Abstract:

Based on 1, 701 precisely relocated seismic events from the 2022 Luding M_S6.8 earthquake sequence, this study systematically delineates the three-dimensional geometric characteristics of the seismogenic faults using a fuzzy clustering-based fault plane determination method, providing critical insights into the complex seismotectonic processes operating along the southeastern margin of the Tibetan plateau. The results identify three prominent aftershock clusters in the main rupture area, each exhibiting distinct spatial distributions and tectonic implications. With respect to the Xianshuihe fault zone and their relative positions, Cluster A displays a slightly elliptical distribution that deviates markedly from the mapped fault trace, making it difficult to fit a single fault plane and suggesting strong control by complex subsurface structures. In contrast, Clusters B and C show well-defined elliptical distributions with pronounced long-axis orientations and high degrees of flattening, and both exhibit excellent agreement with the fitted fault planes, indicating relatively simple and coherent fault geometries consistent with regional tectonic patterns. The B and C faults, extending along the Xianshuihe fault zone, strike NNW-SSE with orientations of 159.9° and 157.2°, respectively, and both are characterized by near-vertical, high dip angles(Fault B: 88.1°; Fault C: 87.1°), consistent with the typical geometry of left-lateral strike-slip faults that dominate the region.

Notably, the area west of the main shock(Cluster A)exhibits pronounced spatial heterogeneity in focal mechanisms used for stress field inversion. Normal fault-type earthquakes account for 21.11% of the total dataset, which is unexpected in a tectonic setting primarily governed by strike-slip and thrust faulting. Clustering of focal mechanism nodal planes reveals a dominant fault plane striking 156.99°, closely aligned with the regional fault orientation despite the contrasting faulting style. Tectonic stress field inversion using a grid-search approach indicates that the main shock area is characterized by a composite stress regime, with near-horizontal compression in the NW-SE direction(P-axis azimuth 101.5°, plunge 0.9°) and vertical extension in the NNE-SSW direction(T-axis plunge 59.0°), consistent with the stress field imposed by the ongoing India-Eurasia collision. Projection of the stress field onto the fault planes shows that Fault B experiences relatively high shear stress(0.719) and compressional normal stress(-0.701), indicating conditions favorable for shear stress accumulation and potential future rupture. Fault C exhibits slightly higher relative shear stress(0.760) but weaker compressional normal stress(-0.625), suggesting spatial variations in fault strength and stress accumulation along the fault system. Both faults have slip angles smaller than 15°, indicating predominantly strike-slip motion with minor dip-slip components; the slip angles of Faults B and C are 13.0° and 14.7°, respectively. These results confirm left-lateral strike-slip as the dominant kinematic behavior, in strong agreement with GPS-derived slip rates of 9~12mm/a and historical strong-earthquake recurrence in the region.

In contrast, the stress field within Cluster A shows an anomalous pattern, characterized by near-vertical compression (P-axis plunge 84.6°, azimuth 210.3°) and near-horizontal extension (T-axis plunge 5.0°), representing an approximately 98° rotation relative to the background regional stress field. This indicates the operation of a fundamentally different local stress regime. We propose that the occurrence of normal fault-type earthquakes in Cluster A may be associated with gravitational potential energy release driven by the high topography of the Gongga Mountain region, producing shallow crustal extension superimposed on deep left-lateral strike-slip motion of the Xianshuihe Fault, thereby generating shallow normal-fault components related to gravitational collapse. Alternatively, or additionally, the presence of concealed secondary faults, highly brittle rock properties, and possible reservoir water infiltration may promote normal faulting through localized stress concentration and pore pressure variations. The relative contributions of these mechanisms, however, require further investigation using integrated geophysical observations and numerical simulations.

Overall, this study provides important constraints on fault activity and seismotectonic mechanisms along the southeastern margin of the Tibetan plateau, highlighting the complex interplay between regional tectonic stress fields and local geological structures. The identification of anomalous normal faulting west of the main rupture zone offers new perspectives on the lateral extrusion of the Bayan Har block and the role of gravitational collapse in high-relief regions, with implications for analogous tectonic environments worldwide. The integrated analysis of fault geometry, stress field inversion, and kinematic behavior contributes to improved seismic hazard assessment in this highly active region and provides a robust scientific basis for earthquake forecasting and risk mitigation.

Key words: Luding earthquake sequence, fuzzy cluster analysis, fault plane determination, fault activity properties

摘要：

北京时间2022年9月5日12时52分, 四川甘孜州泸定县发生 M_S6.8 地震。为研究泸定地震序列所在的鲜水河断裂磨西段形状, 文中首先采用1701个精定位地震事件, 利用基于模糊聚类的断层面形状确定方法得到了鲜水河断裂磨西段上2段沿NNW-SSE向排列断层的形状参数, 分别为: B断层, 走向为159.94°、 倾角为88.10°; C断层, 走向为157.15°、 倾角为87.14°。为了进一步研究这些断层的滑动性质, 文中搜集了发生在鲜水河断裂磨西段及其邻区的地震震源机制, 其中1个丛集中存在相当数量的正断型地震, 经聚类发现这些震源机制的节面聚类结果与鲜水河断裂平行。基于该类群中的震源机制求解了应力场, 发现这里的应力场相对于背景构造应力场发生了较大偏转。采用去除鲜水河断裂磨西段西侧区域异质震源机制的数据, 求解得到了沿主发震断裂的余震区应力场, 其中压轴方位为101.48°、 倾伏角为0.86°, 张轴方位为10.06°、 倾伏角为58.99°, 应力形因子R为0.70, 最大主压应力轴近水平, 最小主应力轴显示垂向拉张分量, 这一应力状态与青藏高原东缘受印度板块NE向挤压、 四川盆地刚性阻挡导致的物质SE向挤出的动力学背景高度吻合。将得到的构造应力场投影到泸定地震拟合的断层面上, 发现B断层的滑动角为13.0°、 C断层的滑动角为14.7°, 两者近垂直, 以左旋走滑为主。以上结果对认识该地区的断裂活动特性和地震动力学有一定意义。

关键词: 泸定地震序列, 模糊聚类分析, 断层面确定, 断层活动特性