How do seismologists predict the time of earthquakes?

Although it is very difficult to predict earthquakes that we usually understand and can give accurate time, place, size, occurrence probability and its range, it is not impossible to predict suboptimal. This situation is like a house. Although we don't know when it will collapse, we can use a very reliable method to determine whether it is already very dangerous and uninhabitable. The direct cause of its collapse may be a storm attack, the destruction of a mole, or even someone accidentally kicked it, but it is doomed to collapse, but there is no doubt.

Seismologists now have many methods to estimate how big an earthquake may occur in a relatively large area (such as northern Hebei Province) in a relatively long time (such as 10 year). This prediction is very useful for the planning of buildings and projects. For example, the seismic intensity zoning map of China compiled by seismologists is a kind of "suboptimal" prediction, which provides important basic data for earthquake prevention and disaster reduction.

Therefore, the same earthquake will inevitably cause greater damage in some areas. For example, during the 1976 Tangshan earthquake, the damage in Langfang area in the east of Beijing was much greater than that in other surrounding areas. The reason is that seismic waves are "focused" here. The characteristic of engineering structures is that they are usually the most "sensitive" to some types of ground motion. For example, traditional houses are particularly "afraid" of high-frequency earthquake acceleration, while overpasses are more susceptible to long-period earthquake displacement. If these laws are made clear, then in the future construction, no matter whether the earthquake prediction is successful or not, even if another big earthquake occurs, casualties and losses will be greatly reduced.

In this field, in fact, three types of experts are working together to solve key problems. The task of seismologists is to find out the probability of an earthquake in a certain area in a certain period of time and what kind of ground motion will happen in a certain place in case of an earthquake. The task of engineering seismologists is to find out how buildings or engineering structures are destroyed by these ground movements by taking seismologists' information as "input" information. The task of seismic engineers is to design better structures that can avoid these damages according to the "input" information provided by engineering seismologists.

Seismologists have set up an earthquake network to monitor seismic activity and record earthquakes. The above-mentioned earthquake risk study and strong ground motion study use the earthquake and ground motion data recorded by these seismic networks as input parameters.

However, the seismic network has another purpose, that is, after the earthquake, it quickly gives a series of information such as where the earthquake occurred and how big the magnitude was, which is very important for the government and social organizations to provide disaster relief. Usually, this information can be reported to the government within a few minutes after the earthquake. However, it is impossible to do it so quickly just by scanning reports from one area to another. In a few places where intelligent "earthquake early warning system" is adopted, the "time difference" between the early arrival seismic wave and the destructive, usually late arrival seismic wave can also be used.