The Da Qaidam−Zongwulongshan fault along the northern margin of the Qaidam Basin in the northeastern Qinghai−Xizang Plateau, has experienced a sequence of moderate-to-strong earthquakes since 2003. The 2003 Delingha M_W6.3, 2008 Da Qaidam M_W6.3 and 2009 Da Qaidam M_W6.3 earthquakes occurred close to this active tectonic belt, and a further M_W6.4 earthquake occurred in 2026. These spatially clustered events raise the question whether static stress transfer from the earlier earthquakes modified the failure conditions of the fault that ruptured, or potentially ruptured in 2026. This study quantitatively evaluates the cumulative Coulomb failure stress change （ΔCFS） induced by the 2003, 2008 and 2009 earthquakes on candidate receiver faults for the 2026 M_W6.4 event. The objective is not to establish deterministic triggering, but to determine whether the preceding earthquakes promoted or inhibited fault failure under explicitly defined source-fault, receiver-fault and frictional assumptions.

The 2003, 2008 and 2009 M_W6.3 earthquakes were represented by uniform-slip rectangular dislocation sources. Their locations, focal mechanisms, centroid depths and scalar seismic moments were taken from the global centroid moment tensor （GCMT） catalogue and related published studies. Rupture length and width were estimated using the empirical scaling relations of Wells and Coppersmith, while average slip was constrained by the scalar seismic moment and a shear modulus of 32 GPa. This simplified source representation preserves the total seismic moment and provides a consistent framework for inter-event comparison, although it cannot reproduce heterogeneous slip patches, rupture initiation or segment-by-segment propagation. The 2003, 2008 and 2009 earthquakes were treated as source faults. For the 2026 event, a centroid focal-mechanism solution derived from multiple institutional solutions by the Seismology Group was adopted as one candidate receiver plane, and the GCMT auxiliary plane was considered as an alternative candidate plane to reduce the dependence on a single focal mechanism solution.

Static elastic deformation and stress changes were calculated using the Okada elastic half-space solution implemented in the Coulomb software. Coulomb failure stress change was defined as ΔCFS＝Δτ＋μ′Δσ_n, where Δτ is the shear-stress change in the expected slip direction, Δσ_n is the normal-stress change with unclamping taken as positive, and μ′ is the effective friction coefficient. A Poisson’s ratio of 0.25 and an elastic modulus of 80 GPa were used. To examine the influence of the frictional parameter, the stress changes were calculated for μ′ values of 0.2, 0.4 and 0.6. Horizontal stress slices were evaluated at depths of 5.0, 7.5, 10.0, 12.5, 15.0, 17.5, 20.0, 22.5 and 25.0 km. In addition, ΔCFS was sampled along the strike direction of the candidate receiver geometry at each depth. Because the finite rectangular receiver fault extends approximately from 7.37 km to 16.63 km depth, the 5.0 km slice represents an upward extension of the candidate plane, whereas the 17.5−25.0 km slices represent its downward continuation and should not be interpreted as stress values strictly within the finite rupture surface.

The calculated stress fields contain alternating positive and negative lobes around the source faults, reflecting the combined effects of source geometry, receiver orientation and depth. Nevertheless, a coherent loading region is present near the 2026 source area and its adjacent fault segments. The most pronounced positive ΔCFS occurs mainly between 10.0 km and 15.0 km depth, broadly consistent with the finite candidate receiver fault in the depth range of 7.37−16.63 km. This agreement suggests that the cumulative static stress contribution of the 2003, 2008 and 2009 earthquakes was concentrated near the likely seismogenic depth of the 2026 event. The shallow 5.0−7.5 km slices show a more limited and spatially fragmented loading pattern, whereas the models diverge more strongly at 20.0 km and greater depths.

The along-fault profiles further reveal systematic differences among the effective-friction models. For μ′＝0.2, the mean ΔCFS values at 10.0, 12.5 and 15.0 km are 172.5, 125.2 and 67.2 kPa, respectively, and the proportions of the profile exceeding the commonly used reference threshold 10 kPa are 63.6%, 72.9% and 76.4%. At 20.0 km and 25.0 km, however, the corresponding mean values decrease to −42.9 kPa and −13.3 kPa, with zero profile coverage above 10 kPa. The μ′＝0.4 model produces the most persistent loading over the tested depth range. Mean ΔCFS remains positive at all calculated depths, including 51.6, 34.4, 25.9 and 27.1 kPa at 17.5, 20.0, 22.5 and 25.0 km, respectively. The proportions of the profile with ΔCFS≥10 kPa reach 85.0%, 90.7%, 94.3% and 92.9% at 15.0, 17.5, 20.0 and 22.5 km. In contrast, the μ′＝0.6 model again shows pronounced loading at the depth of 10.0−15.0 km but reduced or negative stress changes in the deeper continuation. These results demonstrate that ΔCFS does not vary monotonically with μ′; instead, the frictional contribution interacts with the spatially variable shear- and normal-stress components.

The calculations therefore support the interpretation that the three earthquakes increased the failure probability of the 2026 candidate receiver fault under the tested model assumptions. Positive ΔCFS should, however, be interpreted as a promotion of failure conditions rather than proof of deterministic triggering. Dynamic stresses, pore-fluid pressure, postseismic afterslip, viscoelastic relaxation and long-term tectonic loading may also have contributed to the regional stress evolution. Model uncertainty is additionally introduced by the use of uniform-slip rectangular sources and by ambiguity in the receiver-fault orientation. A heterogeneous finite-fault slip model or a different nodal plane could alter the detailed near-field pattern and the amplitude of the calculated stress changes.

In summary, the cumulative static stress loading generated by the 2003, 2008 and 2009 earthquakes is strongest near 10.0−15.0 km depth and overlaps the finite-depth range of the 2026 candidate receiver fault. The μ′＝0.4 calculation yields the most spatially persistent positive loading, while the deeper results for μ′＝0.2 and μ′＝0.6 are weaken or become negative. The finite-fault depth interval provides the principal basis for evaluating the potential loading of the 2026 source, whereas the shallow upward-extension and deep downward-extension slices mainly constrain the depth-dependent behavior of the model. The study highlights both the possible contribution of preceding earthquakes to the failure conditions of the 2026 event and the need to account explicitly for receiver geometry, frictional parameters and postseismic stress evolution in future analyses.