滇西重力变化的二维图象及其与5级（M_s）以上地震的关系

TWO-DIMENSION PICTURES OF GRAVITY CHANGE IN WESTERN YUNNAN AND THEIR RELATIONS TO THE EARTHQUAKES M_s>5.0

滇西实验场布设了以区域网为主包括绝对网、微网和垂直梯度测线在内的综合重力网，总测点数达１５０多个。自１９８５年以来定期进行了复测，获得了大量高精度测量资料。研究了根据空间域内离散的观测数据构制连续变化图象的方法，绘制了２３幅重力二维变化图。这些图件用于构造活动研究比以往习用的段差和点值变化图的优点是：能够消除测量粗差，滤去浅表局部干扰和易于从整个空间域上判断变化特征。研究了重力二维变化图象与Ｍｓ＞５．０级地震的关系。在观测期间。测区及其周边共发生了９次Ｍｓ＞５．０级地震，地震发生在正负异常区变换带的零线附近；震前总有一个１２０ｎｍｓ－２以上的正异常区出现，震级越大异常区的幅值也越大；震前重力场发生变化的时间长度大约３年，完整规律是先上升后下降，在下降过程中发震，从转折到发震的时间在一年之内。

Based on the area network,the synthetic gravity network including absolute network,micro-network and vertical gradient lines spreads in western Yunnan.The total observation stations are more than 150.Since 1985.the gravity network has been repeatedly measured at regular intervals and a great quantity of observational data was obtained.The method of drawing continuative change picture from discrete observation data in the area is studied, 23items of two-dimension pictures of gravity change are given.This method is in advance of using ordinary method including section difference and point values for studying tectonic activity.It can remove the appreciable error from observational data,filter away the disturbances of shallow and part area and be easy to diagnose the change characters in the total area.Then.the relations between the pictures and 9 earthquakes Ms>5.0 which occurred in and around the area during the measurement period are studied too.The earthquakes occurred nearby the zero lines of gravity anomalies.There is a positive anomaly area with above 120 nms-2 gravity change before the earthquakes,and the bigger the earthquake magnitude the more the gravity anomaly.Before earthquakes,the gravity change lasted about 3 years,the gravity was up at first.then was down and the earthquake occured.It is less than a year from the turn of gravity change to the earthquake.