南汀河西支断裂南定河段的大地电磁测深及意义

Magnetotelluric sounding of the Nanding River segment of the west branch of the Nanting River fault and its significance

南汀河西支断裂是中国滇西南地区主要的NE向发震构造之一,在云县盆地和孟定盆地也发现有古地震遗迹.基于野外地质调查,采用EH4大地电磁测深的方法,获得了该断裂南定河段的构造图像.探测结果表明:南汀河西支断裂南定河段是由2条平行的NE向断层组成,剖面上具有阶梯状的组合特征,为逆—左旋走滑断层,2条断层均未切错上覆光释光(OSL)测年为距今(52.1±6.54)ka的第四纪地层,属于晚更新世早期的活动断层.考虑到历史上邻近南定河断裂的东侧孟定第四纪盆地段和西侧缅甸滚弄第四纪盆地段均发生过7级以上大震,南定河断裂属于晚第四纪中期以来的地震地表未破裂段,其未来的地震危险性值得引起高度关注.经构造类比,若南定河断裂全段破裂,估计震级在7级左右.在山区,采用大地电磁测深技术并结合野外地质勘察对活动断裂定位,也是一种行之有效的方法.

The western branch of the Nanting River fault is one of the main NE striking seismogenic struc-tures in Southwest Yunnan,China.Paleoseismic traces have also been found in the Yunxian Basin and the Meng-ding Basin.Based on the field geological survey,this paper uses the EH4 magnetotelluric sounding method to ob-tain the structural image of the Nanding River section of the fault.The detection results show that the Nanding River reach of the west branch of the Nanting River fault is composed of two parallel NE-trending faults.The pro-file is characterized by a stepped combination.It is a reverse-sinistral strike-slip fault.Neither of the two faults is dislocated by the Quaternary strata(52.1±6.54)ka from today,which is an active fault in the early late Pleis-tocene.In view of the large earthquakes of magnitude 7 and above in the eastern Mengding Quaternary Basin sec-tion and the western Gunlong Quaternary Basin section of Myanmar adjacent to the Nanding River fault in histo-ry,the Nanding River fault belongs to the seismic surface unbroken section since the middle of the late Quaterna-ry,and its future seismic risk deserves high attention.Through structural analogy,if the Nandinghe fault breaks,the estimated magnitude is about 7.In mountainous areas,it is also an effective method to locate active faults by using magnetotelluric sounding technology combined with field geological survey.