Soft-Sediment Deformation in Contemporary Reservoir Sediment: A Repository of Recent Major Earthquake Events in Garhwal Himalaya

Abstract

Two discrete 10–20-cm-thick zones of soft-sediment deformation (SSD) structures in horizontal beds occur in a ∼2.35-m-thick sandbar in the Asan reservoir in the northwestern Sub-Himalaya. The sediment column shows uninterrupted fluviolacustrine deposition since 1975. The depositional and deformational features, boundary conditions, and sedimentation rate suggest that the discrete SSD features formed in an apparent reverse density gradient and were triggered by the 1991 Uttarkashi and 1999 Chamoli earthquakes. These triggering events occurred within 100–150 km of the reservoir, which lay in intensity zones V–VI. The observations correspond well to the empirical relationship between distances to a liquefaction zone from an epicenter. The highly liquefiable sediments in the Asan reservoir provide a unique depositional and tectonic setting to record SSD. Triggering of SSD by contemporary seismic events demonstrates the validity of interpretations of similar causes of SSD in older sediments. This is probably the first documented example of earthquake-triggered SSD from the seismically active Himalaya range.