Research on the seismic strain field before strong earthquakes above M6 in Chinese mainland

In this paper, seismic strain field is considered as a variable of seismic activity. The strain field is calculated before the earthquake above M6 in the mainland of China and the abnormal change of time factor before the earthquake is extracted by means of the natural orthogonal function expansion method. The results show that the time factor before a large earthquake will change suddenly, jump up or down on a stationary background. At least 3 typical fields will show abnormal changes in the first 4 typical fields of seismic strain field before the earthquake, having the characteristics of multi-component display of anomalies. The earliest time of abnormal occurrence is about 3 years before the earthquake. Moreover, there are some short-term time factor anomalies before some earthquakes. The results of case studies show that the time factor anomaly extracted by strain field is more useful than seismic energy field in analyzing the anomaly of seismic activity. The difference between strain field and energy field is also discussed.