A STUDY ON THE BIDIRECTIONAL TECTONIC ACTIVITY OF THE HAICHENG EARTHQUAKE

The authors used the information about five foreshocks (Ms>3.5) and eighteen aftershocks (Ms>3.0) of the Haicheng earthquake of February 1975 to determine the fracture directions of these shock faults and fracture speeds according to the method given in reference [2]. It was found that the distribution of the fracture directions in. the Haicheng shock area are basically consistent with the N-E and N-W strikes of local fracture tectonics,with the strikes of surface fissures created by the aftershocks and with the configuration of the macroscopic isoseismic lines. Further,it was also found that not only the N-W fractures consistent with the fault strike of the major shock were strongly active,but also the N-E faults were evidently active before and after Ms 7.3 Haicheng mainshock,and that the faults striking N-E also played an important role in the seismogenic process. Therefore,the occurrence of the Haicheng earthquake seems to be the result of the activity of two nearly conjugate sets of faults.