Abstract:

Three earthquakes occurred in Yangbi County, Dali, Yunnan Province with the maximum magnitude M6.4, on May 21, 2021, and caused huge economic losses and human casualties. In this paper, the existing high-precision gravity data, mobile gravity survey data and EGM2008 model data were fused into high-precision grid data with 2.5km point distance to clarify the seismogenic structure and seismogenic environment of Yangbi earthquake. With the Yangbi earthquake as the center location, two long gravity profiles and 10 short gravity profiles are extracted, and the three-dimensional crustal imaging characteristics in the study area are obtained by the normalized full gradient imaging method, and the deep and shallow contact relationship and deep seismogenic environment along the northern section of Honghe fault zone, Weixi-Weishan Fault, Yongsheng-Binchuan Fault, Eryuan-Heqing Fault in the Yangbi earthquake area are analyzed. In this paper, the vertical and transverse characteristics of the upper crustal structure of the northern section of Red River Fault in Yangbi and its surrounding areas along the gravity profiles were obtained, the deep structural differences of the southern Yunnan block, Sichuan-Yunnan block and large faults were revealed, and the seismogenic structure and environment of the three Yangbi earthquakes were analyzed and discussed. The results of the study are as follows:

(1)The sudden change zone of dip angle and dip direction of the normalized gravity gradient is in good agreement with the medium and large geological faults, such as Nujiang Fault, Lancangjiang Fault, Red River Fault, Anninghe Fault, and Zemuhe Fault, etc.

(2)When the continuity of normalized gravity gradient of the middle and lower crust is good, and the middle and upper crust is in the high-low transition zone, earthquakes greater than M6.0 will occur frequently, especially in the intersection area of Weixi-Weishan Fault, Yongsheng-Binchuan Fault and the northern section of Red River Fault.

(3)Near the epicenter of Yangbi earthquake, there is a strong deformation belt of high and low normalized gravity gradients in the upper crust converging at a depth of about 15km, and the epicenter projection intersected with the Weixi-Weishan Fault and the secondary fault at a depth of about 10km, the continuity of normalized gravity gradient values is very well below the depth of 20km in the crust, it is inferred that the seismogenic structure of the three earthquakes in Yangbi are the Weixi-Weishan Fault and its secondary fault.

(4)Earthquakes of M6.0 or higher normally occur where the geological strata connect and are relatively young. Strong earthquakes occurred at the junction of the Triassic and Permian in the east of Dali. At the same time, analyzing the distribution characteristics of the normalized gravity gradient value(G_h)can provide a reference for the division and correction of stratigraphic boundaries.

(5)In the deformation process of geological structure, when the high-low gradient deformation zones of G_h value are formed in the middle and upper crust, whilst G_h values have good continuity in the middle and lower crust, earthquakes of M6.0 or higher normally occur. These features can be used as an important marker to judge the preparation and occurrence of strong earthquakes.

Based on the geological and geophysical characteristics and the distribution characteristics of M≥6.0 earthquakes, the relationship between the change of G_h values and the occurrence of moderate and strong earthquakes, the stratigraphic boundary, the strike and dip angle of structural faults in the study area were analyzed, and the seismogenic structure and environment of the three Yangbi earthquakes on May 21 in 2021 were discussed. This study can provide a scientific basis and important reference value for determining the seismogenic mechanism and location of moderate-strong earthquakes.

Key words: Yangbi M6.4 earthquake, Sichuan-Yunnan block, normalized total gravity gradient imaging, Bouguer gravity anomaly

摘要：

2021年5月21日云南大理白族自治州漾濞县发生了3次较强地震, 最高震级为M6.4, 造成巨大经济损失和人员伤亡。 为厘清漾濞地震的发震构造与孕震环境, 文中将已有的高精度重力数据、 流动重力测网点数据和EGM2008模型数据融合为2.5km点距的高精度网格数据, 并以漾濞地震为中心, 提取了2条长重力剖面和10条短重力剖面, 采用归一化梯度成像方法获取研究区内三维地壳成像特征, 重点分析了漾濞震区沿红河断裂北段、 维西-巍山断裂、 永胜-宾川断裂和洱源-鹤庆断裂等的深浅接触关系及其深部孕震环境。 结果表明: 1)重力归一化梯度陡变带的倾角、 倾向与地质上的中大型断裂吻合较好, 如怒江断裂、 澜沧江断裂、 红河断裂、 安宁河断裂和则木河断裂等; 2)中下地壳重力归一化梯度连续性较好, 且中上地壳为高低转换带时, M≥6.0地震频发, 特别是维西-巍山断裂、 永胜-宾川断裂及红河断裂北段交会的区域; 3)漾濞地震震中附近, 上地壳存在归一化梯度高低强变形带且相互靠拢, 在深度约15km处会聚, 震中投影与维西-巍山断裂及其次生断裂在深度约10km处交会, 且地壳20km以下重力归一化梯度值连续性较好, 推断漾濞3次地震的发震构造为维西-巍山断裂及其次生断裂。 文中结果对地震发震机制和发震地点的判定具有非常重要的科学意义和参考价值。

关键词: 漾濞M6.4地震, 川滇地块, 归一化梯度成像, 布格重力异常