近地震波速异常与地震预报

利用较大地震前后的地震波特性变化探索地震预报的途径,是当前地震战线上的一个重要研究课题。地震波的运动学特性及动力学特性在较大地震前后均可能发生一定的变化,本文主要探索地震波运动学特性的变化——波速异常。

VELOCITY ANOMALIES OF SEISMIC WAVES FROM NEAR EARTHQUAKES AND EARTHQUAKE PREDICTION

A Simple method for the determination of the locations of near earthquake hypo-centres using data from many seismic stations is given in addition to several methods of finding anomalies of seismic wave velocities. The chief attempt is to determine the variation of the mean wave velocity and velocity ratio in the hypocentral region before a large earthquake,and to draw maps of wave-front and travel-time variations. The anomalies of Vp and Vs and their ratio before and after the Sichi （西吉） earthquake （Dec. 3,1970,M=5.7） are investigated in detail.Some preliminary results are obtained. Among them are: （1） The mean velocitiesVp0 Vs0 of P and S-waves and the ratio Vp0/Vs0 obviously diminish during many months be-fore the forthcoming large earthquakes （M>5） in a zone around their hypocenters where the stress is concentrated; （2） On the-background of the reduced velocity and velocity ratio,the anomalous area shows a spindle-shape,approximately corresponds to the distribution of the directions of the initial motions of the P-waves of the main earthquake; （3） During the period of anomalous velocity before the large earthquakesthe mean ratio Vp0/Vs0 within the Earth's crust and in the upper mantle of the wholeregion is also anomalous.The physical implications of the velocity anomalies before and after the large earthquakes and the changes within the earth's crust are discussed.Finally certain preliminary methods of using velocity anomalies to predict earthquakes are suggested.