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

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