STRESS DROP OF ZHANGBEI EARTHQUAKE AND ITS LARGER AFTER SHOCK IN 1998

Using the method to estimate stress drop with the semiperiod of seismic wave's first motion,we calculated the stress drop of 25 larger earthquakes in Zhangbei earthquake area,and analyzed the stress drop of after shocks.The stress drop of the M L3.5 earthquake occurred on February 23 was high to 26.2×10⁵ Pa.This anomaly can be regarded as the precursor of March 11's Ms5.6 earthquake.