青海当江公元1738年M6(1/2)(?)地震同震变形特征再研究

Re-study on the Coseismic Deformation Characteristics of the A.D.1738 M6½Earthquake in Dangjiang,Qinghai

公元1738年发生在青海玉树西北当江地区的一次强烈地震造成人员死亡约260余人。但受高海拔缺氧与交通不便等条件所限,对其同震变形特征研究尚存争议,属于历史疑难地震。此前的地震目录将本次历史地震的宏观震中定在玉树附近,震级估算为6(1/2)级。本次研究根据卫星影像解译与野外地质调查,发现该次地震在甘孜-玉树断裂带当江段形成了明显的长约120 km的同震地表破裂带,最大水平同震位移为3.6±0.1 m。破裂带沿线发育了一系列左旋断错、河流阶地断错及断层陡坎等地貌特征。通过震例类比和同震地表破裂长度与最大同震位移经验公式综合分析,估算此次地震震级可达M 7(1/2)级。讨论认为当江段1738年地震的离逝时间已接近其强震复发周期,当江断裂与玉树断裂之间存在明显的破裂空区,其大震活动潜势不容低估。

A strong earthquake occurred in Dangjiang area,northwest of Yushu,Qinghai province,which caused nearly 260 casualties in 1738 AD.The previous earthquake catalog determined the epicenter of this historical earthquake is nearly Yushu with an estimated magnitude of 6½.However,because of the conditions such as high altitude and inconvenient transportation,there is still controversy on coseismic deformation characteristics.Based on the interpretation of satellite images and the field geological survey,we found that the Dangjiang segment of the Garzê-Yushu fault zone was clearly activated,which forming a left-lateral strike-slip coseismic deformation zone of about 120 km in length,with the maximum coseismic displacement of 3.6±0.1 m.A series of modern gully faults,terraces and fault scarps have been developed along the fault zone.Based on the analogy of earthquake cases and the empirical formula of coseismic surface rupture length and maximum coseismic displacement results,the magnitude of this earthquake is re-estimated at M7½.It is discussed that the elapsed time of the 1738 earthquake in the Dangjiang segment is close to its strong earthquake recurrence interval,and there is an obvious rupture gap between the Dangjiang fault and the Yushu fault,and its potential for large earthquakes cannot be underestimated.