Abstract:

Understanding the stress evolution of earthquake sequences is critical for elucidating the physical mechanisms driving earthquake nucleation and rupture. This study investigates the spatiotemporal variations in the stress field following the Dingri M_S6.8 earthquake in Tibet, using seismic data from both permanent and temporary mobile stations. Double-difference relocation was performed using HypoDD 2.1, incorporating phase data from temporary seismic networks for improved accuracy. Focal mechanism solutions were determined for 189 events with well-constrained P-wave first-motion polarities and adequate azimuthal coverage(≥8 observations), using the polarity method. The SATSI algorithm was subsequently applied to invert the orientations of the principal stress axes and estimate the stress ratio R.

The relocation results indicate that the mainshock ruptured the southern segment of the Dengmecuo Fault, with aftershocks propagating northward along the fault’s N-S trending structure. The aftershock distribution reveals a westward-dipping fault geometry. In the central portion of the rupture zone, both eastward- and westward-dipping fault branches are present, while the southern segment exhibits intersecting NW- and NE-striking faults, suggesting multiple rupture planes. The mainshock likely occurred near the eastern boundary between the east- and west-dipping segments, consistent with surface ruptures observed in the field.

Stress inversion results indicate a normal faulting regime. The maximum principal stress(σ₁)has a trend of 142° and a plunge of 67°, while the minimum principal stress(σ₃)trends at 110°(W)with a shallow plunge of 7°, and the intermediate stress(σ₂)trends at 17° with a plunge of 21°. The optimal stress ratio(R=0.22)suggests a dominantly extensional regime, consistent with the regional tectonic setting of N-S compression and NE-SW extension. Temporally, the orientation of σ₁ evolved from 135°(SSE)to 180°(S), and σ₃ shifted from NEE to an EW orientation, reflecting a post-seismic adjustment toward a stable regime of NS compression and EW extension. The R-value initially decreased from 0.5 to 0.05, followed by a gradual increase to 0.25, indicating early release of horizontal extensional stress and an increasing influence of vertical σ₁—typical of normal faulting sequences. Aftershock activity diminished within seven days and stabilized thereafter, indicating progressive dissipation of residual stress. Spatially, the source region was divided into southern, central, and northern clusters, with respective dominant strike orientations of NNW, NNE, and NNW. The southern cluster, which recorded the most events and the most diverse focal mechanisms, yielded well-constrained stress inversions(narrow confidence intervals). However, the plunge of σ₁ in this zone was only 31°, deviating from the near-vertical orientation typical of pure normal faulting. This deviation likely reflects complex fault geometry and secondary fracturing, which may have induced localized strike-slip components. In the central and northern zones, σ₃ remained horizontally oriented toward the SWW. In the northern cluster, σ₁ rotated to a NE orientation, likely influenced by increased strike-slip activity near the Nongqu Fault. Zone Ⅱ exhibited unstable inversion results, with overlapping σ₁-σ₂ confidence intervals, indicating a more complex local stress field. A northward increase in R suggests a transition from dominantly extensional to more strike-slip-dominated deformation.

The region remains in a phase of post-seismic stress adjustment and has not yet returned to its pre-mainshock stress state. Continued seismic monitoring, particularly of the structurally complex southern fault system and the northern strike-slip segments, is essential for assessing future seismic hazard and stress accumulation.

Key words: Tibet Dingri earthquake sequence, stress tensor, maximum principal stress, focal mechanism, R

摘要：

掌握地震序列的应力演化过程对于理解地震孕育和发生的物理机制至关重要。通过震源机制解反演震源区的构造应力场, 能够了解震后应力的释放和调整状态。文中利用P波初动极性方法反演了西藏定日 M_S6.8 地震序列中189个地震的震源机制解, 基于SATSI算法反演了主应力轴的方向和应力比R值, 分析了震源区应力场的时空演化特征。结合地震序列双差定位结果, 发现 M_S6.8 主震发生在登么错断裂的南部, 余震沿SN向的登么错断裂向N扩展。局部应力场向SN向挤压和EW向拉张的方向演化; R值先降低后回升, 反映了拉张作用的增强和应力场的恢复阶段。地震活动主要集中在南部、 中部、 北部3个地震丛集区, 地震展布分别为NNW、 NNE和NNW向。3个区域的最小主应力轴近水平, 均为SWW向, 但最大主应力轴和R值存在一定程度的差异。南部以正断层拉张为主, 而中、 北部拉张作用逐渐减弱。南部、 中部之间的过渡区地震活动较少, 数据对应力轴的约束不足, 也可能反映了断裂结构的复杂性。根据余震分布和应力场的时空演化特征推测, 震源区还处在应力调整阶段, 尚未恢复至主震前的区域应力状态。

关键词: 西藏定日地震序列, 应力张量, 最大主应力, 震源机制, R值