1950年西藏墨脱—察隅8.6级地震震源参数、发震构造及周边地震活动性

Source parameters,seismogenic structures of the 1950 Medog-Zayu M_(S)8.6 earthquake and seismicity in the surrounding areas

印度—欧亚板块的挤压碰撞形成了长达2500 km的喜马拉雅造山带.北京时间1950年8月15日22点09分,在喜马拉雅东构造结的墨脱—察隅一带发生了M_(S)8.6地震,是世界上有历史地震记录以来发生的最大的内陆型地震,整个青藏高原及毗邻的印度平原均有明显震感,给周边地区带来重大的经济和财产损失.我们基于前人在地震灾害、地震定位和震源机制等方面获得的研究成果,对墨脱—察隅大地震的震源参数、震源区地下结构、发震断层的构造特点进行了系统回顾.在此基础上,我们收集了不同机构给出的历史地震目录资料,总结了震前20年和震后10年较为完整的5.0级以上地震目录,分析了4个不同时间和空间发展阶段的地震活动性;同时利用现代地震目录和我们在震源区架设的近台观测波形资料,探讨了1964年以来发生的中小地震、2017年米林6.9级地震和2019年墨脱6.3级地震的发生机制.东构造结地区构造背景复杂,发育了三个不同方向、依次向南迁移的次级构造结,即南迦巴瓦构造结、桑构造结及阿萨姆构造结.在最年轻的阿萨姆构造结地区,印度板块不仅沿着米什米逆冲断裂向NE方向俯冲,同时沿着主前锋逆冲断裂向NW方向俯冲,阿萨姆构造结前缘的察隅岛弧和桑构造结表现出强烈的地壳水平缩短.这种持续不断的高强度的构造挤压应力作用,导致主前锋逆冲断裂和米什米逆冲断裂同时发生破裂,形成M_(S)8.6的墨脱—察隅大地震,并在米什米山和阿博尔山一带发生大范围的余震活动和明显的地表形变.早期地震发生机制的不确定性有待进一步深入分析,地震、地质、遥感和野外调查等多种研究手段的融合可以为其提供较为全面的观测依据.

The collision between the Indian and Eurasian plates formed the 2500 km long Himalayan orogenic belt.At 22:09 on August 15,1950,Beijing time,an M_(S)8.6 earthquake occurred in Medog-Zayu area in the eastern Himalayan syntaxis,which is the largest continental earthquake in the world ever recorded since the historical earthquake records have become available.It has been felt by people across the entire Tibetan Plateau and the adjacent plains of India and caused extensive economic and property damages.Based on previous studies about the seismic hazards,earthquake locations and focal mechanisms,we systematically review the source parameters,deep structures of the earth beneath the source area and the features of the seismogenic faults.In addition,we collected earthquake catalogues provided by different agencies and summarized a relatively complete earthquake catalogue with magnitudes greater than or equal to 5.0 within 20 years before and 10 years after the great earthquake.We then analysed the seismicity in four different special and temporal stages.Using the modern earthquake catalogues and waveform data recorded by seismic stations we deployed in the nearby areas,we discussed the mechanisms of small and medium earthquakes occurring since 1964,the 2017 M_(S)6.9 Mainling earthquake,and the 2019 M_(S)6.3 Medog earthquake.The tectonic background of the eastern Himalayan syntaxis is complex.There are three secondary syntaxis which migrate from north to south in different directions,i.e.,the Namche Barwa syntaxis,the Sang syntaxis and the Assam syntaxis.In the youngest Assam syntaxis,the Indian plate subducts not only toward NE along the Mishmi Thrust but also toward NW along the Main Frontal Thrust,resulted strong compressive crustal deformation in the Zayu island arc and in Sang syntaxis.Under the continuous high strength of tectonically compressional regime,the Main Frontal Thrust and the Mishmi Thrust ruptured simultaneously,led to the formation of the M_(S)8.6 Medog-Zayu great earthquake.Large aftershocks and clear surface deformation are widely observed in the areas of the Mishmi hills and the Abor hills.The uncertainties of the mechanism of early great earthquakes need to be further analyzed.The integration of seismological,geological,remote sensing and field investigations may provide observations in more comprehensive ways.