ON THE ANOMALOUS VARIATIONS OF VELOCITY RATIO OF SEISMIC WAVES IN THE BEIJING REGION

The data of more than one thousand earthquakes occurred within the Beijing seismic station network since 1967 have been analysed, using the Wadati method for calculating the velocity ratio of seismic waves (V_p/V_s) for each earthquake. The results show that:(1) For the time period mentioned above, before 13 earthquakes of magnitude M_L≥3.5 within the seismic network of Beijing, the velocity ratio values obviously drop around the epicentral area before the main shocks and recover after them, (2) Between the magnitudes of the earthquakes and the areas of anomalous values of V_p/V_s, there appears a linear logarithmic relation, while the lasting period of anomalous velocity values does not have such a relation with the magnitudes. (3) A waiting period appears following the conclusion of the velocity ratio anomalies till the outbreak of each of the main shocks. Its duration does not have any clear connection with the velocity ratio anomalies. It seems to indicate the very complicated nature of the earthquake mechanism and difficulty involved in using anomalous velocity ratio variation as a precursory phenomenon for the prediction of earthquakes.