Outer-arc active tectonic deformation off the Nias - Nicobar Trench: Insights from lithospheric stress and seismic slip models

This study investigates the seismic behavior and deformation characteristics in the outer-arc region off the Nicobar trench using stress inversion and teleseismic finite-fault modeling. The spatial and depth-wise variations in stress field in this region indicate variations in post-seismic deformation scenarios along the arc. The lateral shear, similar to the north Wharton basin, is the major deformational field in the Nicobar segment, favoring earthquake faulting along -ESE or -NNE nodal planes here. Shallow oblique-normal faulting and deeper oblique-reverse regimes are observed in the Nias region. These observations suggest that the plate bending effects directly influence the ongoing tectonics in the Nias region. Moreover, variations in the stress fields across the arc could be indicative of an uncoupled plate interface. Our finite-fault modeling analysis indicates dominant -ESEWNW fault plane orientations for selected events from this region. This suggests the possibility of faulting within similarly oriented active shear structures within the northern Wharton Basin, contrasting the prevailing -N-S faulting pattern in that area. Besides, the centroid depths of most of these earthquakes are typically within the 600 degrees C isotherm. However, the seismic slip may extend deeper by rupturing the crust, occasionally reaching upper mantle depths.