Impact of coseismic and postseismic stress adjustments from the 2015 <italic>M</italic>_W7.8 Nepal earthquake on Himalayan seismicity

The 2015 MW7.8 Nepal earthquake significantly altered the seismic activity in the Himalayan region, highlighting the crucial role of coseismic and postseismic stress adjustments in earthquake triggering. Observations show that seismic activity near the rupture zone increased dramatically between 2015 (after the event) and 2017, while the 2025 MW7.1 Dingri earthquake region, located farther away from the 2015 rupture area, exhibited continuously enhanced seismicity over the subsequent 10 years compared to the pre-earthquake period, suggesting that postseismic stress adjustments may play an important role in triggering earthquakes. This study employs a regional three-dimensional (3D) viscoelastic finite element model constrained by long-term GNSS observations to systematically analyze the spatiotemporal evolution of stress fields induced by coseismic rupture, postseismic afterslip and viscoelastic relaxation from lower crust and upper mantle. The results indicate that the coseismic event induced substantial Coulomb stress perturbations of up to 15 MPa on the Main Himalayan Thrust (MHT), while postseismic processes (e.g., afterslip and viscoelastic relaxation) contributed to long-term stress adjustments, with a maximum cumulative Coulomb stress reduction of 3.7 MPa over 10 years within the postseismic afterslip zone on the MHT. Furthermore, the model results reveal the impact of postseismic stress adjustments on the seismic hazard of active crustal faults in the Himalayan region, showing that major faults near the up-dip and down-dip portions of the rupture zone experienced varying degrees of stress loading, indicating potential possibility of seismic hazards. For example, this Nepal earthquake and its MW7.3 aftershock contributed ~18 KPa of coseismic Coulomb stress accumulation in the 2025 MW7.1 Dingri earthquake region, followed by gradual postseismic stress increases, indicating its triggering effects on subsequent seismicity in adjacent regions. This study gains insights into the tectonic stress evolution of the Himalayan region and helps seismic hazard assessments, emphasizing the importance of postseismic stress evolution in the earthquake cycle.