Abstract: In the paper, seismic frequency are taken as the indications of the seismicity before strong earthquakes above M7, which are calculated by means of the natural orthogonal function expansion method, and the abnormal change of time and space factor are extracted before the strong earthquakes. Results show that, before the strong earthquake, the time factor has a high or low value anomalies exceeding mean square error, most abnormal distributes in the first four typical frequency fields, showing the characteristics of multi-components. Amorry of them, the first typical field anomalies is most contribution (generally 40%~60%). Before strong earthquakes, mose cases have long-term anomalies in 5~8 years, some cases have medium-term anomalies in 1~2 years, and a few cases have short-term or imminent anomalies. The area with high gradient vortex in frequency contours in the abnormal region, in which the area with isoline value greater than 0.9 and overlaping with activce tectonics indicates the possible position for future mainshock. In addition, the comparison between the frequency field and the 3 months frequency by the traditional seismic method shows that the time factor anomaly of the frequency field has its own unique advantages and disadvantages, the differences of these methods have been discussed.

Key words: Strong earthquakes above M7, The seismic frequency field, Time factor, Contour map