关键词: 构造应力场, 震源机制解, 静态库仑应力变化, 触发效应

Abstract:

The Jinggu earthquake sequence exhibits two dominant spatial orientations. Immediately after the mainshock, aftershocks propagated along the fault plane in a NW-SE direction. Subsequently, following two M_W5.5 strong aftershocks, the sequence expanded primarily in a NNW-SSE direction. These observations imply relatively complex triggering processes between the mainshock and the two M_W5.5 events, as well as substantial interactions among aftershocks. Clarifying triggering relationships within the sequence therefore requires a stress-based analysis.
To investigate these relationships, we first compiled focal mechanism solutions for 698 historical earthquakes and inverted the regional tectonic stress field in southwestern Yunnan using a 1°×1° grid. We then inverted the local stress field of the seismogenic fault using focal mechanisms of M_W≥3.0 events within the Jinggu sequence. The regional and local stress fields were used to constrain static Coulomb stress calculations on the optimally oriented fault plane, thereby improving result robustness. Based on the mainshock rupture model, we used Coulomb3.3 to evaluate the sensitivity of static Coulomb stress change to different friction coefficients and centroid depths. We further constructed source-fault models for 20 M_W≥3.0 aftershocks using empirical scaling relationships and calculated Coulomb stress transfer among these events. This workflow aims to quantify stress triggering from the mainshock to aftershocks and among aftershocks themselves.
The results indicate that the regional stress field in the area 23°~24°N and 99.5°~100.5°E is broadly consistent with the local stress field inferred for the Jinggu seismogenic fault. The maximum principal stress axis indicates NNE-SSW compression, with dominant azimuths of 12°(regional) and 25°(local). The minimum principal stress axis indicates NWW-SEE extension, with dominant azimuths of 102°(regional) and -65°(local). Stress shape ratios(R) of 0.50 and 0.59 suggest overall stress regimes approximating a uniaxial extension-uniaxial compression state and a biaxial extension-uniaxial compression state, respectively.
Coulomb stress-change patterns computed using different friction coefficients show consistent spatial trends. As the friction coefficient increases, the likelihood of mainshock-triggered aftershocks increases, but the effect becomes weak once the coefficient exceeds ~0.4. Accordingly, we adopt the commonly used empirical value for subsequent calculations. Coulomb stress-change patterns are also broadly consistent across tested depths; therefore, we use a centroid depth of 5km. Under these assumptions, ~73.46% of aftershocks are located within stress-loading zones. Cross-sections along the sequence trend indicate Coulomb stress changes extending vertically to ~25km depth and laterally for ~40km along strike. The concentration of aftershocks within stress-loading areas is consistent with the inferred mainshock rupture process.
Within the M_W≥3.0 aftershock set, ~55% of events are classified as triggered. The two M_W5.5 strong aftershocks are associated with stress loadings of 0.251MPa and 0.376MPa, respectively, far exceeding the commonly cited triggering threshold of 0.01MPa. Coulomb stress calculations for the M_w≥3.0 sequence further indicate a multiphase triggering process governing its spatiotemporal evolution. Aftershocks initially expanded preferentially toward the NW and later shifted toward the SE. The first M_W5.5 event marks a key turning point: it not only directly triggered a subsequent cluster of aftershocks extending approximately NS, but also altered the overall migration pattern of the sequence.
Mutual triggering among aftershocks is already pronounced prior to the M_W5.5 event; specifically, 8 of the 13 preceding events show clear triggering effects on subsequent earthquakes. This behavior is primarily attributed to superposition of static Coulomb stress perturbations generated by multiple events, which can operate through two end-member mechanisms. When stress changes from different sources are similarly oriented, they act constructively on the receiver fault and expand the affected region beyond the initial perturbation. Conversely, when stress orientations differ, they generate complex spatial variations in stress magnitude and direction on the receiver fault, producing a heterogeneous aftershock distribution. As a result, aftershocks not directly triggered by the mainshock may still be promoted by stress transfer from other aftershocks. The M_W5.5 aftershock sequence may have occurred on a branching structure with a different orientation from the mainshock fault, or on an unmapped blind fault. The distinctive triggering behavior of the Jinggu sequence provides useful constraints for assessing post-seismic evolution and for seismic hazard analysis.

Key words: tectonic stress field, focal mechanism solution, static Coulomb Stress, triggering effect