THE STUDY ON SHEAR-WAVE SPLITTING OF DATONG EARTHQUAKE,NORTH CHINA

The aftershocks over 15 days after two large earthquakes, in October 1989 in Datong Area, North China, were recorded by temporal small apertural network consisted of three component digital seismographs.P-wave codas are remarkable feature of P-wavetrains and are expected to be caused by concave sediment layer and transit layer under stations.Shear wave splitting could be identified in the wider range of take-off angles for this particular near-surface structures.Polarizations of faster shear-waves have the alignment consistent with the direction of nearly horizontal regional maximum compression.The relative delays for two sequences of foreshock, mainshock and aftershock during one day are further recog-nized as being quickly variations before and after the significant events, using the method of comparing theoretical calculation and the observation records.This phenomenon could be in-terpreted in term of EDA-cracks and its temporal changes induced by changes of regional stress.The variation of relative delay in short time interval illustrated the faster response of EDA-crack of upper crust to the changes of regional stress.