Study on the evolution characteristics of seismicity pattern before strong earthquakes

75 earthquakes with M≥6.5 happened in China's mainland since 1966 are divided into 52 groups.Based on the spatio-temporal scaning of their seismicity patterns with M≥4 and M≥5 respectively,we chose typical patterns and analyzed their characteristics of evolution,consistency and difference according to North China-Northeast China,Southwest China and Xinjiang tectonic regions.It comes to a conclusion that the anomalous pattern transition of two stage happened before strong earthquakes is a common phenomenon.The anomalous patterns with M≥4 after the seismicity patterns with M≥5 may be a sign of transition from the medium term to the short term during preparation of strong earthquakes.