Precursory specialties of apparent stresses in Yunnan earthquake series

On the basis of the assumption that ω2 model accords with source displacement spectra,we have obtained the mathematical expressions for calculating apparent stresses of moderate-small shocks from low-frequency flat level and corner frequency. By using digital seismic records,apparent stress values are calculated for 823 moder-ate-small shocks of 4 earthquake series in Yunnan area following corrections for instrument response,propagation influence and site effect. The results show that for the 4 earthquake series in Yunnan area,apparent stress hints precursory information,which means that if a moderate-small shock occurs with apparent stress larger than 1 MPa in an earthquake series,a moderate-strong earthquake will occur afterwards; and if there is not moderate-small shock with apparent stress larger than 1 MPa after a moderate-strong event in an earthquake series,strong after-shock will not occur. The research also indicates that the average apparent stress value is 0.8 MPa in Yunnan area,therefore,apparent stress is not obviously related to seismic magnitude.