1950年西藏察隅M8.6强震序列震源参数复核

Redetermination of the source parameters of the Zayü,Tibet M8.6 earthquake sequence in 1950

1950年8月15日西藏察隅发生M8.6巨大地震.我们利用收集到的全球239个台站的P波资料,利用我国国家测震台网的常规定位方法和模型,对察隅M8.6强震序列进行了重新定位,并在此基础上重新计算了震源机制解.重定位后的结果表明,察隅M8.6强震序列显示出不同时段的震中分区分布特征:第1阶段是前震,1950年2月23日在墨脱北部雅鲁藏布江大拐弯的顶部发生;第2阶段是1950年8月15日─1950年8月18日,发生主震和之后3天内的余震,都分布在察隅附近,并且这些震中呈北西条带分布;第3阶段的余震是1950年8月22日─1950年9月13日,它们扩展到南部的印度和缅甸地区;第4阶段的余震是1950年9月30日─1951年4月15日,发生在西部的墨脱、错那等地.这四个分区的关联特点为顺时针旋移.重新计算后的震源机制解显示出:主震的NWW走向的节面与主震后2区内余震震中的NWW分布方向一致;序列中所有的压应力轴P和张应力轴T,都接近于水平向,其倾伏角大都小于20°;察隅主震和2区内余震的压应力轴P为近南北向,张应力轴T为近东西向;但3区和4区余震的P轴为近东西向,T轴为近南北向.反映出该强震序列中余震震源机制解的差异比较大.

The location parameters of the Zayü,Tibet M8.6earthquake sequence in 1950 in the published catalogues are poor,which cannot help further study of this great event.In order to provide more precise location results and some features of their spatial-temporal distributions,and to reveal the stress field in the corresponding region,we attempted to relocate this earthquake sequence and to determine the focal mechanisms.The data of global seismic stations from International Seismological Summary（ISS）and the Catalogue of China Earthquakes（BC 1831─AD 1969）were collected.The 16 events with M≥6of the Zayü,Tibet M8.6sequence were relocated by P arrivals at 239 worldwide stations using the routine location method and model of Chinese National Seismographic Network：Improved Geiger and the JB time tables.The corresponding fault plane solutions and composite fault plane solutions were determined by first motions of P waves based on the new relocations.（1）The relocations of 16 strong earthquakes of the Zayü M8.6 sequence have been completed.It is shown that the epicenters in different periods appeared in different sub-regions：A foreshock which occurred on 23 Feb 1950appeared in the 1st sub-region,the top of the great bend of the Yarlung Zangbo River,which lies at the north of Motuo.The mainshock and the following 7aftershocks in 3days which occurred from 15 Aug 1950to 18 Aug 1950were located in the 2st sub-region near Zayüand distributed along the north-west belt.The 3aftershocks in period 3which occurred from 22 Aug 1950 to 13Sep 1950,were located in the 3st sub-region extending to the south,India and Myanmar.The 3aftershocks in period 4which occurred from30 Sep 1950to 15 Apr 1951 appeared in the 4st sub-region at Motuo and Cuona,west of the aftershock epicenter area.Another aftershock occurred on the 110 th day after the mainshock,the epicenter of which returned to the place near the mainshock.（2）The focal mechanism solutions determined based on the relocations indicate that the NWW strike of a nodal plane of the mainshock is consistent with the NWW direction of the major axis of the aftershock epicenter area in the 2st sub-region.All compressive axes Pand tensional axes Tof the earthquake sequence are nearly horizontal,of which most dip angles are less than 20°.The compressive axes of the mainshock and the aftershocks in the 2st sub-region are nearly north-south direction,and tension axes are nearly east-west direction.But in the 3st and 4st sub-region,the compressive axes of aftershocks are nearly east-west direction,and tensional axes are nearly north-south direction.The 16 events of the Zayü,Tibet M8.6earthquake sequence in 1950 have been relocated.The new results are quite different from the parameters in the previous catalogues of which the locations of many aftershocks were exactly the same as the mainshock,i.e.only one point.The relocations indicate that the epicenters in different periods appeared in 4different sub-regions rather than distributing along the determined rupture.The obvious correlation of these 4subregions is clockwise rotation movement.The focal mechanism solutions determined based on the relocations display that the differences of focal mechanisms among the aftershocks of the sequence are large.