ANOMALIES OF SHORT-PERIOD GEOMAGNETIC VARIATIONS AND DISTRlBUTION OF HIGH CONDUCTIVITY LAYER IN THE BOHAI SEA AREA

In this paper, analyses of short-period geomagnetic events such as sudden commencementand geomagnetic bay, together with some quiet day, disturbed day and geoinagnetic stormvariations, recorded at 25 repeat stations in the Bohai Sea area during the years 1973--1979are presented. It is found that there are anomalies of short-period variations in this areaincluding the Tangshan earthquake region, and that local anomalies with periods rangingfrom 5 minutes to 2 hours along the Changli- Tangshan-Ninghe-Baodi belt (positive △Z ) andat Yantai (negative △Z) are most obvious. By applying the numerical theory of three-dimensional electromagnetic induction, an attempt is made to fit these anomalies to the struc-ture of high electrical conductivity in the upper mantle. The result brings to light an upliftof high conductivity layer, at the northern margin of which the Tangshan earthquake islocated. The additional thermal stress due to such an uplift calculated to amount to a thou-sand of bars may be an important possible source of generating force for the Tangshan earth-quake.     

渤海地区地磁短周期变化异常、上地幔高导层的分布及其与唐山地震的关系

本文分析了渤海周围25个地磁台站1973—1979年急始、湾扰等短周期事件,以及静日、抗日和磁暴的频谱成分,发现包括唐山震区在内的渤海地区存在地磁短周期变化异常。北部的昌黎、宁河、宝坻一带异常最强,垂直分量的变化△Z为正,南部的烟台附近异常最强,△Z为负,周期为5分钟至2小时的异常变化最为明显。应用三维电磁感应的数值理论,拟合了与这种短周期地磁异常变化相应的上地幔高导层电性结构,发现渤海地区有一上地幔高导层的局部隆起,唐山地震位于隆起部位的北侧边缘。与隆起构造相应的附加热应力,其量级高达千巴,很可能是唐山地震的重要动力来源。