2021年青海玛多M_(S)7.4地震震源区上地壳三维精细速度结构

Three-dimensional fine velocity structure of the upper crust of the 2021 Madoi M_(S)7.4 earthquake,Qinghai

精细的三维速度结构是揭示深部孕震环境的重要信息.为探究2021年5月22日青海玛多7.4级地震深部物性结构及其孕震机制,本文基于近震P波走时数据,开展体波走时成像,获取了玛多地震震源区上地壳三维精细的P波速度结构.研究结果表明江错断裂周边的速度结构存在明显的非均匀性和分段性.在鄂陵湖南段的地表破裂与发震断裂存在一定夹角,破裂空间展布与地震精定位吻合.速度图像显示该段破裂带北部区域存在大规模的高速异常体,高速体下方存在明显低速层,地震发生在低速层上覆的高速体内,推测鄂陵湖南段可能受到北边高速体的阻挡作用,导致应力在南侧释放,地表形成大规模的破裂.野马滩大桥到黄河乡段深部存在明显的低速区,该段发震深度主要集中在低速区上覆区域,深度浅、频次低.黄河乡以东约20 km处存在大规模的高、低速分界带,地震发生频次最高,发震深度集中在8~12 km,说明该区域可能受局部构造影响,应力大规模释放,地震活动性强.

Obtaining a fine three-dimensional velocity model is an important means to study the seismogenic environment.In order to explore the crustal structure and mechanism of the Qinghai Madoi M_(S)7.4 earthquake on May 22,2021,we constructed a three-dimensional fine crustal model for the Madoi M_(S)7.4 earthquake area by using the local seismic P-wave traveltime data.The results show that the velocity structure around the seismogenic fault presents obvious inhomogeneity and segmentation.There is a certain angle between the surface rupture and the seismogenic fault in the southern part of the E'ling Lake.The spatial distribution of the rupture coincides with the relocated earthquake.The velocity images show that there is a large-scale high-velocity anomaly in the northern region of the rupture zone,and there is an obvious low-velocity layer below the high-velocity layer.The earthquake occurred in the high-velocity anomaly overlying the low-velocity layer.It is speculated that the southern section of the E'ling Lake may be blocked by the high-velocity anomaly in the north,resulting in the release of stress in the south and the formation of large-scale ruptures on the surface.An obvious low-velocity zone is found from Yematan Bridge to Huanghe Township.The earthquakes in the section are mainly concentrated in the overlying area of the low-velocity zone,with shallow earthquake depth and low frequency.The velocity maps show that there is a large-scale high-velocity and low-velocity boundary area about 20 km east of Huanghe Township,with the highest frequency of earthquakes and a concentrated earthquake depth of 8~12 km.It is speculated that this area may be affected by local structures,with large-scale stress release and strong seismicity.