EVALUATION OF THE DIRECTION OF TECTONIC STRESS IN NORTH CHINA FROM RECORDED DATA OF A LARGE NUMBER OF SMALL EARTHQUAKES

Based on the first motion polarization records of earthquakes with magnitude less than 5 obtained from local Seismological stations in North China during the period of 1966 -1978, the directions of the principal axes of crustal tectonic stress in 13 subregions are inferred by computing the composite nodal plane solutions.A numerical computation test shows the possibility of inferring the principal axes of tectonic stress from the P, B, T axis of earthquakes with randomly oriented fault planes. The advantage of using microearthquake polarity data over using the data from large earthquakes in estimating the direction of tectonic stress is argued. By use of the trial and error method, the 3-dimensional parameter space of all possible average nodal plane solutions is tested with the scan step of 555, the computation time is saved by improving the program.