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recently, a series of earthquakes in hefei have attracted attention. the 4.7 magnitude earthquake on september 18th was the strongest earthquake many people have ever felt in their lives. some people are worried that there will be bigger earthquakes in the future. in my previous video, i also analyzed it from a geographical perspective. generally speaking, there is no need to worry. when the earthquake occurred, many people received earthquake warning information. some people said that they received earthquake warnings a few seconds before the earthquake, while others said that they received warning information after the earthquake. so how does the earthquake warning system work? why is there a difference in the order of the information received? if a major earthquake occurs, how much time can we buy to escape? today, let's talk briefly about related topics.

earthquake, as the name implies, is the vibration of the earth, which is mainly caused by the movement of the earth's crust, but can also be caused by volcanic movement, meteorite impact and human factors. when an earthquake occurs, seismic waves are generated. every earthquake will definitely produce solid waves, which are mainly longitudinal waves and transverse waves, or p waves and s waves. p waves are the fastest type of seismic waves, about 5-8 kilometers per second, and s waves are about 3-4.5 kilometers per second. due to the different propagation methods, the main damage in an earthquake comes from s waves. the propagation speed of electromagnetic waves is the speed of light, which is 300,000 kilometers per second. the early warning system works based on the time difference of wave speeds.

when an earthquake occurs, the seismic network first detects the p wave, and based on this, calculates the epicenter location and earthquake intensity. within seconds, the system will quickly estimate the impact range and degree of damage, that is, decide how large a range of people to issue earthquake warnings to. before the destructive s wave reaches the warning area, the electromagnetic wave, that is, the mobile phone that each of us uses, or the tv at home and other devices, will send us an alarm to buy some time to escape.

from the working principle, we can see that the closer to the epicenter, the shorter the time, and even as mentioned at the beginning, the warning may arrive later than the earthquake. in simple terms, if you are within 10 kilometers from the epicenter, the warning system will have little effect on ordinary people. it takes only about 2 seconds for s waves to propagate 10 kilometers, and more than 1 second for p waves. there is only a time difference of more than 1 second in between, and it may take 2 seconds for the system to make a judgment and send a signal. if you are 50 kilometers away from the epicenter, you will have a few seconds to 14 seconds to escape, which is completely different.

in general, earthquake early warning systems are still very important for buying time to escape, and the farther away from the epicenter, the more useful they are. however, if you encounter a magnitude 7 or above earthquake and are within 10 kilometers of the epicenter, the effect is very limited.