Abstract: Based on natural orthogonal function expansion method and taking seismic strain as variable, we studied the seismic strain before the earthquakes above magnitude 7 in China since 1980, and extracted temporal and spatial anomalies of strain field before the earthquakes. The results show that the time factor before earthquakes above magnitude 7 show jump-up or jump-down changes, and under normal circumstances, at least 3 of the the first 4 strain fields show abnormal changes with multi component distribution characteristics, and the earliest time of abnormality is about 3 years before the earthquake, a few occurred 1~6 months ago. Contrasting the time factor of strain field with seismic energy within the time factor, abnormal shape similarity, the consistency of time in each grid cell (0.5°×0.5°) , we can find the exact location of the anomalous seismic strain space. The spatial anomalies before most major earthquakes are distributed around the main earthquake. The spatial anomaly of a few large earthquakes is far away from the main earthquake epicenter. The reason for this phenomenon is that the short and short term or impending earthquake groups and significant earthquakes occur in the cell grid with different distances from the main earthquake before the major earthquake. Finally, the reasonableness of calculating the space-time anomaly of strain field to delete the aftershock is discussed.

Key words: Seismic strain field, Natural orthogonal function expansion method, Spatiotemporal anomaly, Chinese mainland