Abstract: Based on random field theory, a new method named multi-arameter synthetic variables of seismic activity has been proposed. This menthod is to do the natural orthogonal function expansion on the random field function of seismic activities. And then the synthetic variables consisted of linear combinations of 4 activity parameters, e.g., the seismic strain release E, the average earthquake distance D, the mean time interval of earthquake T and the rate of earthquake occurrence N, have been studied. Through the synthetic variable analyses of the field, the seismicity anomalies before strong earthquakes have been drawed. As examples, the 1990 M7.0 earthquake occurred in Gonghe, Qinghai and the 2010 M7.1 earthquake occurred in Yushu Qinghai have been discussed. The results show that before the two magtitude 7 earthquakes, the major synthetic variables all show marked abnormalities and better correspondences with these two earthquakes. The synthetic variables of seismic activity field can show up centrally the not very obvious differences of all primitive variables, making the reflected changes as obvious as possible. We think that the synthetic variable method is a kind of potentially effective analytical method in earthquake prediction research.

Key words: Seismic activity field, Natural orthogonal function expansion, Synthetic Variables, Anomaly