1On July 28th, 976, a strong earthquake of magnitude 7.8 occurred in Tangshan City, Hebei Province, killing more than 240,000 people. One of the reasons for such a devastating earthquake is that it is strong, and the other is that it happened after three o'clock in the morning, and most people are still asleep and have no time to take refuge.
Through the statistics of a large number of historical earthquakes and present earthquakes, it is found that the distribution of seismic activity in time is uneven: many earthquakes with large magnitude occur in a period of time, which is called seismic active period.
There is still a period when there are fewer earthquakes with smaller magnitude, which is called the quiet period of seismic activity; It shows the periodicity of seismic activity. There may be many earthquakes of magnitude 7 or above in each active period, or even large earthquakes of magnitude 8 or so.