BASIC CHARACTERISTICS OF CURRENT STRAIN FIELD IN THE EPICENTRAL AREA OF XINGTAI,BOHAI,HAICHENG AND TANGSHAN STRONG EARTHQUAKE

The principal strain parameters of current strain field in epicenter area corresponding to 4 Ms≥7 earthquakes,which was Xingtai,Bohai,Haicheng and Tangshan earthquake,are calculated with the GPS data observed in North China GPS network in past decade.The results show that the directions of principal compressive axes from GPS data in the four areas are consistent or basically consistent with that of the direction in epicenter and its adjacent area before the earthquakes.It indicates that,after the adjustment for 30 to 40 years,the local stress field in epicentral area is basically coincident with the regional strain field.At present,the principal compressive strain rate in Xingtai earthquake region is very small,with a magnitude of-0.352×10^-9/yr,and the principal tensile strain rate is relatively large,with a magnitude of 3.306×10^-9/yr,which shows that the tensile strain in this area is predominated,and the crust is in a tensional state;in Bohai earthquake region,the principal compressive strain rate is-2.593×10^-9/yr,which is a little bit smaller than the tensile strain rate(2.947×10^-9/yr);in Haicheng earthquake region,the principal compressive strain rate(1.276×10^-9/yr) is smaller than the tensile strain rate(2.940×10^-9/yr),where is dominated by tensile strain;in Tangshan region,the principal compressive strain rate is 4.539×10^-9/yr,and the tensile strain rate is 3.447×10^-9/yr,where the crust dominated by compressive strain.