PRECURSORY VELOCITY VARIATIONS BY ARTIFICIAL WAVE SOURCES-TEMPORAL CHANGES OF APPARENT VELOCITY-RATIO PRIOR TO THE LIYANG JIANGSU EARTHQUAKE OF JULY 9, 1979

Kelation between apparent velocity-ratio =ts11-ts/tp11-tpand real velocity-ratio of thecrustal medium has been discussed in the present paper. Theoretical values of the apparent velocity-ratio have been calculated supposing that the crust consists of two horizontal layers and that within the upper layer of the crust there exists a zone of anomalous seismic velocity, upper and lower boundaries of which are horizontal. The results obtained indicate that in certain cases the anomaly fails to be detected by horizontally propagating direct waves (as, for example, direct waves do not pass through the anomalous zone, or the horizontal extent of the anomalous zone is rather small and so on). However, on the basis of variations of values it is still possible to be detected if the reflected waves pass through the anomalous zone.Seismic data recorded by seismic station at Liyang, Jiangsu from explosions at Maanshan Iron Mine have been studied using the above mentioned method. Negative anomaly of values is detected prior to the Liyang, Jiangsu earthquake (July 9, 1979, Ms =6.0). Average amplitude and duration of the anomaly are 5% and 15-17 months respectively. The anomaly could well be explained, if the path length of the direct waves within the anomalous zone is rather small, or they do not pass through the anomalous zone at all.The present paper indicates that by means of artificial wave sources in detecting precursory variations of seismic wave velocities the results might be improved, if reflected waves are used.