to the speed of nail growth. Although it is very slow, it will continuously accumulate energy. When the energy reaches a certain level, an earthquake will occur. The principle of earthquakes is roughly as follows: due to the relative motion of two plates in different directions, deformation will occur. Long term movement and deformation, exceeding the strength of the plate itself, will cause sudden fractures, which is called earthquakes.

Earthquakes in China are mainly distributed in the western region, indicating that plate movements in these areas are relatively active. All earthquakes occur in areas where plate movements are more active. According to the theory of plate tectonics, seismologists have concluded that the majority of earthquakes in the world are distributed in three seismic zones, with 80% of global seismic energy released in the Pacific Rim seismic zone, 15% in the Eurasian seismic zone, and 5% in the Hailing seismic zone.

When it comes to earthquakes, there are some basic concepts that are easily confused, such as magnitude and intensity. We often mention how many magnitudes of earthquake have occurred in a certain place, which actually refers to the magnitude, which is the total energy released by the earthquake. It does not change with the distance from the epicenter. For example, an earthquake with a magnitude of around 6 releases energy equivalent to the energy of a Hiroshima atomic bomb explosion, while a magnitude 7 earthquake may be 30 times greater than a magnitude 6 earthquake. Intensity refers to the degree of ground shaking caused by an earthquake, generally referring to the degree of vibration felt under your feet or the place where you stand. It usually decays as the distance from the epicenter increases. So, an earthquake has one magnitude and multiple intensities, with higher intensity in areas closer to the epicenter and lower intensity in areas farther away from the epicenter.

Overall, there are very few large earthquakes and more small earthquakes. According to statistics, there are about 200 earthquakes with a magnitude greater than 6 on Earth each year, about 18 earthquakes with a magnitude greater than 7, and an average of only 1 earthquake with a magnitude greater than 8. The situation in our country is that there are an average of 4 earthquakes with a magnitude of 6 or above each year, and an average of 20 earthquakes with a magnitude of 5 or above each year.

Now, we can use the latest digital methods to simulate the Tangshan earthquake in 1976 and the Wenchuan earthquake in 2008, restoring what kind of situation the earthquake occurred at that time, such as the propagation of seismic waves, how long the earthquake lasted, the magnitude of the earthquake, and the entire process. We can also conduct local simulations, such as simulating the local amplification of the Wenchuan earthquake. The Wenchuan earthquake occurred in the mountainous areas of western Sichuan, and the terrain of the mountains played a significant role in amplifying the earthquake, triggering many landslide phenomena.

"Simulating earthquakes to provide a basis for rescue"

Let's talk about the issue that everyone is concerned about: how to defend against earthquake disasters?

Firstly, earthquake warning. Earthquake warning and prediction are different. Prediction is like weather forecasting, telling you that an earthquake will occur next month. Earthquake warning is not such a concept. Earthquake warning is a warning given to areas where earthquakes have already occurred or are currently occurring, before the seismic waves have arrived. After an earthquake occurs, seismic waves propagate outward at a finite speed, so in distant places, seismic waves may arrive more than ten or twenty seconds later. Earthquake warning is the process of detecting the source of an earthquake and issuing a warning as soon as it occurs. People in other areas can take preventive measures in advance to reduce the losses caused by the earthquake.

Secondly, seismic design. Just like lightning rods can avoid lightning, buildings and infrastructure also need to be well designed for earthquake resistance. When encountering an earthquake, they can withstand the shaking of the earthquake, avoid collapse, and reduce casualties. Seismic design is a very important means of earthquake prevention and disaster reduction, and scientists and engineers in the fields of civil engineering in China have done a lot of work for it. Because the primary purpose of disaster prevention is to reduce casualties, followed by economic losses, reducing casualties is the most fundamental point. Through our research, we can understand the motion patterns of earthquakes, which is of great help in designing earthquake resistant buildings. In the future, if seismic design is vigorously promoted, China can greatly improve its earthquake resistance and disaster reduction in urban construction.

Thirdly, earthquake disaster prevention methods based on earthquake process simulation. In the past, once an earthquake occurred, there was no other way but to summarize the experience afterwards. But today, we can use scientific methods to actively simulate the earthquake process, see what kind of damage it will cause, and thus develop earthquake prevention and disaster reduction plans. This is the simulation of earthquake processes.

So far, earthquake prediction has been much more difficult than we imagined, and this will be a long-term task. The energy accumulation of earthquakes can range from several decades to hundreds of years, to thousands or even tens of thousands of years, and earthquakes are instantaneous events. Why is earthquake prediction so difficult? Because it is a typical multi-scale, nonlinear problem that is very complex, it is difficult to solve through calculation. However, after an earthquake occurs, we can simulate the physical process of the entire earthquake using computational methods.

Once an earthquake occurs, seismologists can use a computer to simulate the process of the earthquake within a period of more than ten minutes, calculate the propagation of seismic waves, the damage caused by the earthquake, and calculate the distribution of earthquake intensity and its impact on surface buildings. Based on this, they can conduct disaster assessments and provide contingency plans for government earthquake prevention and disaster reduction.

For example. On September 5, 2022, a 6.8 magnitude earthquake occurred in Luding, Sichuan. After the earthquake, we collaborated with Peking University and they quickly calculated the rupture process of the earthquake source. Based on earthquake data simulation, we completed the simulation and evaluation of the earthquake process in half a day. The evaluation results show that the maximum intensity of the earthquake is 9 degrees, with Luding County and Shimian County being the most severely affected, with an estimated 89 deaths and an economic loss of approximately 23.2 billion yuan. Afterwards, according to the news released on the website of the Sichuan Provincial Government on September 11th, the highest intensity of the earthquake was 9 degrees, resulting in 88 deaths. A few days later, the number of fatalities increased to 93. The report also mentioned that the counties with the most severe losses are Luding County and Shimian County, which is completely consistent with our predicted results. The relatively accurate predictions we made in a short period of time provide scientific basis for the government to carry out emergency rescue.

So, what can we do before an earthquake occurs? For example, the Guangdong Hong Kong Macao Greater Bay Area is not located in the seismic zone and rarely experiences major earthquakes, but this does not mean that earthquakes will never occur here. About 100 years ago, earthquakes with a magnitude of 7 or higher occurred in areas such as Jieyang in eastern Guangdong; Hundreds of years ago, there were also major earthquakes in Haikou, Hainan and Nan'ao, Guangdong. So, we must actively do a good job in earthquake defense. How to prevent it? Earthquake simulation methods can also be used. By simulating the entire process of the coastal seismic fault zone in the Guangdong Hong Kong Macao Greater Bay Area, we can know where the fault is and what its current shape is. Based on our understanding of its physical evolution process, we can simulate the damage caused by an earthquake once the fault occurs. On the basis of this pre evaluation, we will further take preventive measures. We call this method active earthquake assessment, which means putting work ahead before an earthquake occurs.

In addition, people have been paying more attention to the issue of recent tsunamis. The global tsunami zones are roughly consistent with earthquake zones. The basic principle of a tsunami is relatively simple. Due to the rapid rise and fall of the seabed terrain, a plate in a subduction zone is dragged downwards for a long time and bounces back, pushing the water body to cause the propagation of huge waves. The propagation of water waves is called tsunamis. The simulation of a tsunami is relatively simple compared to earthquake simulation, and we can also simulate the entire process of a tsunami and evaluate its destructive power.

In terms of simulating the entire process of a tsunami, both domestically and internationally, it has become relatively mature. In terms of simulating the entire process of earthquakes, our team currently has advanced technology and is in a leading position internationally.

"Seismology is not only used to defend against earthquake disasters"

The so-called seismology is a comprehensive science that studies the occurrence laws of earthquakes in solid Earth media, the propagation laws of seismic waves, and the macroscopic consequences of earthquakes. This science was initially developed by humans to avoid or defend against earthquake disasters. Early work focused on describing earthquake damage and studying the geographical distribution of earthquakes. It was not until the 20th century that scientists discovered that seismic waves emitted by earthquakes were actually the most effective tool for revealing underground conditions.

In terms of the utilization of seismic waves, seismology has developed to a relatively mature stage. Today, seismology not only serves the defense of earthquake disasters, but is also an extremely important applied science in economic construction. Seismology has become an important foundation for deep ground exploration and deep space exploration, and has also been widely applied in resource exploration.

Let's first talk about deep ground exploration. The seismic waves emitted by seismic sources can pass through any depth of the Earth and return to the ground, bringing information about the interior of the Earth. Especially, the propagation speed of seismic waves at various depths inside the Earth is related to the density and elasticity of the medium in that area. Therefore, seismic observation is the most basic method for studying the interior of the Earth. Today, the internal structure of the Earth has become common knowledge and textbook knowledge, all of which are obtained through seismological methods.

Deep space exploration includes the exploration of the interior of the moon, and can also be done using seismological methods. Scientists have placed many seismographs on the moon for a long time. As early as Apollo 11 landed on the moon, astronauts placed five seismometers on the moon. Why use a seismograph? I just want to study the internal structure of the moon by monitoring earthquakes on the moon. Our country's future Mars launches also have plans to place seismometers, with the aim of understanding the differences between the internal structure of Mars and the internal structure of Earth. So, seismological methods are also very important in deep space exploration.

In addition, the detection of oil and gas resources also relies on seismological methods. At present, the method of seismology is the most important method for exploring oil and gas resources. It is also imaged through seismic waves, but only in a smaller area. In this process, the seismic source is not a natural earthquake, but is artificially triggered. By manually controlling the seismic source, high accuracy can be achieved in observing the time of seismic wave propagation, allowing for more precise exploration of oil and gas resources. Similarly, seismic methods can be applied to geothermal resource exploration, urban underground space exploration, and safety monitoring.

Seismology can also be applied to the exploration of mineral resources. In addition to magnetic and gravity exploration methods, seismological methods can also locate underground ore veins and mine locations, because seismological methods have the characteristic of high accuracy. After more than 70 years of mining since the founding of the People's Republic of China, China's shallow mineral resources have been decreasing and depleting year by year. The mining of mineral resources, especially metallic mineral resources, is currently in a stage of comprehensive promotion towards deep areas. If the depth of solid mineral exploration in China reaches 2000 meters, the country's resource reserves will double on the current basis. Fine imaging of underground structures based on various geophysical observation data can provide technical support for the development and utilization of deep Earth resources.

Human beings must coexist harmoniously with nature

The Earth has nurtured humanity with a broad mind and precious resources. However, due to endless demands from the Earth and the inability to treat the Earth well, humanity has faced unprecedented retaliation. Ecological crisis, resource depletion, environmental pollution, and frequent disasters have become urgent problems facing the world.

In recent years, natural disasters that occur on Earth have become increasingly frequent, such as volcanic eruptions, earthquakes, landslides, floods, etc. Some of them are normal phenomena during the evolution of the Earth and have little to do with human activities. However, there are also some disasters caused by human overexploitation or inappropriate activities, such as landslides, mudslides, floods, etc., which are closely related to human long-term damage to the environment. If we can pay attention to these issues in the future, we can at least reduce the occurrence of many disasters.

Finally, I would like to say that the Earth is the home of humanity, and the future destiny of humanity is in our own hands. To have a bright future, humanity must manage its relationship with the Earth and the biosphere, coexist harmoniously with nature, and develop in harmony.