波速异常的进一步研究和问题讨论(一)

本文首先清理了近年来我国一些中强地震前后波速比异常概况。在21个震例中,有7次震前提出过不同程度的预报意见,12次是震后总结发现异常的,2次震后总结未发现异常。其次,讨论了利用天然地震资料探索地震波速度异常的几个问题:(1)现有较密的区域电信传输台网需提高观测精度,使得较大的前兆异常量不致于被误差所掩盖;(2)使用多台和达法探索波速比异常时必须注意其方向性效应,而且,若能把单台波速比法与多台波速比法平行对照使用,效果可能更好些;(3)要注意远台与近台出现异常的早晚与反应的程度;(4)高值点反映地震前兆的信息较小,而低值却可能携带某些前兆信息;(5)不会有固定不变的异常区,应该考虑到异常区的变迁。

THE FURTHER STUDY AND DISCUSSION ON THE ANOMALIES OF SEISMIC WAVE VELOCITY

It has been going through a repeated process from hot to cold to apply seismic velocity ratio to predict earthquake.It is attentive how to treat actually it.In this paper we cleaned up seismic velocity ratio method and gave preliminary conclusion.First,we summed up briefly general situation of seismic velocity ratio before and after some moderate and strong earthquakes in China in recent years.There are seven earthquakes which were predicted,twelve earthquakes which found anomalies after earthquakes and two,not any anomalies even after.Secondly,several problems of anomalous seismic velocity ratio have been discussed by means of natural earthquakes data.The paper considers that recent denser regional transmission networks help increase favourable condition in observational precision and the error may not conceal the anomaly.The examples predicted before earthquakes have been presented,especially,two variation diagrams of seismic velocity ratio in two earthquakes which occurred at same situation during two-and half years are quite similar.We considered that the thoughts about seismic velocity ratio anomaly ought not entirely to be abandoned.We must note its directional effect,i,e,anisotropic effect when the Wadati method is used.In fact,it is not suitable when the Wadati diagram is calculated with data,the epicentral distances of which are more different,so that if the method of the single station and of many stations is applied with comparision,the effect may be better.The anomalous time appeared and reactive level amongfar and near stations,higher and lower points,variable anomalous regions,must be noted.Finally,the variations of seismic velocity by blasting data before and after Haicheng M 7.3 earthquake on Feb.4.1975 are studied.The tendency of variation of imaginary wave velocity in Dandong station from 1972 to 1980 is as same as those of natural earthquakes.The level of average seismic velocity ratio after the main event is much lower than that before.The tendency of variation among Yingkou station,Jinzhou station and many other stations are analogous.(S-P) residuum curve of Dandong station shows that the theoretical value before shock is much lower and higher after shock.The relative variation of P,S wave in|Yingkou station is falling Jrom higher value before shock and the level is lower after shock,and the variation of S wave is large.We found that the tendency of variation of P-travel time residue between Yingkou and Jinzhou stations is oppsite.From the foregoing data and analysis,we don''t think that it is suitable to refute velocity ratio lightly.If we improve the condition of observation,strengthen study of its physical substance,especially,and persist in precursor practice,sum up and study continuosly,a new breakthrough is prossibly made.