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.