Present-day fault kinematic around the eastern Himalayan Syntaxis and probable viscoelastic relaxation perturbation following the 1950 Mw 8.7 Assam earthquake

Abstract

Strike-slip and tensile/dip-slip rates around the EHS region from the elastic block model. • An elastic block model was constructed to analyze the fault slip rates and regional kinematic around the Eastern Himalayan Syntaxis (EHS). • The postseismic viscoelastic relaxation effects caused by the 1950 Assam Mw 8.7 earthquake is still significant even after 65 years. • The interseismic slip rate across the eastern Himalaya derived from pure elastic solution may be overestimated if ignoring the viscoelastic relaxation effects. The Eastern Himalayan Syntaxis (EHS) played important role in accommodating the intracontinental deformation and material flow in southeastern Tibet. Quantifying the regional kinematics around the EHS is critical for understanding the deformation mechanism of the Tibetan Plateau. In this study, we compiled all available GPS measurements and constructed an elastic block model to analyze the fault kinematics around the EHS. The results suggest that the deformation characteristics around the EHS can be well explained by a linear spherical block model that considers the intra-block homogeneous strain. The fault slip rates from the block model agree well with the late Quaternary slip rates, suggesting that the GPS-derived slip rates can generally represent the long-term slip rates. However, some relatively large residuals, especially in the frontal EHS region, are still detectable, suggesting that various other factors, such as the postseismic viscoelastic relaxation, may also influence the deformation. We simulated the postseismic viscoelastic relaxation deformation caused by the 1950 Mw 8.7 Assam earthquake endured as the largest intracontinental earthquake ever recorded. The results indicate significant postseismic viscoelastic deformation due to the Assam earthquake even after 65 years. The interseismic convergence rates across the Mishmi thrust and the Arunachal Himalaya derived from pure elastic solution may be overestimated if ignoring the viscoelastic relaxation effects. The spatial variations of convergence rates across the central and eastern Himalayas could be partly related to the transient deformation following great Himalayan earthquakes.