Abstract:

On January 7, 2025, a M_S6.8 earthquake occurred in Dingri, Qinghai-Xizang Plateau. The earthquake occurred in the Shenzha-Dingjie Rift zone with a moment magnitude of 7.2, which is relatively rare for such a large earthquake to occur in the rift zone. To understand the source characteristics of the earthquake, the available seismic moment tensor solutions of the earthquake are collected, by averaging the corresponding moment tensor element, we obtained the central seismic moment tensor of the earthquake. By using the central seismic moment tensor to replace the central focal mechanism solution to understand the source characteristics, it is not only considering the results of seismic moment tensors from different sources but also a simpler algorithm than the previous central focal mechanism solution algorithms. By decomposing the central seismic moment tensor into a dislocation source part and a compensated linear vector dipole part, it was found that the dislocation source part occurs in a mechanical state of near vertical compression and near east-west tension. In contrast, the compensated linear vector dipole part exhibits a moment release mode of simultaneous vertical and north-south compression and east-west tension. A comprehensive analysis of previous geological surveys shows that the Shenzha-Dingjie rift zone is a steeply dipping normal fault. Therefore, we speculate that the non-double couple moment tensor of the Dingri earthquake is a comprehensive result of continuous sliding with a steep fault in the shallow crust and gentle low-dip in the deep part of the crust, which formed a shovel-shaped fault, and the sliding angles gradually change from shallow to deep. The focal mechanism of aftershocks of the Dingri earthquake and seismic moment tensor data of surrounding historical earthquakes are also collected. The comprehensive seismic moment tensors for aftershocks and historical earthquakes are obtained by summing elements of the seismic moment of every earthquake. The same analysis of the seismic moment of the mainshock was conducted with the comprehensive seismic moment tensors for aftershocks and historical earthquakes. It was found that the patterns of the dislocation source part and the compensated linear vector dipole part obtained were consistent with that of the main shock, which supported the analysis results of the source characteristics of the main shock. This is the first time that the total seismic moment tensor elements of earthquakes and aftershocks in geological fault zones have been averaged to study the properties or characteristics of fault zones or earthquake sequences. The obtained results are still encouraging. It provides a comprehensive method for analyzing fault zones or aftershock zones and analyzing fault properties or focal rupture characteristics. There are multiple hypotheses regarding the formation mechanism of the north-south rift zone on the Qinghai-Xizang Plateau. The analysis of the source characteristics of the Dingri earthquake supports the related models of magmatic activity/intrusion of the lower crust bottom splitting and simultaneous squeezing of Indian plate material. Based on the central seismic moment tensor solution of the Dingri earthquake and the geometry shape of the rift zone, it is inferred that the non-double-couple in the seismic moment tensor originates from the changes in bending faults and sliding angles, which provides ideas for intuitively explaining the non-double-couple part of the seismic moment tensor.

Key words: Dingri earthquake, central seismic moment tensor, compensated linear vector dipole, comprehensive seismic moment tensor, characteristics of rift zone

摘要：

2025年1月7日西藏定日发生6.8级地震。该地震发生在申扎-定结裂谷带上, 矩震级达7.2, 在裂谷带发生如此大的地震较为罕见。为理解该地震的震源特性, 文中搜集所能得到的该地震的地震矩张量解, 采用将对应矩张量元素求平均值的方法得到该地震的中心地震矩张量。采用中心地震矩张量理解震源特性, 不仅考虑了不同来源的地震矩张量结果, 而且算法简单, 优于前人的震源机制中心解算法。将中心地震矩张量分解为位错源部分和补偿线性矢量偶极部分, 发现位错源部分在近垂向挤压和近EW向拉张的力学状态下发生, 补偿线性矢量偶极部分呈现垂向和SN向同时挤压和EW向拉张的矩释放模式。通过综合分析前人地质调查得到的申扎-定结裂谷带为陡倾角的正断层, 我们推测定日地震的非双力偶矩张量为浅部倾角较陡、 深部倾角较缓的铲形断裂滑动角自浅到深逐步变化的连续性滑动导致的综合结果。文中还搜集了定日地震余震的震源机制资料和周围历史地震的地震矩张量资料, 分别求得了余震和历史地震的综合地震矩张量, 并对其开展了同样的分析, 发现得到的位错源部分和补偿线性矢量偶极部分的模式与主震一致, 佐证了主震震源特性的分析结果。青藏高原SN向裂谷带的成因机制存在多种假说, 定日地震的震源特性分析支持岩浆活动或下地壳底劈侵入和印度板块物质共同挤入的相关模型。根据定日地震的中心地震矩张量解和裂谷带形状推测, 地震矩张量中的非双力偶源于弯曲断层和滑动角的改变, 为直观解释地震矩张量的非双力偶部分提供了思路。

关键词: 定日地震, 中心地震矩张量, 补偿线性矢量偶极, 综合地震矩张量, 裂谷区特征