利用重力异常分析2021年青海玛多MS7.4地震发震断层与结构特征

Analysis of seismogenic fault and structural characteristics of the 2021 Madoi, Qinghai MS7.4 earthquake using gravity anomalies

基于2021年青海玛多M7.4地震震源区及周边的重力异常计算了全张量梯度,利用倾斜角(TA)、倾斜角总水平梯度(THDR)和断层倾角(β)分析了巴颜喀拉块体及周边断裂带的类型、连续性和分段性等特征.昆仑山口—江错断裂两侧剩余异常轴线发生明显错断,走向与TA零值线过渡带和THDR极值线分布基本吻合.结合活动断裂分布与地表破裂展布,推断此次地震可能的发震构造为昆仑山口—江错断裂.昆仑山口—江错断裂不同段落具有不同的重力异常、识别标志和倾角变化特征.对照地表破裂分段,TA零值线过渡带和THDR极值线能很好的表征沿鄂陵湖南段、黄河乡段和东草阿龙湖段的断裂结构,而昌麻河段的识别特征不明显.最西端的鄂陵湖南段和东端昌麻河段表现为走滑性质,兼有逆断分量;而中间的黄河乡段和东草阿龙湖段则以走滑为主.由于发震断裂走向的平面变化及其深部结构的横向差异,在区域动力环境加载作用下可能产生沿断层的局部应力集中,从而导致此次地震的发生.

We calculate the Complete Bouguer gravity anomalies in/around the focal region of the 2021 Madoi, Qinghai M7.4 earthquake to obtain the gravity gradient tensor. Faulting type, continuity, and segmentation of the active fault zones in Bayan Har block and its surrounding area are analyzed by tilt angle(TA), tilt angle first horizontal derivative(THDR) and dip angle(β). The residual gravity anomalies contour lines show an obvious misalignment across the Kunlun Mountain Pass-Jiangcuo fault. The strike of Kunlun Mountain Pass-Jiangcuo fault almost consists with TA zero-value transition zones and THDR threshold lines. It is inferred that the possible seismogenic fault of Madoi earthquake is the Kunlun Mountain Pass-Jiangcuo fault, combined with active faults and surface fractures. There are different characteristics of gravity anomalies, identification marks, and dip angles on four segments of the Kunlun Mountain Pass-Jiangcuo fault. Compared with surface rupture segments, the subsurface fault structures along the Elinghu south segment, Huanghexiang segment and Dongcaoarlong segment are clearly detected by TA and THDR, while the structure boundary along the Changmahexiang segment is not identified. The Elinghu south segment and Changmahexiang segment are strike-slip properties, also having reverse fault component. The Huanghexiang segment and Dongcaoarlong segment are pure strike-slip faults. Due to the strike variation of seismogenic faults and lateral difference of deep structure, local stress may be concentrated along the fault under the loading of regional dynamic environment, which leads to the occurrence of Madoi M7.4 earthquake.