Abstract: Several strong earthquakes with magnitude 7 up struck China in recent years, causing tremendous losses of life and property. The research of earthquake mechanism and seismic activity has been paid much attention now. Traditional ideas are that an earthquake occurs when it reaches the critical state of stress, after a long period of preparation. Therefore, it seems that as long as there are fully observation and research, we can determine the location, time and magnitude of an earthquake. But it is often not so optimistic under the real condition of an earthquake. In recent years, some of the observed facts show that small earthquakes in some areas can be triggered by surface waves of strong earthquakes in the distance. Laboratory studies show that small normal perturbation could change the original law of fault unstable sliding. These facts indicate that there is a certain degree of uncertainty in the occurrence of earthquakes.