Earthquakes and deformational structures (seismites) in Holocene sediments from the Himalayan-Andaman Arc, India

Abstract Soft-sediment deformational structures from the Himalayan Arc, caused by episodic earthquake recurrences during the Holocene (Recent), are produced by the ongoing tectonic processes. These small-scale, subtle structures occur along some major thrusts in the Garhwal Himalaya and vary from normal faults in the lower zone to compressional structures in the middle zone along the Main Central Thrust in the Tons Valley. The latter includes isoclinal, angular, kink and disharmonic folds (with discrete displacements along their axial surfaces), thrusts with decollement, listric and overstep thrusts in duplex style, and load slumping. The upper zone is marked by very low-angle thrusts with left-lateral displacement and associated displacement shears (D), Riedel shears (R) and thrust shears (P) as well as small-scale sporadic kinks. Three zones reveal changing tectonic character from gravity tectonics in the lower zone to compression in both the middle and upper zones with the maximum principal stress σ 1 oriented N60° to N110° respectively. Along the Tons Thrust, post-depositional structures include drag along normal fault zone characterized by D-, R- and P-shears and conjugate fractures that provide localized orientation of stresses: σ 1 at 75/N60°, σ 2 at 12/N280° and σ 3 at 12/N290°. Earthquake-induced liquefaction and subtle variations in physiomechanical properties of soft sediments have primarily controlled the deformational pattern due to localized tectonic stresses. Ground and sediment deformations during two earthquakes in the Himalayan-Andaman Arc provide additional support for the seismogenic origin of such structures preserved in loose or consolidated sediments.