Abstract:

According to the China Seismic Network, a magnitude M_S6.8 earthquake occurred on January 7, 2025, in Dingri, Tibet. The epicenter was located near the Shenza-Dingjie rift(87.45°E, 28.50°N)at a focal depth of 10km. This earthquake is attributed to compressional forces resulting from the ongoing convergence between the Indian and Eurasian plates, which induce east-west(EW)extensional stress within the Tibetan plateau. In southern Tibet, a series of north-south(NS)trending rift valleys have developed in response to these tectonic processes. In these regions, variations in lithospheric density generate deviatoric stress, which closely correlates with the spatial distribution of seismicity. Areas exhibiting high deviatoric stress tend to experience more frequent tectonic activity. Furthermore, the elastic thickness of the lithosphere(T_e), a key indicator of lithospheric strength and stability, is generally lower in seismically active zones, particularly within transition zones between strong and weak lithosphere. Therefore, analyzing deviatoric stress and T_e in the study area is essential for understanding the mechanisms behind the Dingri earthquake and related seismic phenomena.

To investigate the seismo-tectonic background of this event, this study constructs an equilibrium equation for deviatoric stress based on gravity field data. The admittance and coherence method is applied to estimate deviatoric stress at various depths, elastic lithospheric thickness(T_e), and load ratio(F), using the WGM2012 gravity field model, ETOPO1 topographic data, and CRUST1.0 crustal structure data. The study further analyzes the coupling between deviatoric stress and regional geological structures, examines the spatial distribution of T_e and its tectonic implications, and evaluates the influence of load ratio(F)on deviatoric stress estimation. These analyses form the basis for a comprehensive discussion of the focal characteristics of the Dingri earthquake.

Our results indicate that deviatoric stress in the study area exhibits a clear south-north gradient, with higher values(>15MPa)concentrated in the southern region, particularly south of the Yarlung Zangbo Fault. In the north, elevated stress values are primarily associated with major fault zones such as the Shenza-Dingjie and Yadong-Gulu faults. Deviatoric stress decreases with depth, showing a marked decline at 50km. The elastic lithosphere thickness is generally greater than 40km across the region, with higher values observed in the central and southern areas, consistent with the subsidence and underthrusting of the Indian plate along the southern margin of the plateau. In contrast, lower T_e values in the northeastern part of the study area are likely linked to rifting and lithospheric extension. The load ratio(F)varies between 0 and 1, with surface loads(F<0.4)dominating most of the region. However, higher values are observed in the northern segment of the Yadong-Gulu fault zone, suggesting a significant contribution from lower crustal or upper mantle processes. High load ratios can introduce uncertainties in deviatoric stress estimates, particularly in regions of active deep-seated tectonism.

The epicenter of the Dingri M_S6.8 earthquake is situated within the Shenza-Dingjie rift zone. The stress regime in this area is dominated by strike-slip tectonics, with NNW-SSE compression and NEE-SWW extension. Under this stress configuration, NS-trending faults near the epicenter are susceptible to normal faulting. Deviatoric stress values at crustal depths of 0, 10, 20, 30, and 50km are 11.45MPa, 8.46MPa, 4.36MPa, 2.86MPa, and 1.19MPa, respectively. These results indicate that deviatoric stress is predominantly concentrated within the upper 20km of the crust and is oriented mainly in the NNE direction, consistent with the regional tectonic stress field. Additionally, the epicenter lies within a transitional zone of elastic lithospheric thickness, where stress resistance varies, providing favorable conditions for shallow, NS-oriented normal faulting.

Key words: Dingri earthquake, regional stress, elastic lithosphere thickness, gravity anomaly

摘要： 2025年1月7日西藏定日发生 M_S6.8 地震, 为深入分析此次地震的孕震背景, 文中基于WGM2012重力场模型数据、 ETOP01地形数据及CRUST1.0地壳模型, 计算了定日地震震源区及邻区不同深度的偏应力、 弹性岩石圈厚度(T_e)和载荷比(F)。结果显示: 研究区偏应力呈南北分异特征, 以藏南拆离断裂带为界, 南部整体较高(>15MPa), 而北部仅在申扎-定结、 亚东-谷露等断裂带附近集中表现出高值。随着深度增加, 偏应力逐渐减小, 尤其在50km深度, 偏应力均低于8MPa; T_e由西南部喜马拉雅块体处的70km沿NE向至拉萨块体减小到17km, 表现出明显的横向不均匀性; 区内载荷比较小(F<0.4), 整体以地表载荷为主。结合震例分析认为: 定日地震震中附近偏应力较高(>10MPa), 主要集中在0~20km深度范围内, 低载荷比(F<0.1), 地形或浅部地壳密度变化在挠曲变形中占主导。此外, 震中还处于弹性岩石圈厚度过渡带, 应力抵御能力不均, 整体表现出浅源破裂的可能性。

关键词: 定日地震, 偏应力, 弹性岩石圈厚度, 重力异常