Can earthquakes be predicted?

　　According to statistics, there are more than 5 million earthquakes every year in the world, with an average of more than 10,000 earthquakes every day. Most earthquakes are so small that humans cannot feel them. There are about one or twenty times a year that can really cause serious harm to humans. So, how do earthquakes occur? Can it be predicted?
　　Recently, the 7.8-magnitude earthquake that occurred in Turkey and Syria has attracted worldwide attention. Experts said that the place where the earthquake occurred was at the junction of four major fault zones, and the power of about 130 atomic bombs exploded for 43 seconds.
　　 So, how do earthquakes happen? Can it be predicted?
　　Can the energy of an earthquake be calculated?
　　 According to statistics, there are more than 5 million earthquakes every year in the world, with an average of more than 10,000 earthquakes every day. Most earthquakes are so small that humans cannot feel them. There are about one or twenty times a year that can really cause serious harm to humans.
　　 So how should we describe the size of an earthquake? In 1935, Professors Richter and Gutenberg of the California Institute of Technology formulated the famous Richter Scale, which characterizes the scale of earthquakes by the maximum amplitude of seismic waves. Their setting for a magnitude 0 earthquake is that the earthquake causes the seismograph 100 kilometers away from the epicenter to produce a maximum amplitude of 1 micron, and other earthquakes are based on this to determine the magnitude. When the maximum amplitude recorded by the seismograph 100 kilometers away from the epicenter is 10 microns, the magnitude is 1; when it is 100 microns, the magnitude is 2, and so on. Most of the energy released during an earthquake stays in the source area in the form of mechanical energy (such as rock fracture) and thermal energy (such as frictional heat generation), and the other part spreads in all directions in the form of seismic waves, which is called seismic wave radiation energy. Part of the energy caused damage to the surface.
　　 The value of seismic wave radiated energy is quite astonishing. In 1960, Chile experienced the highest-magnitude earthquake ever recorded by instruments, reaching a magnitude of 9.5, and its energy was equivalent to more than 170,000 Hiroshima atomic bombs.
　　 In addition to magnitude, you may often hear a word called intensity. Intensity indicates the degree of damage after an earthquake in an area. In my country, it is divided into 12 degrees. The higher the degree, the more serious the damage caused by the earthquake.
　　 There is already a magnitude, why set another intensity? This is because under the same magnitude, different regions have different focal depths, different epicentral distances, different building quality, and often different degrees of damage. Therefore, we need to set an indicator to reflect this difference, which is intensity. In order to assess the intensity accurately and standardizedly, my country has promulgated the national standard “China Seismic Intensity Scale” (GB/T 17742-2020).
　　What attempts have been made to predict earthquakes
　　 Human beings have long tried to predict earthquakes, the most famous of which is the Houfeng seismograph made by Zhang Heng, an astronomer in the Eastern Han Dynasty. When an earthquake occurs in a certain direction, due to the disturbance of the seismic wave, the copper beads contained in the dragon’s mouth hanging in that direction will fall. Strictly speaking, the Houfeng Seismograph is a kind of seismic detector.
　　 Modern earthquake prediction did not take off until the 1960s. In 1975, China successfully predicted a magnitude 7.3 earthquake in Haicheng, Liaoning, avoiding a large number of casualties and property losses. But the good times didn’t last long. In 1976, China failed to predict the Tangshan earthquake. It was also from this period that many methods of earthquake prediction were overthrown one after another. To this day, the prediction of the Haicheng earthquake has become the only successful case of earthquake prediction recognized by the United Nations.
　　 Earthquake prediction is divided into long-term, medium-term, short-term and imminent earthquake prediction. For disaster prevention and risk avoidance, short-term and imminent earthquake prediction are more valuable. However, it is very difficult to achieve it. We can only determine the scope of the fault zone based on research, so as to frame the area with high earthquake risk, but we don’t know when, where, and how large an earthquake will occur.
　　 Researchers have tried a variety of methods to make predictions. For example, the seismic light method can be understood as the sparks generated during the frictional discharge process of rock formations, just as lightning is the discharge phenomenon between clouds. If seismic light is produced, it means that the formation friction is strong, and an earthquake is likely to occur, but more often the earthquake has already occurred. So far, there is no method that can effectively predict earthquakes.
　　Can’t predict doesn’t mean can’t warn
　　 Although human beings have repeatedly hit a wall in earthquake prediction, they are still remarkable in earthquake early warning. The principle of early warning is related to the nature of seismic waves. Seismic waves are divided into P waves (waves that arrive first) and S waves (waves that arrive later). The propagation speed of P wave is about 5.5 to 7 km/s, and that of S wave is about 3.2 to 4 km/s. Because P waves travel faster, they can reach the surface earlier, and earthquake warnings were born.
　　 For example, when a place A receives a P wave, it can immediately issue an earthquake warning to the residents, and at the same time notify the B place immediately through electromagnetic signals (television, radio, SMS, WeChat, etc.). Since the propagation speed of electromagnetic waves is much higher than that of seismic waves, it is possible for site B to receive a message and issue an early warning before the arrival of seismic waves.
　　 Although the early warning is not as powerful as the prediction, its role should not be underestimated. Studies have shown that when the earthquake warning time is 3 seconds, it can reduce casualties by 14%; when it is 10 seconds, it can reduce casualties by 39%; when it is 20 seconds, it can reduce casualties by 63%.
　　 Although we are still unable to predict earthquakes today, with the development of science and technology, we believe that the secrets of earthquakes will eventually be revealed, making earthquake prediction a reality.