2021年5月22日青海玛多M_(S)7.4地震地表破裂带及发震构造

Coseismic surface rupture and seismogenic structure of the 2021-05-22 Maduo(Qinghai)M_(S)7.4earthquake

2021年5月22日2时4分,青海省果洛藏族自治州玛多县发生了M_(S)7.4级强烈地震,震中位于巴颜喀拉地块北部边界东昆仑断裂带以南约70 km左右(34.59°N,98.34°E),震源深度17 km。震后的野外现场考察表明,这次地震在海拔4200~4600 m的高原面上形成了一系列由张裂隙、张剪裂隙、剪切裂隙、挤压鼓包和裂陷等多类型破裂雁行状组合而成的复杂同震地表变形带,总体表现为左行走滑运动性质,局部略带正断分量。该破裂带主要沿东昆仑断裂带南部的江错断裂分布,整体呈N105°E走向,全长约151 km。根据破裂带的走向变化和阶区特征,可将其分为四段:西段、中西段、中东段和东段。其中西段分叉为南、北两支,北支破裂走向N112°E,呈不连续分布,长约18 km,南支走向N94°E,呈连续性分布,长约25 km,最大左行位错约2.9 m;中西段全长约52 km,主要由约22 km长、呈N109°E走向的连续分布的地表破裂与稍北分布约30 km长、不连续分布的两支破裂组合而成,最大左行位错约1.9 m;中东段为总体呈N104°E走向的不连续地表破裂,全长约51 km,其中包含长约20 km的破裂空区;东段分叉为北、中、南三支,北支为走向N84°E、长约23 km的连续性破裂,最大左行位错约1.8 m,中间一支为N110°E走向、长约14 km的不连续破裂,南支则表现为零星破裂及系列滑塌,走向N120°E,长约6 km。这种两端发育较大规模分叉的"扫帚"状同震地表破裂在青藏高原已发生的走滑型地震中尚未报道过。这次地震的发震断裂为江错断裂,该断裂向西延伸可与2001年东昆仑MS 8.1地震的发震断层昆仑山口断裂相接,表明昆仑山口-江错断裂带与北部东昆仑断裂带中东部的托索湖-玛沁断裂挤压弯曲段共同构成了巴颜喀拉地块北部的宽阔边缘断裂带,并与南部的玉树-甘孜-鲜水河断裂带协同运动,共同调节着巴颜喀拉地块向东的运动和形变。由于东昆仑断裂带东部的玛沁-玛曲段是历史地震空区,因此可能是未来强震发生的区段。同时需要考虑到近20多年以来,巴颜喀拉地块周缘的强震活动具有跳跃性特征。因此,在未来的强震危险性评价中,应重点关注巴颜喀拉地块北边界带中东段玛沁-玛曲段和南部边界带鲜水河断裂带等的强震活动性及危险性。

At 2:04am on May 22,2021,a M_(S)7.4 earthquake occurred in Maduo County,Guoluo Tibetan Autonomous Prefecture,Qinghai Province.Its epicenter is located at 34.59°N~98.34°E,~70km south of the East Kunlun fault on the northern boundary of the Bayan Har block,with the focal depth is about of 17km.Detailed field investigation from the days right after the earthquake showed coseismic surface ruptures consisting of a series of tension cracks,tension shear cracks,shear cracks,push-ups,and collapses at an altitude of 4200~4600m.The~N105°E-striking surface rupture zone is~151km-long and is characterized by left-lateral strike-slip movement,with minor normal component at places.It follows the Jiangcuo fault located just south of the East Kunlun fault.We divide the surface rupture zone into four segments,according to the geometric changes in strike direction and structural steps/bends between the segments:western,central western,central eastern,and eastern segments.The western segment consists of two branches:the northern splay is discontinuous along~18km and strikes N112°E,while the southern splay is continuous for~25km and strikes N94°E.It is along the southern splay that we found the largest coseismic left-lateral offset:~2.9m.The central western segment is~52km-long,with continuous surface ruptures in the westernmost~22km(striking N109°E)and~30km of discontinuous rupture in the east.The largest left-lateral offset of~1.9mcan be found along that central western segment.The surface rupture along the central eastern segment strikes~N104°E discontinuously for~51km,including a 20km-long gap without any surface rupture.That along the eastern segment consists of three branches:the northern rupture strikes~N84°E continuously for~23km,with the largest left-lateral offset of~1.8m.The central rupture strikes~N110°E discontinuously for~14km.The southern splay only has minor ruptures and collapses striking~N120°E for~6km.The rupture pattern of large-scale branching occurred at both ends of the surface rupture zone and has never been reported along other coseismic surface rupture zones of strike-slip earthquakes on the Tibetan Plateau.The seismogenic fault of this earthquake is the Jiangcuo fault,which may connect with the Kunlun Pass surface rupture zone of the 2001East Kunlun(Kokoxili)earthquake(MS8.1)to the west,suggesting that the Kunlun Pass-Jiangcuo fault and the restraining bend(Tuosuo-Maqin section)of the East Kunlun fault zone jointly form a wide(~100km)marginal fault zone of the Bayan Har block.This marginal fault zone,together with the southern Yushu-Ganzi-Xianshuihe fault zone,accommodate eastward motion and deformation of the Bayan Har block in the northern Tibetan Plateau.Since the Maqin-Maqu segment of the eastern East Kunlun fault zone is a seismic gap,future large earthquakes may occur along that segment.In addition,because large earthquakes along the borders of the Bayan Har block appear to have jumped in the last 20years from one side to another,we infer that the next large earthquake may also occur along its southern boundary:the Ganzi-Xianshuihe fault.