2021年青海玛多M_(S)7.4地震余震序列震源机制解及其发震构造特征

Focal mechanism solutions of the aftershocks of the 2021 Qinghai Madoi M_(S)7.4 earthquake and its seismogenic structure characteristics

2021年5月22日在巴颜喀拉块体内部发生了青海玛多M_(S)7.4地震,而精确的震源机制解和区域应力场有助于判定发震构造和探究地震的孕震机制.本文基于玛多地震科考获取的高质量波形数据,采用CAP方法反演得到了玛多地震震源区18次中小型余震的震源机制解,并进一步获取了震源区的构造应力场特征.震源机制解结果显示,这18次中小型余震包含16次走滑型和2次逆冲型地震,揭示出玛多M_(S)7.4地震的发震断层——江错断裂具有高倾角左旋走滑性质,且沿着破裂带自西向东具有分段性差异,表明发震断层构造形态的复杂性.余震的震源矩心深度均在10 km之上,主要集中于4~8 km,说明后续中小型余震的能量释放主要集中在浅部上地壳.应力场反演结果显示震源区最大水平主压应力轴方位为53°(NEE),倾伏角为13°,与区域构造应力场主压应力方向一致,揭示余震的发生仍主要受区域应力场的控制.综合已有的地球物理和地质资料,推测玛多M_(S)7.4地震的发生与上地壳速度结构的非均匀性及中下地壳软弱物质的挤压和垂向上涌密切相关.玛多M_(S)7.4主震发生之后,震源区应力不断调整,局部地震活动性明显增强.同时在局部速度结构复杂性及独特的断层系统的共同影响下,余震震源机制解类型呈现出多样化特点.

On May 22,2021,an earthquake with magnitude 7.4 occurred in Madoi area of Bayan Har block,Qinghai province.The high-precision focal mechanism solutions and regional tectonic stress field are significant to reveal the rupture properties and explore the seismogenic mechanism of earthquakes.Based on the high quality waveform data of the Madoi earthquake scientific research,focal mechanism solutions as well as the centroid depths of 18 small and medium-sized aftershocks were inverted using CAP method,and the tectonic stress field of focal area was further obtained by fault plane solutions of these aftershocks.The focal mechanism solution results show that these aftershocks include 16 strike slip and 2 thrust earthquakes,and indicate that the Jiangcuo fault,the seismogenic fault of the Madoi M_(S)7.4 earthquake,is characterized by large dip and sinistral strike slip.Obvious segmented differences can be seen from west to east along the surface rupture zone,which suggests the complexity of the seismogenic fault.The focal centroid depths of these aftershocks are all above 10 km and mainly concentrated in 4~8 km,indicating that the energy release of aftershocks is mainly concentrated in upper crust.The stress field result shows that the azimuth of the maximum horizontal principal compressive stress axis is 53°(NEE)with an inclination angle of 13°,generally consistent with the regional tectonic stress field,indicating the generation of aftershocks is mainly controlled by the regional stress field.Based on previous geophysical and geological studies,it is speculated that the occurrence of the Madoi M_(S)7.4 earthquake is closely related to the heterogeneity of the velocity structure of upper crust and the compression and vertical upwelling of weak materials in the mid-lower crust.After the Madoi M_(S)7.4 main shock,the stress in the focal area is continuously adjusted and the seismicity is significantly enhanced locally.Influenced by stress adjustment,complexity of local velocity structure and the unique fault system simultaneously,the types of focal mechanism solutions show diversified characteristics.