Paleoliquefaction evidence and periodicity of large prehistoric earthquakes in Shillong Plateau, India

The tectonic setting and the occurrence of the great Assam earthquake (M=8.7) of 1897 in the Shillong Plateau succeeded by three great earthquakes (1905, 1934 and 1950) in the adjoining Himalayan frontal arc, indicates the vulnerability of the Shillong Plateau to large earthquakes. The lack of seismicity records of the region earlier than 100 years and data on the recurrence of damaging earthquakes led us to investigate the paleoseismicity of the Shillong Plateau. Our paleoseismic investigations in the meizoseismal area of the 1897 earthquake revealed well-preserved liquefaction and deformed syndepositional features at 10 selected sites in the alluvial deposits along two north flowing tributaries of the Brahmaputra river. These features are 14C dated using associated organic samples. As the liquefaction of sediments was an important feature of the 1897 earthquake, we identified this seismic event, and other large/major prehistoric earthquakes through paleoliquefaction and other coseismic structural deformation at the investigated sites. In addition to the 1897 event, we provide geological evidence for at least three large seismic events. Two of them occurred during 1450–1650 and 700–1050 AD, the third predates 600 AD. The analysis of the 14C data suggests a return period of about 400–600 yr for the large earthquakes in the Shillong Plateau. This finding is the first of its kind from the Himalaya and adjoining region.