Soft sediment deformation structures from the Brahmaputra Basin: A window to the eastern Himalayan paleoseismicity and tectonics

The Brahmaputra foreland basin, is one of the most dynamic in terms of tectonism and sediment load, revealed a variety of seismically generated soft sediment deformation structures (SSDS) in the form of clastic dikes, sand sills and related micro-faults. The qualitative and quantitative parameters of these SSDS in the study area such as dimension, angle of intrusion to capping layer, cross-cutting relations, wall shattering, sand sills, and micro-faults seem to be caused by multiple strong seismic events. The causative earthquake with intensity (VIII-X), paleo-peak ground acceleration (0.55g) and magnitude (M ≥7.0) might have caused liquefaction features in the study area. Along with the geometry of various SSDS and Optically stimulated luminescence (OSL) dates derived for the clastic dikes indicate the oldest possible age for the seismic event as 0.94 ka. Further, the intensity and age obtained for the present study indicates a possible near seismic source like Kopili Fault Zone, Mishmi Thrust, and neotectonic lineament with trend NW-SE, which can be traced as micro faults and in clastic dikes. Liquefaction potential studies in the area indicate the likelihood of very high liquefaction potential (>45) with ground failures regardless of earthquake size or epicentral distance. The present study highlights the importance of SSDS for establishing paleoseismology in tectonically active regions where surface ruptures are sporadic due to dense alluvial cover and most of the active faults are blind.