ANOMALOUS VARIATIONS OF SEISMIC VELOCITY RATIO BEFORE THE SONG-PAN(松潘)—PINGWU(平武) EARTHQUAKE (M=7.2) OF 1976

Using the observational data of travel times of the direct seismic waves, recorded at the networks of seismic stations in Sichuan (四川) and Gansu (甘肃) Provinces, the preliminary studies of the velocity ratio anomaly before the Songpan-Pingwu earthquake in Sichuan Province of August, 1976 were made. The obtained results indicate that bofore this earthquake the anomalous variation of the velocity ratio was quite obvious, the durations of the Vp/Vs anomaly and its recovery were rather short. The anomalous region of very large extent was located to the south of the epicentral area of the main shock within Sichuan Province. Certain features of the V_p/V_s anomaly seem to show the connection and also difference between this earthquake and the Songpan-Nanping (M_s=6.5) of August, 1973. In consideration of the characteristics of the velocity ratio anomaly observed in this region, the related problems of its recovery and the superposition of the anomalous regions have been briefly discussed.