Abstract: 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.