A METHOD FOR TESTING CRUSTAL STRESS MEASUREMENTS AND THE OBSERVATIONS BEFORE TWO EARTHQUAKES

The observations from two crustal stress observatories at Qongzhong (Guangdong Province) and Yongjing (Gansu Province) show that the relatively stable regional stress field might have been disturbed during the late preseismic stages before the Yaxian earthquake (M = 4.1) of Jan. 25,1982 in Guangdong Province and the Wuwei earthquake (M = 5.5) of Jan. 6,1984 in Gansu Province. The characteristics of change of the additional stress state in the observation area are: 1. The curves of the major horizontal principal stress show nonlinear changes; 2. A systematic rotation of the principal directions of additional stress takes place; 3. The tendency of change of the stress recovers with considerable speed after the seismic events,but the values of additional stress has decreased. The College of Economic Management,Beijing Union University; Institute of Earth's Crust Dynamics,State Seismological Bureau; Seismological Bureau of Ningxia Huizu Autonomous Region; Seismological Bureau of Guangdong Province; Lanzhou Seismological Institute,State Seismological Bureau.