Historic large earthquake-induced soft sediment deformation features in the Sub-Himalayan Doon valley

Abstract

Well-preserved soft sediment deformation structures were observed at six sites along the course of the Baldi Nadi in the Doon valley of the Garhwal Himalaya. These deformed structures lie in mid- and side-channel bars of the braided Baldi stream. The deformed sediments are composed of unconsolidated alternations of mud, silt and very fine sand with ripple lamination. Deformation is restricted to a single stratigraphic layer bounded by undeformed beds, suggesting synsedimentary deformation. These features can be traced laterally along the course of the braided Baldi Nadi, over a distance of 7 km in isolated bar deposits at one stratigraphic level, 1.5-2 m above the lean season water level. The deformed features display over-steepening of sedimentary strata, folding, graben-type faulting, plume-like intrusion, flame structure, slumping related to disrupted bedding and pear-drop structure. The style and type of deformation vary considerably in the different segments within a single bed, while the intensity may be same at one site. The deformation features of the Baldi Nadi are interpreted to be the product of liquefaction and fluidization of unconsolidated mud and silt during historic earthquake(s), related to the past seismic activity of the Main Boundary Thrust (MBT) and the Himalayan Frontal Fault (HFF) or blind thrust. The intensity of deformation provides an opportunity to reconstruct the felt area magnitude of large historic earthquake(s) and helps in tracing the palaeoearthquake epicentre. The observed earthquake-induced soft sediment deformation features indicate that in addition to the 1905 'great' earthquake, the Doon valley was also affected by historic earthquake(s) of magnitude [similar]7, possibly corresponding to those of 1803 and/or 1720, if not older.