A study of ellipse anomaly of ground tilt vector before the tangshan earthquake

This paper has studied the ground tilt data of Beijing region, Taiyuan and Zhangjiakou stations in 5years before and after the Tangshan Ms 7.8 earthquake in 1976, and the ground tilt data of ShigUai and Huhehot seismic stations in 7 years before an fafter the Anlunbulugesumu Ms 5.5 earthquakes in 1991.It is found that the distridution of the ellipse long axis of the annual variational vector in right angle coordinate is concentrated and stable in normal years, that is, passing initial POint and distributing in two quadrants, it will divert with large amplitude within three years before strong earthquake. These anomalies group with quasi-synchronization cha racteristics in a de fined region c an be u sed as a mid term precusory anomaly for moderately strong earthquakes.