Stress transfer and connectivity between the Bhutan Himalaya and the Shillong Plateau

Within the northern Indian Plate, the Shillong Plateau is a peculiar geodynamic terrane, hosting significant seismic activity outboard the Himalayan belt. This activity is often used as an argument to explain apparent reduced seismicity in the Bhutan Himalayas. Although current geophysical and geodetic data indicate that the Bhutan Himalayas accommodate more deformation than the Shillong Plateau, we aim to quantify the extent to which the two geodynamic regimes are connected and potentially interact through stress transfers. We compiled a map of major faults and earthquakes in the two regions and computed co-seismic stress transfer amplitudes. Our results indicate that the Bhutan Himalayas and the Shillong Plateau are less connected than previously suggested. Major earthquakes in either of the two regions mainly affect transverse faults connecting them, causing up to ~40 bar Coulomb stress change; however, this effect is clearly less on thrust faults of the either region (up to 1 bar only). The MW 8.25 1897 Assam earthquake that affected the Shillong Plateau did not cause a stress shadow on the Main Himalayan Thrust in Bhutan as previously suggested. Similarly, the Mw 8 ± 0.5 1714 Bhutan earthquake had negligible impact on stress accumulation on thrust faults bounding the Shillong Plateau. Furthermore, the main process shaping the regional stress patterns continues to be interseismic loading with complex boundary conditions in a diffuse deformation field involving the Bengal Basin and Indo-Burman Ranges. While both the Bhutan Himalayas and the Shillong Plateau exhibit a compressional regime, their stress evolutions are more weakly connected than hypothesized. Although our modelling suggests lateral increase in stress interactions, from west (less) to east (more), in the Bhutan Himalayas, a clearer picture will only emerge with better constrained fault geometries, slip rates, crustal structure, and seismicity catalogues in the entire region of distributed deformation.