利用人工波源探测震前波速变化——以溧阳六级地震为例

本文讨论了视波速比=ts11-ts/tp11-tp与介质真实波速比的关系。假设地壳为水平双层结构,上层地壳内包含一个水平延伸的波速异常区.计算了的理论值。结果表明,对于以往用水平传播的直达波测不出异常的某些情况(例如直达波没有穿过异常区;异常区的水平尺度很小等),只要反射波穿过异常区,根据值的变化,仍有可能把异常较为有效地检测出来。 应用这一方法研究了溧阳地震台记录的马鞍山人工爆破资料,发现1979年7月9日溧阳6.0级地震前值呈现负异常,平均幅度约为5%,异常时间为15——17个月。为了解释这一异常,直达波在异常区内的路程应当很短,或者根本不穿过异常区。 本文结果表明,主要使用反射波,可能会改善用人工波源方法观测震前波速变化的前景。 

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.