Kinematics of Intra-Plate Strike-Slip Earthquakes in an Oblique Convergent Setting  : Insights from the Eastern Himalayan and Indo-Burman Plate Boundary Systems

Abstract

North-East India comprises a part of the eastern extremity of the Alpine-Himalayan Belt and is one of the most rapidly deforming regions owing to its unique geological setup. The tectonics of this region is dominated by oblique convergence between two nearly perpendicular plates and results in a zone of distributed deformation. This region is associated with a large number of intra-plate strike-slip and oblique-slip (thrust) earthquakes which are not related to any of the plate boundaries. In this study we model the source mechanisms of five recent strong-to-moderate earthquakes (5.5≥Mw≤6.0) using teleseismic P and SH waveforms inversion and use source directivity and back-projection of the high-frequency energy from multiple teleseismic arrays for the largest event, to isolate the fault plane from the auxiliary plane. We then combine these mechanisms with results from previous studies of earthquake source and GPS geodetic velocity vectors and the GPS-derived strain field to build a kinematic model for this region. The depth distribution of the earthquakes reveals that they occur in the lower crust of the underthrusting Indian Plate. The oblique-thrust and thrust events are the result of compressive stresses in the inner arc of the flexed Indian Plate. The oblique convergence of the Indian Plate with respect to Tibet and the slab pull force from the subduction of the Indian Plate beneath Burma combined together are responsible for the strike-slip earthquakes. The region north of the Dauki Fault in the vicinity of the Kopili Fault Zone deforms through dextral strike-slip faulting and anti-clockwise rotation of blocks along NW-SE trending transverse structures. The transitional crust of the Bengal Basin has several NE-SW trending paleorifts which manifest sinistral strike-slip motion and clockwise rotation. The GPS velocity vectors and the strain field indicate that throughout most of the region north of the Dauki Fault there is a strong coupling between the surface deformation and the earthquake faulting whereas towards the south in some areas the coupling is weaker. The strike-slip events in the Indo-Burman Ranges probably occur due to a complex interplay between the trench-normal slab pull forces and lateral shear forces set up by the strike parallel components of the interplate coupling resistance and the mantle drag forces.